Units: 1) General principles

(a) Nature of flammable materials (b) gas grouping (c) basic principles of area classification (d) temperature codes (e) ingress protection

2) Standards, Certification and Marking 3) Flameproof Ex d & EEx d 4) Increased Safety Ex e & EEx e 5) Type ‘n’ protection 6) Pressurisation EEx p 7) Intrinsic Safety EEx i 8) Other methods of protection, EEx o, EEx q, EEx m & EEx s 9) Combined (Hybrid) methods of protection 10) Wiring Systems 11) Inspection & Maintenance to BS EN60079-17 12) Sources of ignition 13) Induction to Competence Validation Testing 14) Permit to Work System and Safe Isolation Appendix 1 Data for flammable materials for use with electrical equipment, ref BS5345:

Part 1: General recommendations. Appendix 2 Self assessment project and apparatus label reading. Appendix 3 Supplementary notes for the selection of flameproof cable glands.

National training and certification of personnel for work on electrical apparatus for use in potentially hazardous atmospheres This package has been compiled with information gathered from current standards and the authors will not be held responsible for any inaccuracies found therein. Acknowledgements: The production of this document would not have been possible without the much appreciated assistance from the following authorities and, therefore, the authors of the document wish to thank and gratefully acknowledge all those who provided material and advice for the production of the package, particularly the following: The British Standards Institute JCE (Aberdeen) Ltd, Aberdeen, Scotland James Scott Ltd, Aberdeen, Scotland Weidmuller (Klippon Products) Ltd., Sheerness, Kent Hawke Cable Glands Ltd., Ashton-under-Lyne, Lancashire Hexagon Technology Ltd., Aylesbury, Buckinghamshire Measurement Technology Ltd., Luton, Bedfordshire Brook Hansen, Huddersfield, West Yorkshire The Design and Presentation Team of Aberdeen College, including all staff involved at the Altens Centre The BASEEFA Crown mark shown in this document is the property of the Health and Safety Executive and should not be interpreted to convey certification. The marks have been reproduced with the kind permission of the EECS (HSE). Copyright of document: No part of this document may be reproduced, stored in a retrieval system or transmitted in any form by any means. i.e. electronic, electrostatic, magnetic media, mechanical, photocopying, recording or otherwise without the absolute permission in writing of the appointed representatives of Aberdeen College.

3rd Edition, January 2000

Introduction About the ‘Ex’ facility Ex training courses have been run in Aberdeen College since 1990 and have developed to the level of sophistication we have today. In it’s present form the CompEx course has been in operation since August 1994 and has been designed and constructed specifically for the National Training of personnel who work with electrical installations and plant in hazardous and/or potentially explosive environments. The facility includes both classroom and simulated work areas, these being designed to give as realistic site conditions as is possible to achieve. The practical work you will be required to carry out will take place in these simulated areas and this is intended to make you feel that you are working under site conditions. Approximately half of your time will be spent in the classroom where the ‘job knowledge’ elements of the course will be delivered by means of presentations incorporating lectures, demonstrations, and photographic slides of good and bad practice on apparatus. The remaining time will be spent on Competence Validation Testing in the simulated hazardous areas. The tests are nationally set for Ex training. The outcome The objective of the training is to introduce you to operating procedures and techniques and to give you and your employers confidence that you are competent to work on electrical apparatus in hazardous or potentially explosive environments. The competence certificates gained by you will provide evidence that you have achieved the standards of competence laid down nationally by industry and through this will help make your industry a safer one. About the programme The need for training in this areas of work is self evident in that the safe operation of electrical equipment in hazardous areas is paramount. It is extremely important for all personnel who operate in these conditions to be competent in the correct techniques and operational procedures. This can best be achieved by means of training by skilled staff in an environment as close to the ‘real thing’ as possible. In addition to this, the job knowledge developed through the course must be put into operation in the actual working situation so that the levels of expertise are increased through experience. The design of the programme The program is divided into two halves, namely:

(a) Job Knowledge; (b) Competence Validation Testing (CVT)

The ‘job knowledge’ component takes place during the first half of the week and provides the information and experience you need to tackle the CVT’s.

Selection, Installation, and Maintenance of Electrical Apparatus for use in Hazardous Locations.

Units: 1) General principles

(a) Nature of flammable materials (b) gas grouping (c) basic principles of area classification (d) temperature codes (e) ingress protection

2) Standards, Certification and Marking 3) Flameproof Ex d & EEx d 4) Increased Safety Ex e & EEx e 5) Type ‘n’ protection 6) Pressurisation EEx p 7) Intrinsic Safety EEx i 8) Other methods of protection, EEx o, EEx q, EEx m & EEx s 9) Combined (Hybrid) methods of protection 10) Wiring Systems 11) Inspection & Maintenance to BS EN60079-17 12) Sources of ignition 13) Induction to Competence Validation Testing 14) Permit to Work System and Safe Isolation Appendix 1 Data for flammable materials for use with electrical equipment, ref BS5345:

Part 1: General recommendations. Appendix 2 Self assessment project and apparatus label reading. Appendix 3 Supplementary notes for the selection of flameproof cable glands.

Course outline The training scheme The training scheme is arranged to prepare candidates for the assessment programme which comprises four discreet Competence Validation Tests (CVT’s) offered as complimentary pairs. The four CVT’s are as follows:

EX01 Preparation & Installation of Ex d, Ex e, Ex n and Ex p Systems EX02 Inspection & Maintenance of Ex d, Ex e, Ex n and Ex p Systems EX03 Preparation & Installation of Ex i Systems EX04 Inspection & Maintenance of Ex i Systems

Job knowledge The classroom (job knowledge) part of the training scheme consists of 12 Units which apply to the four CVT’s as illustrated below. Unit 1: General principles Unit 2: Standards,

Certification and Marking

Unit 3: Flameproof EEx d

EX01 & EX02 EX01 & EX02 EX01 & EX02 EX03 & EX04 EX03 & EX04

Unit 4: Increased Safety EEx e

Unit 5: Type ‘n’ protection

Unit 6: Pressurisation EEx p

EX01 & EX02 EX01 & EX02 EX01 & EX02 (Written Assessment) Unit 7: Intrinsic Safety EEx i

Unit 8: Other methods of protection

Unit 9: Combined (Hybrid) protection methods

EX03 & EX04 (Written Assessment) EX01 & EX02 Unit 10: Wiring Systems Unit 11: Inspection &

Maintenance to BS EN60079-17

Unit 12: Sources of ignition

EX01 & EX02 EX01 & EX02 EX01 & EX02 EX03 & EX04 EX03 & EX04 EX03 & EX04

Unit 13: Induction to Competence Validation Testing

Unit 14: Permit to Work and Safe Isolation

EX01 & EX02 EX01 & EX02 EX03 & EX04 EX03 & EX04

The CVT’s are a series of practical tests which you will undertake within the simulated work areas during the second half of the programme. On successful completion of these tests you will be awarded a Certificate of Core Competence which will indicate the areas the awarding body, Joint Training Ltd. (JTL), has deemed you are competent. During the final half-day of the programme you are required to sit written assessments in the form of multi-choice papers which are related to the practical CVT assessments. The staff who are involved in monitoring the various assessments are present only as observers and not to prompt or offer technical assistance. Their observations of your work is recorded on Nationally written checklists which are processed outwith the Centre and your results cannot be determined until this process is complete. Self-assessment project At the rear of this manual, you will find assessment material which you should complete during the course. This exercise will enable you to determine your prior knowledge of the subject, especially in the area of interpretation of apparatus labelling.

Programme: Electrical Apparatus in Potentially Hazardous Areas

5 - Day Programme

Monday Tuesday Wednesday Thursday FridayPresente

r

8.30

12.30

Course induction Unit 1: General principles Unit 3: Flameproof Ex d Unit 6: Pressurisation Ex p

Unit 13: Induction to Competence Validation Testing Unit 4: Increased Safety EEx e Unit 5: Type ‘n’ protection

Unit 7: Intrinsic Safety EEx i

EX02 CVT Inspection & maintenance

of d, e & n

apparatus

Candidates 7-12

EX01 CVT Preparation &installation of

d, e & n apparatus

Candidates

1-6

EX04 CVT Inspection & maintenance

of type ‘i’

apparatus

Candidates 7-12

EX03 CVT Preparation

& installation of

type ‘i’ apparatus

Candidates

1-6

Break Break Break Break Break13.00

17.00

Unit 10: Wiring systems and cable glanding Unit 14: Permit to work and safe isolation

Unit 2: Standards certification

Preparation & installation of and marking

Unit 11: Inspection & maintenance to IEC 79-17 Unit 8: Other methods of protection Unit 9: Hybrid methods of protection

EX01 CVT

d, e & n apparatus

Candidates

1-6

EX02 CVT Inspection & maintenance

of d, e & n

apparatus

Candidates 7-12

EX03 CVT Preparation &installation of

type ‘i’ apparatus

Candidates

1-6

EX04 CVT Inspection & maintenance

of type ‘i’

apparatus

Candidates 7-12

Job knowledge assessment

EX01 JK - EX02 JK

EX03 JK - EX04 JK

Unit 1: General principles Objectives: On completion of this unit, ‘general principles’, you should know: a) the nature of flammable materials with regard to ‘explosive limits’ (LEL/UEL), ‘flashpoint’,

‘ignition temperature’, the effect of ‘oxygen enrichment’ and ‘relative density’; b) the basic principles of area classification; c) the Grouping of gases according to ‘minimum ignition energy’ (MIE) and ‘maximum

experimental safe gap’ (MESG); d) appropriate T-ratings for apparatus relative to the ignition temperature of a given flammable

material, e) the levels of ‘ingress protection’.

- 1 -

General principles Nature of flammable materials Fire triangle The fire triangle represents the three elements which must be present before combustion can take place. Each point of the triangle represents one of the essential elements which are: (1) Fuel: This can be in the form of a gas, vapour, mist or dust. (2) Oxygen: Plentiful supply since there is approximately 21% by volume in air. (3) Source of ignition: This can be an arc, spark, naked flame or hot surface.

Combustion will take place if all three elements, in one form or another, are present, the gas/air mixture is within certain limits and the source of ignition has sufficient energy. The removal of one element is sufficient to prevent combustion as is the isolation or separation of the source of ignition from the gas/air mixture. These are two techniques used in explosion protected equipment. Other protection techniques allow the three elements to co-exist and either ensure that the energy of the source of ignition is maintained below specific values, or allow an explosion to take place and contain it within a robust enclosure. These techniques are addressed in the various sections of this manual.

- 2 -

Flammable (Explosive) Limits Combustion will only occur if the flammable mixture comprising fuel, in the form of a gas or vapour, and air are within certain limits. These limits are the ‘lower explosive limit’ (LEL), and the ‘upper explosive limit’ (UEL), and between these limits is known as the flammable range. An every day example of this is the carburettor of a petrol engine, which must be tuned to a particular point between these limits in order that the engine may function efficiently.

Lower Explosive Limit: When the percentage of gas, by volume, is below this limit the

mixture is too weak to burn, i.e. insufficient fuel and/or too much air. Upper Explosive Limit: When the percentage of gas, by volume, is above this limit the

mixture is too rich to burn, i.e. insufficient air and/or too much fuel. The flammable limits of some materials are given below.

Material LEL % by Volume

UEL % by Volume

Propane 2 9.5

Ethylene 2.7 34

Hydrogen 4 75.6

Acetylene 1.5 100

Diethyl Ether 1.7 36

Paraffin 0.7 5

Carbon Disulphide 1 60

- 3 -

Flammable (Explosive) Limits (continued) Different gases or vapours have different flammable limits, and the greater the difference between the LEL and the UEL, known as the flammable range, the more dangerous the material. An explosive (flammable) atmosphere, therefore, only exists between these limits. Operational safety with flammable mixtures above the UEL is possible, but is not a practical proposition. It is more practical to operate below the LEL. Sources of ignition Sources of ignition are many and varied and include:

(a) electrical arcs/sparks; (b) frictional sparks; (c) hot surfaces; (d) welding activities (e) cigarettes; (f) static discharges; (g) batteries; (h) exhausts of combustion engines; (i) thermite action; (j) sodium exposed to water (k) pyrophoric reaction; (l) chemical reactions; (m) lightning strikes;

The source of ignition as far as this text is concerned is primarily electrical equipment.

- 4 -

Flashpoint By definition flashpoint is: ‘the lowest temperature at which sufficient vapour is given off a liquid, to form a flammable mixture with air that can be ignited by an arc, spark or naked flame’. Typical values are given below

Material Flashpoint °C

Propane -104

Ethylene -120

Hydrogen -256

Acetylene -82

Diethyl Ether -45

Paraffin 38

Carbon Disulphide -95

The flashpoint of a material gives an indication of how readily that material will ignite in normal ambient temperatures. Reference to the tables of flammable materials from the UK Code of Practice, B55345: Part 1 (see Appendix 1) reveals that different materials have different flashpoints, which vary from well below to well above 0°C. Materials with high flashpoints should not be overlooked as a potential hazard since exposure to hot surfaces can allow a flammable mixture to form locally. Furthermore, if a flammable material is discharged under pressure from a jet, its flashpoint may be reduced.

Amount of vapour released dependant on temperature

- 5 -

Flashpoint (continued)

Kerosene: flashpoint 38 oC

- 6 -

Ignition temperature Ignition temperature is defined as: ‘the minimum temperature at which a flammable material will spontaneously ignite’. Ignition temperature, formerly known as auto-ignition temperature, is an important parameter since many industrial processes generate heat. Careful selection of electrical equipment will ensure that the surface temperatures produced by the equipment, indicated by the T-rating, will not exceed the ignition temperature of the flammable atmosphere which may be present around the equipment. Typical values of ignition temperature are:

Material Ignition Temperature

oC

Propane 470

Ethylene 425

Hydrogen 560

Acetylene 305

Diethyl Ether 170

Paraffin 210

Carbon Disulphide 102

- 7 -

Oxygen enrichment The normal oxygen content in the atmosphere is around 20.95%, and if a given location has a value which exceeds this it is deemed to be oxygen enriched. Typical examples of where oxygen enrichment may occur are gas manufacturing plants, hospitals, and where oxy-acetylene equipment is used. Oxygen enrichment has three distinct disadvantages. First of all, it can lower the ignition temperature of flammable materials as shown in the table below.

Air Increased Oxygen Material

Ignition temperature °C

Ignition temperature °C

Hydrogen sulphide 260 220

Acetylene 305 296

Ethane 512 506

Secondly, oxygen enrichment significantly raises the upper explosive limit (UEL) of the majority of gases and vapours, thereby widening their flammable range. This is illustrated in the following table.

Air Increased oxygen Material

LEL %

UE %

LEL %

UEL %

Methane 5 15 5.2 79

Propane 2.2 9.5 2.3 55

Hydrogen 4 75 4.7 94

Thirdly, oxygen enrichment of a flammable atmosphere can allow it to be ignited with much lower values of electrical energy. Explosion protected equipment will have been tested in normal atmospheric conditions and, therefore, the safety of such equipment in an oxygen enriched atmosphere cannot be assured because of the modified nature of the flammable mixture.

- 8 -

Density If a flammable material is released, it is important to know whether the material will rise or fall in the atmosphere. The different flammable materials are compared with air and allocated a number to denote their relative density with air. Since air is the reference, its relative density is 1 so that for a material twice as heavy as air, its relative density will be 2. Therefore, materials with a relative density less than unity will rise in the atmosphere, and those greater than unity will fall in the atmosphere. Materials which rise in the atmosphere can collect in roof spaces, and those which fall, such as butane or propane, can drift along at ground level and possibly into a non-hazardous location, or may collect in locations lower than ground level without ever dispersing. Such locations should be well ventilated in order to avoid ignition due to a stray spark or a discarded cigarette. Knowledge of where a flammable material will collect ensures that gas detectors when fitted will be located at the correct level and ventilation is directed accordingly.

Material Relative vapour density

Air 1

Propane 1.56

Ethylene 0.97

Hydrogen 0.07

Acetylene 0.9

Diethyl Ether 2.55

Paraffin 4.5

Carbon Disulphide 2.64

- 9 -

Area classification An hazardous area is defined as: an area in which an explosive gas atmosphere is present, or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of apparatus. A non-hazardous area is defined as: an area in which an explosive gas atmosphere is not expected to be present in quantities such as to require special precautions for the construction, installation and use of apparatus. Zones Zoning is a means of representing the frequency of the occurrence and duration of an explosive gas atmosphere based on the identification and consideration of each and every source of release in the given areas of an installation. Zoning will have a bearing on, and simplify the selection of, the type of explosion protected equipment which may be used. Hazardous areas are, therefore, divided into three Zones which represent this risk in terms of the probability, frequency and duration of a release. The three Zones, as defined in BS EN60079-l0: Electrical apparatus for explosive gas atmospheres, Part 10. Classification of hazardous areas, are as follows:

Zone 0 - In this Zone, an explosive gas atmosphere is continuously present, or present for long periods;

Zone 1 - In this Zone, an explosive gas atmosphere is likely to occur in

normal operation. Zone 2 - In this Zone, an explosive gas atmosphere is not likely to occur in

normal operation and, if it does occur, is likely to do so only infrequently and will exist for a short period only.

Although not specified in the standards, it is generally accepted in the industry that the duration of a gas release, or a number of gas releases, on an annual basis (one year comprises circa 8760 hours), for the different Zones is as follows.

Zone 2 - 0 - l0 hours Zone 1 - 10 - l000 hours Zone 0 - over 1000 hours

- 10 -

Area classification (continued) Zones - diagram representation of

- 11 -

Area classification (continued) Fixed roof storage tank

- 12 -

Area classification (continued) Sources of release

- 13 -

Area classification (continued) Platform hazardous areas

- 14 -

Gas / apparatus grouping In the IEC system, the Group allocation for surface and underground (mining) industries are separate. Group I is reserved for the mining industry, and Group II which is subdivided into IIC, IIB and IIA for surface industries using. The representative gases for the sub-groups are shown in the table below. Two methods have been used to ‘group’ these flammable materials according to the degree of risk they represent when ignited. One method involved determining the minimum ignition energy which would ignite the representative gases. In the table below it can be seen that for Group II, hydrogen and acetylene are the most easily ignited and propane the least easily ignited. The other method involved tests using, for example, a special flameproof enclosure in the form of an 8 litre sphere which was situated inside a gas-tight enclosure. Both halves of the sphere had 25 mm flanges and a mechanism enabled the gap dimension between the flanges to be varied. During tests, the area inside and outside the sphere was occupied with a gas in its most explosive concentration in air and, by means of a spark-plug, the gas inside the sphere was ignited. The maximum dimension between the flanges, which prevented ignition of the gas/air mixture, is known as the ‘maximum experimental safe gap’ (MESG), and the values for the representative gases are shown in the table below. The more dangerous a gas, the tighter the gap at the flanges has to be. The table also shows that these flammable materials fall into the same order for both tests, i.e. in a relative context, hydrogen and acetylene present the most risk and propane the least risk in terms of ‘minimum ignition energy’ and ‘MESG’.

Gas Group

Representative

Gas

MESG

(mm)

Maximum Working

Gap (mm)

Minimum Ignition Energy

(µJ)

I Methane (Firedamp) 1.17 0.5 280

IIA Propane 0.97 0.4 260

IIB Ethylene 0.71 0.2 95

IIC Hydrogen & Acetylene 0.5 0.1 20

Note: Apparatus other than flameproof or intrinsic safety, which has no sub-division letter (A, B or

C) after the group II mark, may be used in all hazards.

Apparatus marked IIXXXXX: XXXXX represents the chemical formula or name of a flammable material, and apparatus marked in this way may only be used in that hazard.

- 15 -

Gas / apparatus grouping The group sub-division marking is one of the important considerations during the selection process of explosion protected apparatus. For example, apparatus marked IIA can only be used in IIA hazards such as propane, it can not be used in IIB or IIC hazards. Apparatus marked IIB can be used in IIB and IIA hazards but not IIC hazards. Apparatus marked IIC can be used in all hazards. Apparatus for determination of M.E.S.G.

- 16 -

Gas / apparatus grouping Comparison of BS 229 and IEC BS 229 is an old British Standard which is obsolescent. This means that electrical apparatus, although very few, is still made to this standard.

BS 229 Representative Gas IEC

I Methane I

II Propane IIA

IIIa Ethylene

IIIb Coal Gas IIB

IV Hydrogen & Acetylene IIC

- 17 -

Temperature Classification Approved electrical equipment must be selected with due regard to the ignition temperature of the flammable gas or vapour which may be present in the hazardous location. Apparatus will usually be marked with one of the temperature codes shown in the table below. Temperature codes

Temperature code Maximum surface temperature

T1 450°C

T2 300°C

T3 200°C

T4 135°C

T5 100°C

T6 85°C

In the table below, it will be observed that for each material, the T-rating temperature is below the ignition temperature of the flammable material. Moreover, the T-rating temperatures are based on a maximum ambient rating of 40 °C as far as the UK is concerned. For example, apparatus classified T5, based on a 40 °C ambient rating will have a maximum permitted temperature rise of 60 °C. In order to avoid infringement of the apparatus certification, the ambient rating must be compatible with environmental ambient temperatures, and the temperature rise not exceeded. This is demonstrated on page 18. A further consideration is that apparatus for use in hotter climates, typically found in Middle and Far Eastern countries, will usually require ambient ratings greater than 40 °C.

Material Ignition temperature

T-rating

Methane 595 °C T1 (450 °C)

Ethylene 425 °C T2 (300 °C)

Cyclohexane 259 °C T3 (200 °C)

Diethyl Ether 170 °C T4 (135 °C)

Carbon Disulphide 102 °C T5 (100 °C)

T6 ( 85 °C)

- 18 -

Temperature Classification (continued)

- 19 -

Ingress Protection Enclosures of electrical equipment are classified according to their ability to resist the ingress of solid objects and water by means of a system of numbers known as the ‘International Protection (IP) Code’. This Code, which is not always marked on apparatus, consists of the letters IP followed by two numbers, e.g. IP56. The first number, in the range 0 - 6, indicates the degree of protection against solid bodies, and the higher the number the smaller the solid object that is prevented from entering the enclosure. Zero (0) indicates no protection and 6 indicates the apparatus is dust-tight. The second number, ranging from 0 - 8, identifies the level of protection against water entering the enclosure, i.e. 0 indicates that no protection is afforded, and 8 that the apparatus can withstand continuous immersion in water at a specified pressure. An abridged version of the full table is shown below.

Solid Objects Water

First Numeral Level of Protection Second

Numeral Level of Protection

0 No protection 0 No protection

1 Protection against objects greater than 50 mm 1 Protection against drops of water

falling vertically

2 Protection against objects greater than 12 mm 2 Protection against drops of water

when tilted up to 15°

3 Protection against objects greater than 2.5 mm 3 Protection against sprayed water

up to 60°

4 Protection against objects greater than 1.0 mm 4 Protection against splashed water

from any direction

5 Dust-protected 5 Protection against jets of water from any direction

6 Dust-tight 6 Protection against heavy seas - deck watertight

7 Protection against immersion in water 1m in depth and for a specified time

8 Protection against indefinite immersion in water at a specified depth

- 20 -

Unit 2: Standards, certification and marking Objectives: On completion of this unit, ‘Standards, Certification and Marking’, you should know: a) current British, European and International Standards and also relevant older British

Standards and Codes of Practice; b) the certification process for explosion protected apparatus; c) the methods of marking explosion protected apparatus.

- 1 -

Standards, certification and marking Introduction There are many industries involved in the process of hazardous materials, and these include chemical plants, oil refineries, gas terminals and offshore installations. These industries rely heavily on electrical energy to power, for example, lighting, heating and rotating electrical machines. The safe use of electrical energy in the hazardous locations of these industries can only be achieved if tried and tested methods of explosion protection are implemented and to this end, the authorities involved in the writing of standards, testing and certification of equipment have a very important role to play. Since the early 1920’s, many standards have evolved as a result of careful research, often prompted by incidents such as the Senghennydd colliery disaster in 1913 in which 439 miners lost their lives. The cause at that time was not fully understood but after investigation, was thought to have been due to an electrical spark igniting methane (firedamp) present in the atmosphere. Other disasters include Abbeystead Water Pumping Station in which 14 people lost their lives, once again due to the electrical ignition of methane gas, Flixborough, and more recently Piper Alpha in the North Sea in which 167 men lost their lives. Construction of apparatus to relevant standards coupled with testing by an independent expert test authority will ensure that the apparatus is suitable for its intended purpose. Explosion protected apparatus may be constructed in accordance with relevant standards, but the integrity of such apparatus will only be preserved if such apparatus is selected, installed and maintained in accordance with the manufacturers recommendations. Guidance in this respect has been provided for many years by the UK Code of Practice BS 5345, but this document has been superseded by a new series of five separate harmonised standards based on the IEC79 series of International standards. These five documents cover, (1) installation of apparatus, (2) classification of hazardous areas, (3) inspection and maintenance, (4) repair of explosion protected apparatus and (5) data for flammable gases. These new standards are a further stage in the process to harmonise standards globally. Although BS 5345 has been superseded, it nevertheless remains current until it is completely withdrawn at sometime in the future. In the United Kingdom, manufacturing and testing standards are published by an organisation known as the British Standards Institute (BSI). With regard to Europe, the organisation which publishes harmonised standards for it’s member nations is the European Committee for Electrotechnical Standardisation (CENELEC) and, with a view to global harmonisation the International Electrotechnical Commission (IEC) publishes standards for this purpose. Equipment designs are evaluated and prototypes tested by independent organisations, one of which was formerly known as ‘British Approvals Service for Electrical Equipment in Flammable Atmospheres (BASEEFA), but is now known as ‘Electrical Equipment Certification Service (EECS)’. The acronym, BASEEFA, which has been closely associated with explosion protected apparatus for may years, has been retained by EECS for certification marking purposes. EECS is part of the Health and Safety Executive (HSE). EECS also publishes standards for special applications. The other European nations have, as do other/nations internationally, their own organisations which publish manufacturing and testing standards, and also testing and certification services. Some of these are introduced in the following text.

- 2 -

Reasons for Product Certification 1) To demonstrate product quality with regard to the ability of the apparatus to function safely in

an hazardous environment. 2) To enhance market acceptability by inspiring confidence in those involved in the selection,

purchase, installation, operation and maintenance of approved/certified products. 3) To improve quality and safety control procedures in manufacturing and construction. Certification process The certification process will generally involve submission of drawings of the proposed equipment design to the certification authority. If the drawings are in compliance with the relevant standards, the authority will request a prototype of the equipment in order that tests, which are normally detailed in the standards, can be carried out. A detailed certification report is compiled and retained on file by the test authority and, if all requirements are satisfied, the equipment will be issued with a ‘certificate of conformity’ which allows the manufacturer to display, for example, the EECS ‘crown’ symbol and the European Community symbol - shown later in this text - on the label of the equipment and associated documentation. The manufacturers facilities will also be inspected including quality control procedures and if satisfactory the manufacturer will be allocated a three year licence to manufacture the product. During this three year term, the manufacturer’s facilities will be regularly inspected to ensure that the production of equipment is consistent with the original certified design, and to ensure that quality is maintained to acceptable standards.

- 3 -

Evolution of BS - British Standards

SMRE: Safety in Mines Research Establishment BASEEFA: British Approvals Service for Electrical Equipment in Flammable Atmospheres

- 4 -

European test authorities

- 5 -

Comparison of IEC, European (CENELEC) and British Standards Prior to the closer ties between the UK and Europe, electrical equipment, such as flameproof or increased safety etc., was manufactured in accordance with the British Standard BS 4683. Equipment built and certified to this standard was entitled to display the mark Ex on its label, which indicated that the apparatus was explosion protected. This term should not be confused with term explosion-proof as they are entirely different. In addition to the ‘Ex’ mark, the label was also marked with a ‘crown’ symbol, which is the distinctive mark for the UK test house BASEEFA, or EECS as they are now known. Various American and European symbols are shown in Page 8. Because of the differences in standards, equipment manufactured in the UK could not be used in the other European countries and vice-versa, and hence, equipment made to BS 4683 could only be used in the UK, or certain other countries outwith Europe. Co-operation between the standards writing bodies in the UK and Europe resulted in the development of ‘Harmonised’ standards, also known as ‘Euronorms’ of which the English version was published as BS 5501 comprising nine separate parts and shown in the middle column below. The Euronorm equivalents, written in French, are shown in the first column. Column four shows the new numbers for the revised standards which will replace BS 5501. Note: The draft European (CENELEC) standard, EN50 021 for type of protection ‘n’, which had

been under consideration for many years, was finally approved on 1 August 1998 and issued as BS EN50 021 in April 1999. As a consequence, the UK standard BS 6941 may eventually be withdrawn.

CENELEC EURONORM

(EN) NUMBER

INTERNATIONAL STANDARDS

BRITISH STANDARD

(BS) NUMBER

REVISED STANDARD

(BS EN) NUMBER

TYPE OF PROTECTION

EN 50 014 IEC 79-0: Pt.0 BS 5501: Pt. 1 BS EN50 014 General Requirements

EN 50 015 IEC 79-6: Pt.6 BS 5501: Pt. 2 BS EN50 015 Oil Immersion ‘o’

EN 50 016 IEC 79-2: Pt.2 BS 5501: Pt. 3 BS EN50 016 Pressurised Apparatus

‘p’

EN 50 017 IEC 79-2: Pt.5 BS 5501: Pt. 4 BS EN50 017 Powder Filling ‘q’

EN 50 018 IEC 79-1: Pt.1 BS 5501: Pt. 5 BS EN50 018 Flameproof Enclosure ‘d’

EN 50 019 IEC 79-7: Pt.7 BS 5501: Pt. 6 BS EN50 019 Increased Safety ‘e’

EN 50 020 IEC 79-11: Pt.11 BS 5501: Pt. 7 BS EN50 020 Intrinsic Safety ‘i’

EN 50 028 IEC 79-18: Pt.18 BS 5501: Pt. 8 BS EN50 028 Encapsulation ‘m’

EN 50 039 BS 5501: Pt. 9 BS EN50 039 Intrinsic Safety Systems

‘i’ EN 50 021 IEC 79-15: Pt.15 BS EN50 021 Type of Protection ‘n’

BS 6941 Type of Protection ‘n’

- 6 -

Other (older) British Standards The standards listed below are those which preceded the harmonised European standards listed in the previous table. These standards, and in particular BS 4683, are not entirely obsolete, and older designs of equipment may still be manufactured to these standards. It is, therefore, important that reference to the correct standard is made before maintenance is carried out on such apparatus. BS 5345 is included because it is from this document that the table of data for flammable materials - see Appendix 1 - is taken.

BS 229 Flameproof enclosure of electrical apparatus (Obsolescent)

BS 889 Flameproof electric light fittings (Withdrawn)

BS 1259 Intrinsically safe electrical apparatus and circuits for use in explosive atmospheres (Obsolescent)

BS 4683: Part 1 Classification of maximum surface temperature

BS 4683: Part 2 The construction and testing of flameproof enclosures of electrical apparatus.

BS 4683: Part 3 Type of protection ‘N’.

BS 4683: Part 4 Type of protection ‘e’.

BS 4683: Part 15 Machines with type of protection ‘e’.

BS 4683: Part 16 Type ‘N’ electric motors

BS 5345 UK Code of practice for the selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres.

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Standards for Selection, Installation and Maintenance As previously stated, the UK Code of Practice BS 5345, which has for many years provided recommendations for the selection, installation and maintenance of explosion protected apparatus for use in potentially explosive atmospheres (other than mining applications or explosives processing and manufacture), has been superseded but remains current until completely withdrawn. The table below illustrates its component parts.

UK CODE OF PRACTICE TYPE OF PROTECTION

BS 5345: Part 1 General Recommendations

BS 5345: Part 2 Classification of Hazardous Areas

Installation and maintenance requirements for electrical apparatus with type of protection:

BS 5345: Part 3 ‘d’ Flameproof enclosure

BS 5345: Part 4 ‘i’ Intrinsically safe apparatus and systems

BS 5345: Part 5 ‘p’ Pressurisation, continuous dilution and pressurised rooms

BS 5345: Part 6 ‘e’ Increased safety

BS 5345: Part 7 ‘N’ (Non - incendive)

BS 5345: Part 8 ‘s’ Special protection

BS 5345: Part 9 ‘o’ Oil immersion ‘q’ Powder filling

The standards which supersede the Code of Practice BS 5345 are illustrated in the table below

BS EN / IEC Nos. Electrical Apparatus for Explosive Gas Atmospheres:

BS EN60079-10: 1996 (IEC 60079-10: 1995)

Part 10: Classification of hazardous areas

BS EN60079-14: 1997 (IEC 60079-14: 1996)

Part 14: Electrical installations in hazardous areas (other than mines)

BS EN60079-17: 1997 (IEC 60079-17: 1996)

Part 17: Inspection and maintenance of electrical installations in hazardous areas (other than mines)

BS EN60079-19: 1997 (IEC 60079-19: 1996)

Part 19: Repair and overhaul for apparatus used in explosive atmospheres (other than mines or explosives)

BS EN60079-20: (IEC 60079-20:)

Data for flammable gases

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Certification symbols The following symbols are used to identify apparatus approved/certified by recognised European and American authorities. European marks 1)

Equipment marked with this symbol may only be used for underground (mining) applications in the UK.

2)

This is the EECS (BASEEFA) symbol and used to identify equipment for surface industry use only.

3)

Equipment marked with this symbol, the European Community mark, in addition to the above symbol (2), indicates that the apparatus has been constructed and tested in accordance with the CENELEC/EURONORM standards.

4)

The symbol used by the German certification authority PTB.

UL mark 5)

The most common UL listing mark.

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Apparatus marking Apparatus approved/certified as providing a method of protection for use in hazardous locations are required to display the following markings.

(a) The symbols Ex or EEx; and (b) The type of protection used, e.g. ‘d’, ‘e’, ‘N’, and (c) The gas group, e.g. IIA, IIB or IIC; and (d) The T-rating, e.g. T1, T2 etc.

Examples: i) Ex d IIB T3 ii) EEx d IIC T4 iii) EEx e II T6 In example (i), equipment marked thus (Ex), as far as Europe is concerned, can only be used in the UK because it has been constructed to the British Standard BS 4683, which is not a harmonised European standard. Apparatus constructed to this standard, however, is used in other countries outwith the European Community. Such equipment would also be marked with the EECS certification authority symbol (2) on the previous page. For apparatus marked EEx as in examples (ii) and (iii), the additional letter ’E’ indicates that the apparatus has been constructed to a harmonised European standard. Such apparatus would be marked with the EECS certification authority symbol (2) as well as the European Community mark (3). Sample labels are shown below, and it should be noted that the construction standard to which the equipment has been manufactured to, i.e. BS 4683: Part 2, BS 5501: Parts 1 & 5 and EN50 014 & EN50 018 are also given on the labels. For BS 4683 equipment, the IEC equivalent standard, i.e. IEC 79-1 in example (a) below, is usually included.

(a) BS 4683: Pt.2 (IEC79-1) (b) BS 5501: Pt.1 & 5 (EN50 014 & EN50 018)

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Certification marking

- 11 -

Certificate number

- 12 -

ATEX ATEX represents the European Union’s Directive 94/9/EC which specifies the new requirements which manufacturers of, for example, explosion protected equipment must comply with. These requirements are wide ranging and beyond the scope of this section but, what is important is the influence the directive will have on the marking of explosion protected apparatus. This will be the most obvious difference to those involved in the selection, installation and maintenance of explosion protected apparatus. The marking required by the EU Directive 94/9/EC is illustrated below and is additional to the marking requirements already discussed.

The categories are defined overleaf.

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Category definitions Group II Category 1: Very high level of protection. Equipment with this category of protection may be used where an

explosive atmosphere is present continuously or for long periods, i.e. Zone 0 or Zone 20.

Category 2: High level of protection. Equipment with this category of protection may be used where an

explosive atmosphere is likely to occur in normal operation, i.e. Zone 1 or Zone 21.

Category 3: Normal level of protection. Equipment with this category of protection may be used where an

explosive atmosphere is unlikely to occur or be of short duration, i.e. Zone 2 or Zone 22.

Group I Category Ml: Very high level of protection. Equipment can be operated in the presence of an explosive

atmosphere. Category M2: High level of protection. Equipment to be de-energised in the presence of an explosive

atmosphere.

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Unit 3: Flameproof Ex d & EEx d Objectives: On completion of this unit, ‘flameproof Ex d & EEx d’ apparatus, you should know: a) the principle of operation and causes of pressure piling; b) the general constructional requirements including types of joints; c) the installation requirements with regard to thread engagement of cable entries and

stopping devices, circuit protection, obstruction of flamepaths and additional weatherproofing methods in accordance with BS EN60079-14;

d) the inspection requirements with regard to BS EN60079-17.

Flameproof EEx d or Ex d Flameproof is one of the original methods of explosion protection developed for use in the mining industry. It has a wide range of applications, typically junction boxes, lighting fittings, electric motors etc. The letter ‘d’, which symbolises this type of protection, is from the German word ‘druckfeste’ (kapselung), which roughly translated means ‘pressure tight’ (enclosure).

Flameproof apparatus, when properly installed in the intended location, enables components such as switches, contactors and relays etc. to be safely used in hazardous areas. Flameproof is the only one of the nine different methods of explosion protection in which an explosion is permitted. This explosion, however, must be contained by the robustly constructed flameproof enclosure.

Standards

BS EN50 018 Flameproof enclosure ‘d’.

BS 5501: Part 5: 1977 Flameproof enclosure ‘d’.

BS 4683: Part 2: 1971 The construction and testing of flameproof enclosures of electrical apparatus (Ex d).

BS 229: 1957 Flameproof enclosures of electrical apparatus

IEC 79-1, Part 1: 1971 Construction and Test of Flameproof Enclosures of Electrical Apparatus (Ex d).

BS EN60079-14: Part 14 Electrical apparatus for explosive gas atmospheres: Part 14. Electrical installations in hazardous areas (other than mines).

BS EN60079-17: Part 17 Electrical apparatus for explosive gas atmospheres: Part17. Inspection and maintenance of electrical installations in hazardous areas (other than mines).

BS 5345: Part 3 Code of Practice for the Selection, Installation and Maintenance of flameproof apparatus.

Definition The construction standard BS EN50 018 defines flameproof as:

‘A type of protection in which the parts which can ignite an explosive atmosphere are placed in an enclosure which can withstand the pressure developed during an internal explosion of an explosive mixture and which prevents the transmission of the explosion to the explosive atmosphere surrounding the enclosure’.

Zone of Use: 1 & 2 Ambient conditions Flameproof enclosures are normally designed for use in ambient temperatures in the range -20°C to +40°C unless otherwise marked

Principle of operation Flameproof enclosures are not gas tight and a gas or vapour will enter the enclosure where, for example, joints or cable entries exist. Since these enclosures are designed to contain components which are an ignition source, ignition of the gas or vapour may occur, and the resulting explosion pressure can reach a peak value of around 150 p.s.i. The enclosure must, therefore, be strong enough to contain this explosion pressure, and the gaps at the joints and threads of cable entries must be long and narrow to cool the flames/hot gases before they reach, and cause ignition of a flammable atmosphere which may exist outwith the enclosure. Typical materials used for the construction of flameproof apparatus include cast iron, aluminium alloys, and where corrosion resistance is required, gun metal bronze, phosphor bronze and stainless steel may be used. Plastic materials are also used but the free internal volume must not exceed 10 cm3. Both standards specify that ‘THERE SHALL BE NO INTENTIONAL GAP AT THE COVER JOINTS’ and that the average roughness Ra of the flamepath surfaces must not exceed 6.3 µm.

Gap dimensions Although the standards specify that there shall be no intentional gap at the joints of flameproof equipment, gaps will normally exist due to manufacturing methods, tolerances and economics, but must not be in excess of the dimensions specified in the tables of the relevant standards for a given hazard. Factors which influence the dimension of the gap are:

(a) the width of the joint; (b) the gas group; (c) the internal volume of the enclosure; (d) the type of joint.

Flamepath joints The diagrams below show three types of joints specified in the British standard BS EN50 018 for use in flameproof apparatus. In a flanged joint, the machined surface on the cover makes face-to-face contact with the corresponding surface on the base to give a gap dimension normally less than that specified in the tables when the cover is properly bolted down. This type of joint will be used at the covers of, for example, junction boxes. Spigot joints will be used at junction box covers and motor endshields. Threaded joints are used for cover joints, cable gland and conduit entries. An adequate flamepath length is normally achieved with a thread engagement of five full threads. This standard does not permit the use of flanged joints when a IIC gas such as acetylene is the hazard. Flanged joints may be used for other IIC gases or vapours but the enclosure volume must not exceed 500 cm3.

(a) Flanged joint

(b) Spigot joint

(c) Screwed joint

Flamepath joints types (rotating machines) (d) Cylindrical (shaft gland) joint

(e) Labyrinth joint for shafts

Flamepath joints (other examples) Flamepaths other than those at cover joints are also necessary where, for example, an actuator spindle passes through the wall of an enclosure, or where a cable gland or conduit enters an enclosure. Examples are shown below. Push-button spindle

Cable (gland) entry

Entry by cable or conduit The thread engagement requirements for cable and conduit entries are specified in BS EN50 018 and apply to the three sub-groups IIA, IIB and IIC. Only threaded entries are permitted for all cable glands or conduit entering flameproof enclosures - clearance entries are not permitted.

Volume

≤ 100 cm3 > 100 cm3

Thread engagement

Axial length

Thread engagement

Axial length

≥ 5 full threads ≥ 5 mm ≥ 5 full threads ≥ 8 mm

Unused cable or conduit entries It is important that unused cable/conduit entries in flameproof enclosures are closed using appropriate stoppers as specified in the standards or supplied by the manufacturer. These must be ‘component certified’ metal stoppers - plastic stoppers are unacceptable - which are fully engaged by 5 full threads. The construction standard specifies suitable types, examples of which are illustrated below.

Flamepath gap dimensions - BS EN50 018 Table 1

Flamepath gap dimensions - BS EN50 018 Table 2

Pressure piling

If a flammable mixture is compressed prior to ignition, the resulting explosion will be considerably higher than if the same mixture was ignited at normal atmospheric pressure. Pressure piling can materialise as a result of sub-division of the interior of a flameproof enclosure, which prevents the natural development of an explosion. An explosion at one side of an obstacle pre-compresses the flammable mixture at the other side, resulting in a secondary explosion which can reach an explosion pressure around three times that of the first or normal explosion pressure. Manufacturers, guided by relevant construction standards, must ensure that, in any cross-section within an enclosure, there is adequate free space (typically 20 - 25% of the total cross-section) around any potential obstruction, which may be a large component or a number of components. This will ensure that pressure piling is kept under control.

Pressure piling in flameproof motors

In rotating electrical machines, sections with appreciable free volume normally exist at each end within the main frame of the machine. These sections are linked by the airgap between the stator and rotor cores. In the illustration of a flameproof machine in the diagram below, an explosion in section ‘1’ must be prevented from migrating to, and causing ignition of the flammable mixture in section ‘2’ which will have been pressurised by the initial explosion. The airgap, therefore, also acts as a flamepath.

Obstruction of Flamepaths The UK Code of Practice BS 5345 Part 3 recommended that obstruction of flameproof enclosures, particularly those with flanged joints, should be avoided. This recommendation is also given in BS EN60079-14: Electrical installations in hazardous areas (other than mines). A solid obstruction such as a wall, steelwork, conduit, brackets, weatherguards or other electrical apparatus etc., in close proximity to the opening at the joint can, in the event of an internal explosion, reduce the efficiency of the flamepath to the extent that ignition of the external gas or vapour could occur.

The minimum distances between the flamepath opening and an obstruction, as specified in BS EN60079-14 and BS 5345: Part 3 are:

Group Distance

IIA 10 mm

IIB 30 mm

IIC 40 mm

Weatherproofing Flameproof equipment must have a level of ingress protection to suit the environmental conditions in which the equipment is installed. Equipment should have, as part of their approved design, seals or gaskets to prevent the ingress of water and/or dust. Where environmental conditions are extreme, consideration of additional measures may be necessary if this is permissible after consultation with relevant standards, or the manufacturer or other authority. These measures are specified in BS EN60079-14, the Standard which gives recommendations for the installation of electrical equipment in hazardous areas. This document specifies the limitations of use for non-hardening grease bearing textile tape (typically Denso tape) as detailed below, and non-setting grease or compounds. The use of non-setting grease on the machined surfaces of flamepaths has two advantages since, in addition to providing an additional level of ingress protection, it also inhibits the formation of rust on these surfaces. Silicone based greases require careful consideration in order to avoid possible damage to the elements of gas detectors. For flameproof equipment, the limitations for the use of non-hardening tape are specified as follows. (a) Non-hardening tape may be applied around the flamepath of apparatus allocated to

group IIA using a short overlap. (b) The Code of Practice BS 5345: Part 3 (now superseded but still current)

recommended that expert advice be sought when considering the use of non-hardening tape on group IIB or IIC equipment installed in locations containing group IIB gases or vapours. The replacement document, BS EN60079-14, did not maintain this recommendation but, a supplementary document PD60079-14: 2000: Guide to the application of BS EN60079-14 was published to clarify issues not addressed in the latter standard.

With regard to the use of non-hardening tape on group IIB apparatus, PD60079-14: 2000 permits it’s use providing the flamepath gap does not exceed 0.1mm regardless of the length of flamepath. This would also apply to IIC enclosures used in IIB hazards.

(c) Non-hardening tape must not be used on group IIC equipment installed in locations

containing group IIC gases or vapours. (d) The machined surfaces of flanged joints must not be painted. An enclosure may be

painted after assembly even if paint is likely to enter the flamepath gap. Paint on the machined surfaces of flamepaths, however, must be removed prior to re-assembly.

Ingress protection methods The diagrams below illustrate the location of gaskets or rubber ‘O’ rings for ensuring a high level of ingress protection. The gaskets etc. must be an integral part of the original design, i.e. they cannot be added at a later date to an enclosure manufactured without gaskets. Typical examples for outdoor use are illustrated below.

Direct / indirect entry The selection of cable glands for flameproof apparatus is influenced by several factors, one of which is the method of entry into the apparatus. There are two entry methods, namely direct and indirect, examples of which are shown below. Direct entry comprises a single flameproof chamber within which components such as switches, relays or contactors may be installed. Flameproof apparatus with indirect entry has two separate chambers, one of which contains only terminals for connection of the conductors of incoming cables or conduit. Connection to the arcing components in the second compartment is made via these flameproof terminals which pass through the flameproof interface between the two compartments.

Direct entry Indirect entry

Electrical protection Flameproof enclosures are tested for their ability to withstand internal gas explosions only; they are not capable of withstanding the energy which may be released as a result of an internal short-circuit. In order to avoid invalidation of the certification, it is important that properly rated/calibrated electrical protection, e.g. fuses and/or circuit breakers, are utilised.

Modification of flameproof enclosures Flameproof enclosures are normally supplied complete with all internal components fined and certified as a single entity by a recognised test authority. The testing procedure will take into consideration the free internal volume after all the components have been fitted, the temperature rise (determined by the maximum power dissipation), creepage and clearance distances, and the rise in pressure as a result of an internal explosion using a gas/air mixture in its most explosive proportions. The certification, therefore, “seals” the design of the apparatus so that any unauthorised modifications will effectively invalidate the approval/certification. Modifications will modify the original test results recorded by the test/certification authority and, consequently, the following points should be observed. (a) Replacement components should always be exactly the same as the original

specified components in order to avoid infringement of the certification. For example, a component larger or smaller than the original will affect the internal geometry of the enclosure. Pressure piling is a possibility if a larger component is fitted, and increased volume will result if a smaller component is fitted.

Note: illustrations are for demonstration only and must not be carried out

Original arrangement

Replacement of ‘A’ with a larger item

Replacement of ‘A’ with a smaller item

(b) adding components is also forbidden because of the possibility of increased explosion pressure as a result of pressure piling.

Addition of component ‘C’

(c) The removal of components should also be avoided since an increase in the free

internal volume will result. The original test results, prior to certification, would be compromised as a result of a modification such as this.

Removal of component ‘B’

Note: Illustrations are for demonstration only and must not be carried out

(d) Drilling and tapping of cable gland/conduit entries should only be carried out by the manufacturer of the enclosure, or his approved agent. The threads of the entries are required to be compatible with those of cable glands or conduit in terms of type of thread, thread pitch and clearance tolerance since flamepaths exist at these points.

Correct alignment of the threaded entry is also important since the flamepath length at one side will be reduced if the cable gland or conduit is not fitted perpendicular to the face of the enclosure.

The strength of a flameproof enclosure may be impaired if the number and size of entries exceeds that permitted in the original design certified by the test authority. Compliance with the original design is paramount with regard to number, size and location of entries to ensure the enclosure will contain an internal explosion.

(e) Gaskets can only be replaced; they must not be added retrospectively if not

included as part of the original design.

The use of unauthorised sealants should also be avoided when it is required to maintain or improve the IP rating.

BS EN 60079-17: Table 1: Inspection Schedule for Ex‘d’, Ex‘e’, and Ex‘n’ Installations (D = Detailed, C = Close, V = Visual)

Unit 4: Increased Safety Ex e & EEx e Objectives: On completion of this unit, ‘Increased Safety Ex e & EEx e’ apparatus, you should know: a) the principle of operation; b) the principle design features; c) the methods for estimating terminal content of enclosures; d) the installation requirements according to BS EN 60079-14; e) the inspection requirements according to BS EN 60079-17.

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Increased Safety Ex e or EEx e The explosion protection concept Increased Safety was invented in Germany where it has been widely used for many years. It is has become popular in the UK mainly because it has a number of advantages for certain applications over the traditional flameproof method of explosion protection. America has traditionally relied on the use of explosion-proof enclosures in hazardous locations, and the prospect of using an Increased Safety enclosure, which is not designed to withstand an internal explosion, as an alternative, has probably been viewed with a little trepidation. This method of protection has a good safety record and comparable with the other methods of protection. The letter ‘e’ which symbolises this method of protection is taken from the German phrase Erhohte Sicherheit, which roughly translated means ‘increased security’. Typical applications are induction motors, lighting fittings and junction boxes.

Standards

BS EN50 019 Increased Safety enclosure ‘e’

BS 5501: Part 6 Increased Safety enclosure ‘e’

BS 4683: Part 4 Type of protection ‘e’

IEC 79-7 Construction and Test of Electrical Apparatus, Type of Protection “e”

BS EN60079-14 Electrical apparatus for explosive gas atmospheres: Part 14 Electrical installations in hazardous areas (other than mines)

BS EN60079-17 Electrical apparatus for explosive gas atmospheres: Part 17 Inspection and maintenance of electrical installations in hazardous areas (other than mines)

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Definition ‘A protection method in which increased measures are taken to prevent the possibility of excessive HEAT, ARCS or SPARKS occurring on internal or external parts of the apparatus in normal operation’. Zones of use: 1 & 2 Ambient temperatures Increased Safety enclosures are normally designed for use in ambient temperatures in the range -20 °C to +40 °C unless otherwise marked.

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Principle The safe operation of Increased Safety apparatus is dependent on the prevention of any source of ignition, i.e. excessive surface temperatures, arcs or sparks, which might otherwise be produced by internal or external parts of the apparatus. Special design features are, therefore, incorporated in the apparatus by the manufacturer and are as follows. 1) Mechanically strong enclosure resistant to impact - tested to 4 or 7 joules impact energy

depending on application. 2) Ingress protection against solid objects and water - at least IP 54. 3) Terminals manufactured from high quality insulation material. 4) Specified creepage and clearances incorporated in design of terminals. 5) Terminal locking devices to ensure conductors remain secure in service. 6) Certified de-rating of terminals. 7) Terminal population of enclosure limited by circuit design. 8) Close excess current circuit protection. Increased Safety Terminals The terminals installed in an Increased Safety enclosure must be ‘component certified’ terminals. They will be manufactured from good quality materials such as Melamine, Polyamide and, for special applications, Ceramic. These materials, which have good thermal stability, have been subjected to a ‘Comparative Tracking Index (CTI)’ test to determine their resistance to tracking. The following definitions are relevant: Clearance distance: The shortest distance through air between two conductors. Creepage distance: The shortest distance between two conductors along the surface

of an insulator. Tracking: The leakage current which passes across the contaminated

surface of an insulator between live terminals, or live terminals and earth.

Comparative Tracking Index: The numerical value of maximum voltage, in volts at which an

insulation material withstands e.g., 100 drops of electrolyte (usually ammonium chloride solution in distilled water) without tracking.

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Increased safety terminals Test criteria - Comparative Tracking Index (CTI) The Comparative Tracking Index (CTI) test criteria are given in the table below. Four grades of materials ‘a’, ‘b’, ‘c’ and ‘d’ are considered, the highest quality material being ‘a’ which is subjected to the greatest number of drops of electrolyte falling between the test electrodes, and the highest voltage applied across the electrodes from the variable voltage source. Each material must withstand the specified number of drops of the electrolyte at the specified voltage for it to be acceptable. Thus, the combination of high quality materials and good design, which incorporates specified creepage and clearance distances, ensures that Increased Safety terminals have a greater resistance to tracking to prevent arcing or sparking.

Grade of material C.T.I. Test voltage Number of drops

a - 600 > 100

b 500 500 > 50

c 380 380 > 50

d 175 175 > 50

Creepage and Clearance Distances

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Increased safety terminals Creepage and Clearance Distances

- 6 -

Increased safety terminals Creepage distances relative to voltage and grade of insulation The following table shows the creepage distances relative to the grade of material and applied voltage.

Minimum creepage (mm)

Insulation Grade to IEC 112 Rated

Voltage (V)

a b c d

Minimum Clearance

(mm)

33 3 3 3 3 -

66 3 4 5 6 3

275 6 8 10 12 5

418 8 10 12 15 6

550 10 12 15 18 8

726 12 16 20 25 10

1100 20 25 30 36 14

Increased Safety terminal types and ratings The terminals are de-rated so that the maximum current for Increased Safety applications is nearly half that for standard industrial applications as illustrated in the following table for enclosures manufactured to BS 5501 Part 6. This de-rating, along with other considerations, ensures that internal and external surface temperatures are kept within prescribed limits. The table below also shows the maximum conductor size for each terminal type.

Terminal type Conductor size

Increased Safety maximum current

(amps)

Industrial maximum current

(amps)

SAK 2.5 2.5 15 27

SAK 4 4 21 36

SAK 6 6 26 47

SAK 10 10 37 65

SAK 16 16 47 87

SAK 35 35 75 145

SAK 70 70 114 220

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Increased safety terminals Terminal locking device It is essential that conductors are securely connected in the terminals to prevent sparks occurring as a result of loose connections. The illustration below shows how this is achieved.

- 8 -

Estimation of terminal population The number of terminals which can be installed in a given size of enclosure is limited. Several methods have been developed by manufacturers for this purpose. These are: Enclosure factor: A method used in apparatus manufactured to BS 4683 Part 4 in

which the terminal content is assessed by dividing the ‘enclosure factor’ by the certified current rating of a given terminal.

Load limit: Similar to ‘enclosure factor’ but used only on apparatus manufactured

to BS 5501 Part 6. Kelvin rating: Normally used for high current applications and apparatus

manufactured to BS 4683 Part 4 and BS 5501 Part 6. In this method, enclosures and terminals are assigned a temperature rating. Enclosures will normally be limited to a temperature rise of 40K for a T6 temperature rating, but the temperature for the terminals will be dependent on their type, rated current, size of associated conductor, and the size of enclosure in which they are installed. This involves the use of tables which are provided by the manufacturer. Once the terminal ‘K’ rating has been established, it is divided into the ‘K’ rating for the enclosure to give the number of terminals of one type which may be installed.

Max dissipated power: This is a method which will replace the current ‘load limit’ method and

applies to apparatus manufactured to BS 5501 Part 6 and BS EN50 019. In this method, enclosures are assigned a ‘watts dissipation’ rating, but the rating of the terminals is determined by use of a unique table (provided by the manufacturer) for the enclosure. This table provides the ‘watts dissipation’ of the terminal through consideration of conductor size and load current. The terminal content is determined by dividing the ‘watts dissipation’ value for the terminal into that for the enclosure.

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Sample calculation using ‘Load limit’ The ‘Load Limit’ will be specified on the certification label of an Increased Safety enclosure, as illustrated below, and represents the sum of all the circuit currents the enclosure is able to carry without exceeding the temperature classification. Thus, the number of terminals of one type which can be installed in a given enclosure is simply the ‘Load Limit’ divided by the Increased Safety current rating of the terminal type to be used as demonstrated in the following calculation.

TYPE TB11 S/No D779

EEx e II T6

BASEEFA CERT No Ex 84B3299X

BS 5501 Pt 6 (EN50 019)

LOAD LIMIT 600

Enclosure Load Limit = 600

SAK 2.5 Ex e terminal rating = 15 A

Number of SAK 2.5 terminals = ratingterminal2.5SAK

LimitLoad

= 15600

= 40 SAK 2.5 terminals

Where the circuit current is below the certified current rating of the terminals, it may be possible to base the terminal population on the circuit current provided it will not exceed the assigned value. Assuming a circuit current of 10 A, the calculation is as follows.

Enclosure Load Limit = 600

Circuit current = 10 A

Number of SAK 2.5 terminals = currentCircuitLimitLoad

= 10600

= 60 SAK 2.5 terminals

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Terminal Assemblies

Component approved terminal group 1. Mounting rail; 2. Terminals - certified components; 3. End plate; 4. End bracket; 5. Distance sleeve; 6. Partition; 7. Copper cross-connection; 8. Zinc plated screw; 9. Copper cross-connection; 10. Copper cross-connection.

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Terminal Assemblies

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Installation, inspection and maintenance It is essential that Increased Safety enclosures are installed and maintained in accordance with the relevant Standards and Codes of Practice in order to comply with the Certification. The following list specifies the main points. 1. Enclosure content should not be modified without consulting the manufacturer. 2. Only components specifically approved should be fitted in the enclosure. 3. All terminal screws, used and unused, should be tightened down. 4. Conductor insulation should extend to within 1 mm from the metal throat of the terminal. 5. Partitions should be fitted at either side of terminal linking assemblies. 6. Only one conductor should be fitted to each terminal side. 7. An additional single conductor, min 1.0 mm2, may be connected within the same terminal way

when an insulated comb is used. 8. Only the conductors from each cable entry shall be loomed together. 9. The insulation of cables shall be suitable for use at least 80°C for a T5 temperature class. 10. The individual earth continuity plates within plastic enclosures must be bonded together and

locknuts used to secure glands to the continuity plates. For clearance holes, serrated metal washers must be used between locknuts and the glandplate.

11. When Intrinsic and Increased Safety circuits occupy the same enclosure the two types of

circuit must have at least 50 mm clearance between them. 12. There must be adequate clearance between adjacent enclosures to allow proper installation

of cables and glands. 13. All unused cable entries should be closed using suitable plugs. 14. The schedule of the appropriate certificate should be consulted before cable entry holes are

drilled. 15. Cable glands or conduit entries must maintain the minimum ingress protection of

IP 54. 16. All lid and gland plate bolts must be fully tightened after installation.

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Increased safety EEx e motors These motors are similar in appearance to standard industrial motors and inspection of the certification/rating plate is usually necessary to identify them. These motors are not designed to withstand an internal explosion and hence have special design features to prevent arcs, sparks and excessive surface temperatures occurring both internally and externally. The principal design features are:

1) Special attention to airgap concentricity and clearance of all rotating parts; 2) Impact testing of motor frame; 3) Temperature rise 10 °C lower than normal; 4) T2 or T3 surface temperature limitation; 5) Compliance with tE characteristic; 6) Special terminal block with specific creepage/clearance distances and locking

devices on terminals; 7) Minimum ingress protection to IP54;

Under stall (locked rotor) conditions, the rotor surface temperature will normally increase faster than that of the stator windings, and hence, the T rating applies to both internal and external surface temperatures. Under fault conditions, the motor must trip within the tE time specified on the motor data plate. tE time Defined as: ‘the time taken to reach the limiting temperature from the temperature reached in normal service when carrying the starting current IA at maximum ambient temperature. In the graph shown, ‘OA’ represents the maximum ambient temperature and ‘OB’ the temperature reached at maximum rated current. If the rotor locks as a result of a fault, the temperature will rise rapidly towards ‘C’ as shown in part 2 of the graph, which is less than the T rating of the motor. The time taken to reach ‘C’ from ‘B’ is known as the tE time, and during fault conditions the thermal overload device in the motor starter must trip out the motor within this time. Increased safety motors are intended for continuous duty only, i.e. they are unsuitable for applications which require frequent stopping and starting and/or long run-up times.

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Determination of tE time

Limiting Temperature Temperature limited either by selected T-Rating or Limit for Class of Winding Insulating Material

A = maximum ambient temperature; B = maximum temperature at rated current; C = limiting temperature; θ = temperature; (1) = temperature rise at rated current; (2) = temperature rise during locked rotor test; tE = time from maximum temperature (B) at rated current to limiting

temperature (C).

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Tripping characteristic of thermal overload The thermal overload will be selected for suitability according to its tripping characteristic. The tE time and IA/IN current ratio are influential in the selection of the device and are marked on the motor nameplate.

IN = rated current of motor; IA = locked rotor current of motor. Example 1: IA/IN = 5 and tE time = 10 secs The above characteristic would trip the motor after 8 secs, which is within the

tE time and therefore acceptable. Example2: IA/IN = 4.5 and tE time = 8 secs For these values the tripping time is 10 secs, which is outwith the tE time

assigned to the motor, therefore an overload device with this characteristic would not be suitable for the values specified.

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BS EN 60079-17: Table 1: Inspection Schedule for Ex‘d’, Ex‘e’, and Ex‘n’ Installations (D = Detailed, C = Close, V = Visual)

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Unit 5: Type n apparatus Objectives: On completion of this unit, ‘Type ‘n’ apparatus’, you should know: a) the principle of operation; b) the principle design features; c) the protection methods applied to arcing/sparking components to enable their use in

enclosures etc.; d) the installation requirements according to BS EN 60079-14; e) the inspection requirements according to BS EN 60079-17.

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Type ‘n’ protection Since the presence of a flammable gas or vapour is less likely in Zone 2, the constructional requirements for electrical equipment used in these hazardous locations are not as strict as those for equipment used in Zone 1. A method of protection which falls into this category is type ‘n’ apparatus, which is basically similar to increased safety type “e” apparatus except that there is a relaxation in the constructional requirements. Type ‘n’ protection, a UK innovation, has been under consideration for some time by the European standards organisation CENELEC who have recently accepted the draft standard EN50 021. Prior to acceptance of this standard by CENELEC, this type of protection was symbolised by the (upper case) letter ‘N’ and, as far as Europe is concerned, was only acceptable for use in the UK. Now that EN50 021 has been approved, this type of apparatus will be symbolised by the (lower case) letter ‘n’ and will also display the European Community mark thus enabling wider use of this type of protection in the EC. Standards

BS EN50 021 Type of protection “n”

BS 6941: 1988 Electrical apparatus for explosive atmospheres with type of protection N

BS 4683: Part 3: 1972 Type of protection ‘N’

BS EN60079-14 Electrical apparatus for explosive gas atmospheres: Part 14. Electrical installations in hazardous areas (other than mines)

BS EN60079-17 Electrical apparatus for explosive gas atmospheres: Part 17. Inspection and maintenance of electrical installations in hazardous areas (other than mines)

Definition The definition for Electrical apparatus with type of protection “n” as given in the CENELEC Standard BS EN50 021 and also BS 6941 states:

‘A type of protection applied to electrical apparatus such that, in normal operation, it is not capable of igniting a surrounding explosive atmosphere and a fault capable of causing ignition is not likely to occur’.

Zone of use: Zone 2

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Ambient conditions Type ‘n’ apparatus is normally designed for use in ambient temperatures in the range -20 °C to + 40 °C unless otherwise marked. Principle In Zone 2 hazardous locations, the presence of a flammable gas or vapour is not likely to be present, or if it is present it’s duration will be for a short time only. This fact allows the use of less expensive methods of protection, i.e. non-incendive or type ‘n’ protection. As previously stated, type ‘n’ protection is similar in concept to increased safety type ‘e’ protection. The design features for this type of protection ensure that, in normal operation, sources of ignition in the form of excessive surface temperatures, arcs or sparks are prevented from occurring either internally or externally. Since the design requirements are not as strict as those for increased safety type ‘e’ protection, it is possible for the manufacturer to install within type ‘n’ apparatus, components which produce hot surfaces, arcs or sparks, providing these components have incorporated in them additional methods of protection. These additional methods are described later in this unit. The principal design features for type ‘n’ apparatus are as follows.

1) Enclosures and motor fan guards, where exposed to high risk of mechanical damage, to have resistance to impact of 3.5J;

2) Minimum ingress protection IP54 where an enclosure has exposed live parts internally;

3) Use of certified terminals; 4) Terminals manufactured form high quality insulation material; 5) Specified creepage and clearance distances incorporated into the design of the

terminals; 6) Terminal locking devices to ensure conductors remain secure in service.

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Additional protection measures As previously mentioned, components which produce arcs/sparks or hot surfaces may be installed in type ‘n’ apparatus provided additional protection measures are provided. These are explained below. Energy limited apparatus and circuits Energy limited apparatus and circuits achieve safety by ensuring voltages and currents are maintained at safe levels, which is synonymous with IS protection. Energy limited apparatus: Apparatus of this type containing normally sparking contacts,

which has been assessed as a single entity (external circuit conditions not considered), and deemed suitable for use in an hazardous area, is required to be marked ‘Apparatus containing energy-limited circuits’.

Methods employed to limit voltage and current in this type of

apparatus will include the use of zener diodes and series resistors etc. Where the supply to the apparatus is mains voltage via a transformer, an upward tolerance of 10% must be assumed unless alternative measures allow dispensation of this requirement.

Energy limited circuits: If the assessment for this type of apparatus, which contains

normally sparking contacts, considers partly or wholly external influences such as inductance or capacitance of, for example, cables or connected apparatus, the apparatus must be marked ‘Apparatus for connection to energy-limited circuits’.

The apparatus will be marked with the symbol ‘X’ to indicate

that ‘special installation conditions’ apply. These installation conditions will be specified in the documentation to enable safe installation of the apparatus. This information will include the maximum values of voltage, current, inductance, capacitance, and external cable inductance and capacitance.

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Enclosed break device This technique is used in, for example, the lamp holders of type ‘n’ apparatus. The example below shows a typical lamp holder in which there are two sets of contacts. One set of contacts is enclosed in what is effectively a flameproof enclosure in which the free internal volume must not exceed 20 cm3. This enclosure is designed to withstand an internal explosion and the voltage and current limitations are 600 V and 15 A respectively.

Hermetically sealed device A device which prevents an external gas or vapour gaining access to the interior by sealing of joints by fusion, e.g. welding, soldering, brazing, or the fusion of glass to metal. The example of hermetic sealing shown below is a reed switch which comprises a set of contacts hermetically sealed within a glass envelope.

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Sealed device A device containing normally sparking components or hot surfaces constructed in such a way that opening is prevented in normal operation and in which the sealing effectively prevents access by a flammable gas or vapour. The free internal volume must be less than 100 m3. Encapsulated device The device in this instance will be totally sealed by an encapsulating material, typically ‘epoxy resin’, to prevent entry of a flammable gas or vapour. Restricted breathing A technique mainly used in type ‘n’ lighting fittings whereby entry to the interior of a flammable gas or vapour is restricted by virtue of good sealing at all joints and cable entries. This type of protection is suitable for use in Zone 2 only. Sub-division of Type ‘n’ apparatus

Type ‘n’ apparatus variations CENELEC/IEC marking

Restricted breathing enclosures R

Energy limited apparatus L

Simplified pressurised enclosure P

Contacts of sparking apparatus protected by methods other than R, L or P C

Non-sparking apparatus A

The above table shows the marking on Type ‘n’ apparatus to indicate the method applied to either eliminate or control spark energy and/or hot surfaces.

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BS EN 60079-17: Table 1: Inspection Schedule for Ex‘d’, Ex‘e’, and Ex‘n’ Installations (D = Detailed, C = Close, V = Visual)

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Unit 6: Pressurisation Ex p & EEx p Objectives: On completion of this unit, ‘Pressurised Ex p & EEx p’ apparatus, you should know: a) the principle of operation and the importance of purging; b) control measures required to ensure the safe operation of apparatus and systems; c) variations of pressurisation methods; d) action on loss of overpressure; e) the installation requirements according to BS EN 60079-14; f) the inspection requirements according to BS EN 60079-17.

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Pressurised equipment Introduction Pressurisation is a simple technique for providing explosion protection. If the interior of an enclosure is at a pressure above that externally, any flammable gases around the enclosure will be prevented from entering the enclosure. Components which are a source of ignition, i.e. they produce arcs/sparks or hot surfaces, are permitted within the enclosure and, clearly, safety is dependent on the maintenance of the safe gas. The safe gas is the medium which ‘segregates’ the flammable gas from the source of ignition, and its continued presence will be confirmed by an approved/certified ‘fail-safe’ control/monitoring system. A slight over-pressure is usually adequate to maintain safe operation.

Standards

BS EN50 016 Pressurised Apparatus ‘p’

BS 5501: Part 3 Pressurised Apparatus ‘p’

IEC 79-2 Pressurised enclosures ‘p’

BS EN60079-14 Electrical apparatus for explosive gas atmospheres: Part 14. Electrical installations in hazardous areas (other than mines)

BS EN60079-17

Electrical apparatus for explosive gas atmospheres: Part 17. Inspection and maintenance of electrical installations in hazardous areas (other than mines)

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Definition Pressurisation is defined as:

‘The technique of guarding against the ingress of the external atmosphere, which may be explosive, into an enclosure by maintaining a protective gas therein at a pressure above that of the external atmosphere’.

Zones of use: 1 & 2

Applications Pressurisation has a wide range of applications, i.e. it can provide explosion protection for a diverse range of instrument or electrical apparatus, there being no limit to size, within reason, which can be accommodated. Typical examples are transformer/rectifier cabinets, oil drilling control consoles, visual display units (VDU’s), gas analysis equipment, control rooms, switch rooms and workshops. With regard to flameproof apparatus, and in particular rotating machines, there is a maximum practical limit above which handling becomes difficult and manufacturers may overcome this difficulty by the use of a pressurised enclosure. A pressurised machine would be significantly lighter than a flameproof machine of the same rating.

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Principle The basic principle of operation involves raising and maintaining the internal pressure of the enclosure to a level slightly above the atmospheric pressure outwith the enclosure. This ensures that any flammable gases or vapours outwith the enclosure cannot enter the enclosure. The minimum over-pressure specified in the standards is 0.5 mBar or 50 Pa. The safe gas used to provide the over-pressure will normally be air but an inert gas such as nitrogen may also be used in certain instances.

Purging When a pressurised system has not been in use for some time it is important that electrical apparatus inside the enclosure is not energised prior to what is known as the ‘purge’ cycle. Purging, which must occur automatically, involves passing a quantity of the safe gas through the enclosure for a specified time in order to remove any flammable gases which may have entered the enclosure. The standards specify that the minimum quantity of the safe gas required to achieve adequate purging is equivalent to 5 times the internal volume of the enclosure and associated ducting. The purge duration will be controlled by a timer in association with a flow-rate sensor in the control circuit. Manufacturers may, however, recommend a greater number of air changes. Very large systems, which are installed on site, will require on-site tests to establish the purge duration necessary for safe operation. If loss of pressure occurs during operation, the control system must automatically purge the enclosure again.

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Enclosures The European and IEC standards require a minimum level of ingress protection for pressurised enclosures to IP 4X but, not all enclosures are suitable for pressurisation. An enclosure may have ingress protection to IP54 but, it’s lid seal, for example, is designed to prevent entry of contaminants and not to maintain an over-pressure within the enclosure. Enclosures must, therefore, be appropriately designed, i.e. be strong enough to withstand impact tests and the internal over-pressure with regard to the strength of the walls, and have effective and correctly orientated door seals. The enclosure and its associated ducts must be capable of withstanding, in normal operation, an over-pressure equivalent to 1.5 times the maximum working over-pressure declared by the manufacturer. Alternatively, the enclosure must be capable of withstanding the maximum over-pressure obtained when all outlet ducts are closed. In either instance the minimum overpressure will be 2 mBar (200 Pa). Protective gas The protective gas, which is normally air except for certain applications, may be an inert gas such as nitrogen or another suitable gas. When air is the protective gas, it may be provided by either a motor driven fan, a compressor, or from storage cylinders. The protective gas must be non-toxic and free from contaminants such as moisture, oil, dust, fibres and chemicals, and other contaminants which could jeopardise the safe operation of the system. Normally, the temperature of the safe gas entering the inlet duct should not exceed 40 °C. Where temperatures above or below this value are required, the pressurised enclosure will be marked with this temperature. When air is used as the safe gas its oxygen content must not be greater than that normally present in the atmosphere, i.e. 20.9%. A duplicate supply of the protective gas is also desirable when, on loss of pressure, it would be more dangerous to de-energise the electrical apparatus within the enclosure. When an inert gas such as nitrogen is used as the protective gas and personnel can gain access to enclosures, it is essential that doors/covers are fitted with warning labels since there is a danger of asphyxiation. Doors should also be fitted with suitable locks. Enclosure covers/doors Where the interior of a pressurised enclosure can be accessed via doors/covers without the use of tools or keys, an interlock is required to automatically de-energise the electrical supply when the door/cover is opened, and restore the electrical supply only when the doors/covers are closed. When a pressurised enclosure contains components which have hot surfaces, or are capable of storing energy, e.g. capacitors, doors/covers should be fitted with a warning notice which states the time delay after isolation of the electrical supply to the components before opening the doors/covers.

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Control circuit/safety devices The level of overpressure will be monitored by an overpressure sensor or switch and located at a point in the enclosure which has been found by test or experience to be the most difficult to maintain the overpressure, e.g. the internal circulation fan in a pressurised machine. The exact point must be specified either on the enclosure or on the certificate. The rate of flow through the enclosure will be monitored by a flow-rate sensor or switch. A pressure gauge is also desirable and should be located where it can be easily read.

1) Over-pressure monitoring device; 2) protective gas flow-rate monitoring device; 3) pressure gauge; 4) pressure relief valve: setting 75% of maximum declared safe over-pressure

When the safe gas is provided from compressed-air cylinders, failure of the regulator could result in distortion of the pressurised enclosure due to excessive overpressure, and to overcome this risk it is recommended that a pressure relief valve is installed. The setting of the relief valve is required to be 75% of the maximum safe overpressure declared by the manufacturer.

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Ducts The entry of the inlet duct must be positioned in a non-hazardous location (except where cylinders provide the protective gas) and this location must be periodically reviewed in case plant modifications have altered its classification. The exhaust duct, ideally, should have its outlet situated in a non-hazardous location in which there are no sources of ignition, but may be located in a hazardous location if a spark/particle arrestor is fitted. The table below offers guidance in this respect. Where inlet or outlet ducts pass through hazardous areas, they are required to be free of leakage if there is a possibility that the pressure of the protective gas is below the minimum requirement specified in the standards or that specified by the manufacturer. It is essential that both the inlet and outlet ducts are arranged in such a way that they cannot be obstructed causing restriction of the flow of the protective gas. The ducts should also have adequate mechanical strength, be located where accidental damage is unlikely and have adequate protection against corrosion. Spark particle barriers: conditions which require use of

Type of apparatus within enclosure Zone in which exhaust duct is located A B

Zone 2 Required Not required

Zone I Required * Required *

A: apparatus which may produce ignition-capable sparks or particles in normal operation.

B: apparatus which does not produce ignition-capable sparks or particles in normal operation.

* A device to prevent rapid entry of a flammable gas into the enclosure upon

loss of pressure should be fitted if the surface temperature of apparatus within the enclosure is likely to be a source of ignition.

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Ducts (continued) Duct arrangements The density of the safe gas relative to the flammable gas has an influence on the position of the inlet and outlet ducts on the enclosure. This will speed up the rate of displacement of the flammable gas and so ensure efficient purging of the system. If the safe gas is heavier than the flammable gas the inlet duct will be positioned at the bottom of the enclosure and the exhaust duct at the top. If the safe gas is lighter than the flammable gas the positions of the ducts will be reversed. 1) When the safe gas is more dense than the flammable gas:

(2) When the safe gas is less dense than the flammable gas:

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Types of pressurisation Several variations of pressurised systems are available. These are:

a) Static pressurisation. b) Pressurisation with continuous flow; c) Pressurisation with leakage compensation; d) Pressurisation with continuous dilution;

a) Static pressurisation This form of pressurisation has limited applications and, therefore, not widely used. The technique involves pressurising and sealing the enclosure in a non-hazardous area prior to transportation into a hazardous area. Clearly the seals of the enclosure must be very good to minimise leakage once the source of the safe gas is disconnected. b) Pressurisation with continuous flow In this variant the internal over-pressure is maintained as a result of continual flow of the safe gas through the enclosure. The safe gas in this instance has a dual purpose. In addition to maintaining the over-pressure, it may also be used to cool hot parts within the enclosure such as thyristors, or the windings of a pressurised rotating machine. The rate of flow of the safe gas is set at a level which will prevent the temperature of the hot parts exceeding their temperature limit, thereby ensuring that the pressurised enclosure operates within it’s T-rating.

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c) Pressurisation with leakage compensation This method of pressurisation is used when enclosures are poorly sealed at their joints. The system is purged in the usual manner with the damper at the exhaust duct open but, on completion, the damper is closed and the flow of protective gas reduced to a level sufficient to compensate for leakages occurring at the seals/joints of the enclosure.

d) Continuous dilution The analysis of flammable gases on, for example, an offshore platform may take place in pressurised enclosures. A sample of gas will be drawn into a gas analyser and, after analysis, will be expelled into the interior of the pressurised enclosure. The safe gas therefore has two functions. In addition to maintaining over-pressure during and after the initial purge, the rate of flow of the safe gas will be adjusted to ensure that the concentration of the gas/air mixture within the enclosure is well below the lower explosive limit (LEL). Purging may be disregarded in Zone 2 if the concentration of the flammable gas released within the enclosure is considerably below the lower-explosive limit, e.g. 25% LEL. Gas detectors may be installed to verify that the atmosphere within the enclosure remains non-hazardous.

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d) Continuous dilution (continued)

Types and magnitude of internal release The recommended action when loss of pressure occurs in pressurised apparatus using continuous dilution is more comprehensively addressed in IEC79-2 than in BS EN60079-14. The recommendations are as follows. 1) Normal release

None No release of flammable gas or vapour.

Limited A release of flammable gas or vapour which is limited to a value which can be diluted to well below the lower explosive limit (LEL).

2) Abnormal release

Limited A release of flammable gas or vapour which is limited to a value which can be diluted to well below the lower explosive limit (LEL).

Unlimited A release of flammable gas or vapour which is not limited to a value which can be diluted to well below the lower explosive limit (LEL).

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Combination of release

Combination 1 No normal release, limited abnormal release

Combination 2 No normal release, unlimited abnormal release

Combination 3 Limited normal release, limited abnormal release

Combination 4 Limited normal release, unlimited abnormal release

The above combinations of release are applied in the table on page 12 which specifies the action necessary on loss of pressure within an enclosure using the technique of continuous dilution. Action on loss of pressure 1) No internal source of release The over-pressure within the enclosure is monitored by a pressure switch/sensor, and a flow-rate switch/sensor, located at the exhaust duct, is used to monitor the rate of flow of the safe gas through the enclosure. Loss of over-pressure or rate of flow will activate either an alarm or shutdown of the internal electrical components, the action taken being dependent on:

a) The Zone in which the system is located; b) The type of apparatus/components within the enclosure.

For a system which does not have an internal source of release and contains electrical equipment, BS EN60079-14 specifies the action to be taken on loss of pressure as follows.

Area classification

Enclosure contains ignition-capable apparatus

Enclosure contains apparatus which does not produce a source of ignition in normal

operation

Zone 1 Alarm and switch off Alarm

Zone 2 Alarm Internal pressurisation not required

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Action on loss of pressure (continued) 2) With internal source of release

Internal release Combination

Normal Abnormal

Area classification

Ignition-capable apparatus

Apparatus with no sources of ignition

Zone 1 Alarm and switch off Alarm

1 None Limited Zone 2 or non-

hazardous Alarm

No protective measures required

Zone 1 Alarm and switch off Alarm

2 None Unlimited Zone 2 or non-

hazardous Alarm

No protective measures required

3 Limited Limited Zone 1

or Zone 2

Alarm and switch off Alarm

4 Limited Unlimited Zone 1

or Zone2

Alarm and switch off Alarm

Externally mounted electrical apparatus Electrical apparatus mounted on the exterior of a pressurised enclosure must be explosion protected in accordance with the Zone in which the enclosure is situated. Typical examples are pressure/flow rate sensors or switches which may use EEx i apparatus, junction boxes may use EEx d, EEx e or Ex N methods of protection. This requirement also applies to the motor, and its starter, of the fan which provides the flow of air, unless they are situated in a non-hazardous area. It is preferable that the motor and its starter are located in a non-hazardous area. Apparatus energised during absence of overpressure An anti-condensation heater may be used in an rotating electrical machine to prevent the internal surfaces and atmosphere becoming cold, thereby preventing the formation of moisture in the windings. Because the heater will be energised when the machine is without over-pressure, it is essential that it is explosion protected. Emergency lighting will normally be installed in pressurised control rooms, cabins etc. and energised when there is loss of over-pressure, hence, these fittings must also be explosion protected, typically EEx e. Solenoids for fire dampers will be EEx d protected. Alarms, over-pressure and flow-rate sensors may use IS protection. EEx d enclosures will be used for control panels.

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Temperature classification The temperature classification of pressurised apparatus is determined by consideration of one of the following methods.

a) The external surface temperature of the enclosure. b) The surface temperature of apparatus within the enclosure remaining energised when

the enclosure is without over-pressure. A typical example is a flameproof anti-condensation heater inside a pressurised machine.

When the surface temperature of apparatus within the enclosure exceeds the T rating of the system, the following methods may be used to overcome this difficulty.

a) The joints of the enclosure and its ducts are capable of preventing the ingress of a flammable gas coming into contact with the hot surfaces before they have cooled to below the T rating.

b) By the introduction of a secondary ventilation system. c) By encapsulating the hot surfaces or enclosing them in gas-tight containers.

Marking The apparatus marking must be visible and contain the following information:

a) the manufacturers name; b) the manufacturers type number; c) the manufacturers serial number; d) the symbol EEx p; e) the gas group symbol II; f) the temperature class, or the maximum surface temperature, or both, e.g. T3, or 200

°C, or 200 °C (T3); g) the name or acronym of the testing station; h) the test station certificate number; i) the internal free volume excluding the ducts; j) the protective gas (when a gas other than air is used); k) the minimum quantity of the safe gas necessary to purge the enclosure; l) the minimum permissible over-pressure; m) the minimum flow of protective gas.

Note: In order to ensure adequate purging of the system the user must increase the volume

of the safe gas to compensate for the additional volume of the ducts.

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BS EN60079-17: Table 3: Inspection Schedule for Ex ‘p’ Installations (pressurised or continuous dilution)

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Unit 7: Intrinsic Safety Ex i & EEx i

Objectives: On completion of this unit, ‘Intrinsic Safety Ex i & EEx i’ apparatus, you should know: a) the principle of operation; b) the difference between ‘ib’ and ‘ia’ categories of IS; c) the importance of zener and galvanic interfaces; d) the installation requirements according to BS EN 60079-14; e) the inspection requirements according to BS EN 60079-17.

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Intrinsic safety Ex i or EEx i Intrinsic Safety is a widely used method of explosion protection. It is used for very low power applications only, and typical examples are control and instrumentation circuits.

Standards

BS 1259: 1958 Intrinsically safe electrical apparatus and circuits for use in explosive atmospheres

BS 5501: Part 7. 1977 (EN50 020)

Intrinsic safety ‘i’

BS 5501: Part 9 1982 (EN50 039)

Intrinsically safe electrical systems ‘i’

BS EN50 020 intrinsic safety ‘i’

BS EN50 039 Intrinsically safe electrical systems ‘i’

BS EN60079-14 Electrical apparatus for explosive gas atmospheres: Part 14. Electrical installations in hazardous areas (other than mines)

BS EN60079-17 Electrical apparatus for explosive gas atmospheres: Part 17. Inspection and maintenance of electrical installations in hazardous areas (other than mines)

BS 5345: Part 4 1977 (superseded but remains current)

Code of Practice for the selection, installation and maintenance of electrical apparatus with a type of protection ‘i’.

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Definition BS EN50 020 defines an intrinsically safe circuit as: ‘A circuit in which no spark or any thermal effect produced in the test conditions prescribed in this standard (which include normal operation and specified fault conditions) is capable of causing ignition of a given explosive atmosphere’. Zones of use: 0, 1 & 2 (Exi‘a’ & Eexi‘a’) 1 & 2 (Exi‘b’ & Eexi‘b’) Basic principles of IS Intrinsically Safe circuits achieve safety by maintaining very low energy levels such that hot surfaces will not be produced, and electrical sparks, if they occur, will have insufficient energy to ignite the most easily ignitable concentration of a flammable mixture. This is achieved by limiting the voltage and current supplied to the apparatus in the hazardous area. To maintain safety, it is of paramount importance that these levels of voltage and current are not exceeded under normal, or even fault conditions. The circuit parameters, i.e. voltage, current, resistance, inductance and capacitance are factors which have to be considered in the design of an IS circuit. Consultation with the characteristic ignition curves given in the construction standard, and reproduced in this section, and the application of appropriate safety factors, will ensure that safe values are established for these parameters during the design stage.

An IS system, which usually comprises a safe to hazardous area interface, cables, junction boxes and field (hazardous) area apparatus, must also be designed in such a way as to guard against the possibility of particular faults occurring. In contrast to other methods of explosion protection, intrinsic safety is a system concept which applies to the whole

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system and not to any one item only. Apparatus in the safe area connected directly to apparatus in the hazardous area is known as ‘associated apparatus’, and each item making up the system will have a Certificate of Conformity. Associated apparatus may be used in the hazardous area if installation is within another method of explosion protection, e.g. flameproof. In addition, the system may be covered by an overall system certificate. The maximum operating voltage for safe area apparatus is 250 Vrms. Advantages of IS are:

(a) live maintenance is possible; (b) cost effective - certified enclosures not required and ordinary wiring

may be used; (c) Safe - low voltage not harmful to personnel; (d) can be used in Zone 0.

The Zener barrier The faults which can jeopardise the security of IS systems are either overvoltage or overcurrent, and protection against these conditions is afforded by the use of an interface, typically a Zener barrier, the construction of which will be considered in terms of its individual components. The interface, which is connected between the safe area and hazardous area apparatus, is normally located in the safe area and situated as close as possible to the boundary with the hazardous area, but may be located in the hazardous area if installed in a flameproof enclosure.

A simple zener barrier has three principal components, (1) a resistor, (2) a zener diode, and (3) a fuse, all of which must have infallible properties. Infallibility, with regard to the current limiting resistor, means that in the event of it failing, failure will be to a higher resistance value or open-circuit. Clearly, failure to a lower resistance value or short-circuit would allow more current to flow in the IS circuit, which is contrary to the concept of this type of protection. Infallibility will be satisfied by the use of a

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quality wire-wound or metal film resistor, and its operating power, as required in the standards, should not exceed 2/3 of its maximum quoted rating for a specified ambient rating. The next component for consideration is the zener diode, the purpose of which is to limit the voltage available to the apparatus in the hazardous area. The zener diode, as a single item, is not considered to be an infallible component, must also be operated at only 2/3 of its maximum stated rating. For infallibility to be satisfied, the zener diode is required to fail to a short-circuit. Failure to a higher resistance or open-circuit could allow voltage levels beyond safe limits to “invade” the hazardous area. Note: Tests by manufacturers have shown that diodes virtually always fail to a short-circuit state,

but there can be no guarantee of this. Diodes can only be considered infallible when two or more are connected in parallel as discussed later.

The third component, a fuse, is located at the input (safe) end of the zener barrier, its purpose being to protect the zener diodes, and not to protect against, for example, a short-circuit in the field apparatus. Infallibility of the fuse is assured by the use of a sand-filled ceramic type capable of operating properly even when exposed to a prospective fault-current of up to 4000 A. A fuse of this type avoids the problem which can occur with other types of fuses when they rupture, namely vapourisation which can allow the fuse to continue to conduct. As required by the standards, the fuse is encapsulated along with the other components of the barrier to deter replacement. The repair of Zener barriers is not permissible, even by the manufacturer. Zener barrier operation In the event of a short-circuit developing in the apparatus in the hazardous area, or across the IS wiring, the series resistor in the zener barrier will limit the short-circuit current to a safe level so that the integrity of the system is maintained.

If a voltage greater than the normal maximum voltage of the IS system invades the circuit at the input terminals of the zener barrier, this will trigger the zener diode, and the resulting fault current will be shunted to earth. The excessive voltage is, therefore, prevented from reaching the apparatus in the hazardous area.

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Categories of IS Two categories of intrinsic safety are available, ‘ia’ and ‘ib’, the level, of safety provided by each being dependent on the number of component faults which are considered. The first category, ‘ib’, will maintain safety in the event of one fault occurring. The second category, ‘ia’, is required to maintain safety should two simultaneous faults occur. Clearly, for the zener barrier (interface) to maintain safety with one or two faults, additional zener diodes are necessary since they are the components most likely to fail.

Therefore, the addition of a second zener diode, connected in parallel with the first, will satisfy the requirements of category ‘ib’ intrinsic safety in which safety is assured with one fault. A third zener diode connected in parallel with the other two will satisfy the conditions for category ‘ia’ intrinsic safety in which safety is assured with two faults.

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Category ‘ib’ intrinsic safety may be used in Zones 1 and 2, but not Zone 0, and category ‘ia’ intrinsic safety is permitted in Zones 0, 1 and 2. Minimum ignition current curves Since it is necessary to limit the voltage and current in an IS circuit to ensure operational safety, the design of the circuit will be based on the minimum ignition current curves given in the construction standard and reproduced on page 7. Pages 10 and 11 also illustrate the curves for determining the maximum circuit inductance and capacitance respectively. Resistive circuits For a purely resistive circuit, if the voltage is known, the maximum circuit current can be determined from the graph, which enables selection of the correct interface. Thus, for a purely resistive circuit for operation in a IIC hazard, it is intended that a 28 V, 300 Ω zener barrier will be used. A safety factor of 10% must be applied to the voltage of this device since a rise in its temperature may raise the triggering voltage of the zener diodes. Applying the safety factor of 10% (1.1 x 28 V = 30.8 V) produces a value of 30.8 V, which is then located on the horizontal (voltage) axis of the graph. Moving vertically from this point towards the IIC curve, and then moving horizontally from the point of intersection with the curve towards the vertical (current) axis, gives a safe current of 140 mA. A safety factor of 1.5 must be applied to this value, i.e. 2/3 of 140 mA is equal to 93.33 mA. By applying ohm’s law, 28V/93.33 mA = 300 Ω, the same resistance as the zener barrier, it has been verified that the 28 V, 300 Ω interface is suitable for maintaining the integrity of the IS circuit in a IIC hazard.

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Minimum ignition current curves resistive circuits

- 8 -

Simple apparatus The spark energy of an IS circuit, during normal or fault conditions, will be insufficient to cause ignition of a surrounding hazard. The introduction of a switch, which in normal operation produces sparks and does not dissipate power, will not alter the situation, and in fact, any device which is resistive by nature and non-energy storing may be added to the circuit without jeopardising the integrity of intrinsic safety. Devices such as these are referred to as simple apparatus and not required to be certified. Such passive devices include switches, junction boxes, terminals, potentiometer and simple semiconductor devices. Simple apparatus may also include sources of stored energy, for example, capacitors and inductors having well defined parameters, the values of which must be considered during the design stage of an IS installation. Sources of generated energy, typically thermocouples and photocells, may also be described as simple apparatus providing they do not generate more than 1.5 V, 100 mA and 25 mW. Any capacitance or inductance in these devices must also be considered during the design stage of an installation Simple apparatus is normally allocated a T4 temperature classification, but junction boxes and switches may be rated T6 because they do not contain heat dissipating components. Since simple apparatus is not required to be certified, justification for it’s use must be included in the system documentation. Enclosures The minimum ingress protection for enclosures of IS circuits is IP20, but environmental conditions may require a higher rating. Energy storage Energy storing devices such as inductors and capacitors have the potential to upset the security of an IS system. Energy can be stored in these devices over a period of time and then released in a surge of greater amplitude at, for example, a break in the IS cables due to a fault or live disconnection at terminals. This could occur regardless of the design constraints on voltage and current, and cause ignition of a surrounding flammable gas. Measures must, therefore, be applied to counteract this problem at the design stage. Field apparatus which have energy storing capability, i.e. they have some internal inductance and capacitance, are termed ‘non-simple’ and are required to be certified. Cables, especially long runs between the interface and the apparatus in the hazardous area, will have appreciable inductance and capacitance which must be taken into consideration at the design stage. Energy will be stored under normal operating conditions, but will be greater under fault conditions. The voltage will influence which parameter is predominant, i.e. for a voltage of around 5 V, the inductance will be predominant, but at 28 V, the capacitance will be predominant.

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Where ‘simple apparatus’ only is used in the field, the inductance and capacitance present will be due to the cables only, and if the cable runs are short these parameters will be negligible. The parameters for typical instrument cables with twisted or adjacent cores rarely exceed the following values.

Inductance, (L) 1 µH/m Capacitance, (C) 110 pF/m Inductance/resistance ratio (LIR) 30 µH/Ω

Where field apparatus has both appreciable inductance and capacitance, it is important that the combined inductance and capacitance of the field apparatus and cables does not exceed the values specified by the manufacturer of the interface. Evaluation of cable parameters Inductance The maximum inductance of the interconnecting cables can be established from the inductive circuit curves after first of all evaluating the maximum source current. Assuming an interface with a maximum output of 28 V and 300 Ω resistance, the maximum source current is: 28 V/300 Ω = 93.33 mA Applying a safety factor of 1.5: 1.5 x 93.33 mA = 140 mA From the graph, the maximum safe inductance for the interconnecting cables, assuming connection to ‘simple apparatus’ in the hazardous area, is found to be approximately 4.0 mH. This value is found by projecting vertically from 140 mA on the current axis, and then horizontally towards the inductance axis from the point of intersection on the IIC curve. Capacitance For capacitive circuits, the procedure is exactly the same. A safety factor of 1.5 is applied to the zener barrier voltage of 28 V.

i.e. 1.5 x 28 V = 42 V Using the IIC curve in the graph, the maximum safe capacitance for the interconnecting cables, assuming that connection is to ‘simple apparatus’ in the hazardous area, is found to be 0.08 µF approximately. Comparison of the above values with the data provided by the cable manufacturer will establish if the interconnecting cable run is satisfactory.

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Inductive circuit curves

- 11 -

Capacitive circuit curves

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Earthing A dedicated high-integrity earth is a vital factor in maintaining the security of IS circuits, particularly when zener barriers are used. Galvanic barriers, however, operate on a different principle (discussed later in this section) which does not require a high-integrity earth, but earthing may be used for interference suppression. The earth bars on which zener barriers are mounted are insulated from the surrounding metalwork and connected directly to the main earth point via separate earthing conductors. Two cables, each secured at separate points at either end, are normally used to connect the barrier earth bar to the main earth point to facilitate earth resistance tests which must be periodically carried out. The resistance between the barrier earth bar and the main earth point should not be greater than 1 Ω. A value of 0.1 Ω is not unrealistic. The earth cable must be insulated, and the insulation undamaged, along it’s entire length so that contact with the plant metalwork is avoided: Where the risk of damage is high, mechanical protection for the cables should be provided. The earth conductors must have a rating capable of carrying the maximum fault current and have an appropriate cross-sectional area (csa) by means of:

(a) at least two separate 1.5 mm2 (minimum) copper conductors, or (b) at least one 4 mm2 (minimum) copper conductor.

Note: The IS circuit in the hazardous area must be able to withstand a 500 V insulation resistance

test to earth.

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Earthing and Bonding

- 14 -

Earthing and Bonding

- 15 -

Galvanic isolation Although zener barriers have been, and continue to be, widely used in industry, they have particular limitations which are:

(a) a dedicated high-integrity earth is necessary to divert fault currents away from the hazardous area;

(b) a direct connection exists between the hazardous and safe area circuits and earth, which tends to apply constraints on the rest of the system;

(c) hazardous area apparatus must withstand a 500 V insulation resistance test to earth. Devices which overcome these difficulties are isolation interfaces typically relays, opto isolators and transformers. Relay/transformer isolation In the example below, isolation between the hazardous and safe areas is achieved by means of an high integrity component approved transformer and component approved relay. The design of these devices ensures that high voltage invasion of the IS circuit will be prevented from reaching the hazardous area apparatus.

Opto-coupler/transformer isolation This method comprises a component certified opto-isolator and a component approved transformer. Light (or infrared) emitted from the LED when it is forward biased falls onto the phototransistor which is shielded from external light.

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Installation of IS apparatus The apparatus which make up an IS installation, i.e. field apparatus, associated apparatus and interface units, are required to be certified items which have been manufactured in accordance with relevant standards (see page 1). Such apparatus, including interconnecting cables, must be installed in accordance with the manufacturers instructions and with regard to the recommendations in BS EN60079-14. Installation requirements for cables The conductors of IS cables are required to be insulated with elastomeric or thermoplastic insulation which has a minimum thickness of 0.3 mm. Alternatively, mineral insulated cable may be used. The conductors of cables in the hazardous area, and this includes the individual strands of finely stranded cables, must not have a diameter less than 0.1 mm. Separation of the individual strands of cables must be prevented by, for example, the use of core-end ferrules. Though not a mandatory requirement, the colour for IS cables (and terminals) is blue. Minimum conductor sizes Cables must operate within the temperature class established for the IS system when carrying maximum current during fault conditions. The following table specifies the maximum current and minimum cross-sectional area for copper conductors for temperature classifications within the range T1 - T4.

Maximum Current (A) 1.0 1.65 3.3 5.0 6.6 8.3

Minimum csa (mm2) 0.017 0.03 0.09 0.19 0.28 0.44

Mechanical protection The interconnecting cables of an IS circuit are required to have an overall sheath in order to maintain the integrity of the system, i.e. to prevent contact with the cables of other circuits, or earth, as a result of damage. and to ensure the circuit parameters in terms of inductance and capacitance are not exceeded. Armouring or screening of cables for mechanical protection is not required except for IS circuits with multi-core cables in Zone 0. Where IS cables and the cables of other circuits share the same duct, bundle or tray, both types of circuit must be segregated by means of an insulated or earthed metal partition. Separation is not necessary if either the IS cables or the cables of the other circuit are armoured, screened or metal sheathed. The armouring of cables should be securely bonded to the plant earth.

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Cable screens Where the interconnecting cables of IS circuits have overall screens, or groups of conductors with individual screens, the screens are required to be earthed at one point only, usually the barrier earth bar. The individual screens must be insulated from one another and, prior to connection of the screens to the barrier earth bar, an insulation resistance (IR.) test should be carried out between each pair of screens. The test readings should not be less than 1 MΩ/km when measured at 500 V at 20 °C for 1 minute. Overall screens are required to be insulated from the external metalwork, i.e. cable tray etc. Induced voltage Generally, induced voltage in IS interconnecting cables is not likely but may occur if the IS cables are placed parallel to and in close proximity to single-core cables carrying heavy current, or overhead power lines. Adequate segregation between the different circuits will overcome this difficulty as will the use of screens and/or twisted cores. Multi-core cables IS circuits and other types of circuits must not be run in the same multi-core cable. Where a multi-core cable, which is located in Zone 0, has more than one IS circuit, it is essential that no combination of faults between the IS circuits within the cable will cause an unsafe condition. An exemption to this requirement applies if:

(a) the risk of mechanical damage to the cable is minimal or, where the risk of damage is high, additional protection is provided; and

(b) the cables are firmly secured along their length; and (c) each IS circuit uses adjacent cores in the cable throughout it’s length; and (d) none of the IS circuits can operate during normal or fault conditions at more than 60 V

peak; or (e) the cores of each IS circuit are within a screen which is insulated and earthed as

previously discussed. Unused cable cores Unused cable cores should be connected to the IS earth at the interface, and insulated elsewhere by means of connection to terminals which are identified in the documentation.

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Clearance distances

Peak voltage

(V)

Minimum clearance in air between terminals of separate circuits

(mm)

Minimum clearance in air between terminals and earth

(mm)

0-90 6 4

90-375 6 6

Where IS circuits and non-IS circuits share the same enclosure, for example measurement and control cabinets, adequate segregation must exist between the two sets of terminals. This can be achieved by means of a partition or a 50 mm gap. Segregation also applies to the cables of the two types of circuit.

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BS EN60079-17: Table 2: Inspection Schedule for Ex ‘i’ Installations

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IS Inspection Record No. C Junction Boxes in Hazardous

Area

Location

Junction Box No.

Junction Box Connection Drawing No.

Instrument Loop Drawing Numbers

Correctly labelled

No damage

Weather sealing OK

Clean and dry inside Box and Lid

Unused holes plugged

Cable gland Securing cable OK

Correctly labelled

No damage Screens correctly connected Cable

No unspecified cables

Correctly labelled Connected to correct terminals

Cable cores

Crimped OK and tight in terminal blocks

No damage Terminal blocks Creepage and

clearance OK

Date

Inspector’s Initials

Comments:

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IS Inspection Record No. D Hazardous Area

Apparatus

Location

Junction Box No. Junction Box Connection Box Drawing No.

Instrument Loop Drawing No.

Tag No.

Correctly labelled

Securely mounted Apparatus (External)

No damage

Weather sealing OK Apparatus (Internal) Clean and dry inside

Cable gland Securing cable OK

Correctly labelled

No damage Cable Screen insulated from earth

Correctly labelled Connected to correct terminals

Cable Cores

Crimped OK and tight in terminal blocks

No damage Terminal blocks Creepage and

clearance OK

Date

Inspector’s Initials

Comments:

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Inspection of IS Earth System

IS Inspection Record No. 1 Check that the IS earth bars are correctly mounted on insulating blocks.

2 Check that the IS earth bars are firmly supported.

3 Check that the IS earth bars are protected against accidental connection to any

non-IS earth (e.g. supporting rack or cubicle).

4 Check that the IS earth bars are labelled ‘IS EARTH’.

5 Check that the IS earth bars are connected together in accordance with the

approved drawing.

6 Check that the cables connecting IS earth bars have the correct conductor size ( refer to drawing ) and an insulating sheath which is undamaged.

7 Check that the cable connections to the IS earth bars are clean and tight.

8 Check that the main IS earth bar is connected back to the

sub-station or switchroom earth bar in accordance with the approved drawing.

9 Check that the cables ( there should be two ) connecting the main IS earth bar to the sub-station or switchroom earth bar have the correct conductor size ( refer to drawing ) and an insulating sheath which is undamaged along its full length and not in contact with unarmoured cables. These cables should be inspected along their entire route.

10 Check that cable connections to the main IS earth bar and the substation or switchroom earth bar are clean and tight.

Date Inspector’s Initials Comments:

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Unit 8: Other methods of protection EEx o, EEx q, EEx m & Ex s Objectives: On completion of this unit, ‘Other methods of protection EEx o, EEx q, EEx m & Ex s’ apparatus, you should know: a) the principle of operation of each type of protection; b) typical applications for each type of protection.

- 1 -

Other methods of protection Oil-immersion Ex o or EEx o Oil-immersion is not a popular method of explosion protection but is typically used for heavy duty transformers and switchgear. Standards

BS EN50 015 Oil immersion ‘o’

BS 5501: Part 2 Oil immersion ‘o’

IEC 79-6: Part 6 Oil-immersed apparatus

BS 5345: Part 9 Installation and maintenance requirements for electrical apparatus with type of protection ‘o’ oil-immersed apparatus

Definition The definition for this type of protection is: ‘A type of protection in which the electrical apparatus or parts of the electrical apparatus are immersed in oil in such a way that an explosive atmosphere, which may be above the oil or outside the enclosure, cannot be ignited’.

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Zones of Use: 1 & 2 Principle The oil level is used to completely cover the components within the apparatus which arc/spark or produce hot surfaces during normal operation, thereby effectively establishing a barrier between the components below the oil and any flammable gases which may be present above the oil or outside the enclosure. A particular advantage of this method of protection is that circulation of the oil, by convection, enables hot-spots to be dispersed. One function of the oil is to quench arcs occurring at the contacts and, where mineral oil is used, a by-product of this process is the production of hydrogen and acetylene. This condition was considered to be undesirable for apparatus intended for use in hazardous locations, which may explain why, until recently, its use was limited to Zone 2 in the UK. The revised standards, however, have stricter specifications and this type of protection is now permitted in Zone 1. Construction The construction standard requires a breather to be fitted to the apparatus to allow release of the flammable gases produced during arc quenching, and thereby preventing the build-up of these gases in the space above the oil, whilst simultaneously preventing the ingress of dust or moisture, and hence, contamination of the oil. The enclosure ingress protection will be IP66. It is also a requirement that the apparatus is fitted with a gauge which can display the highest and lowest levels of oil, and that the apparatus is installed in such a way that the gauge can be easily read while the apparatus is in service. In the event of breakage of the gauge, even at it’s lowest point, the minimum depth of oil remaining above the arc/heat producing components, after leakage of oil at this point, should not be less than 25 mm. The standard specifies unused mineral oil which complies with IEC 296 for the protective liquid, but other types may be used, e.g. Askarels or silicone liquids tested to IEC 588-2 and IEC 836 respectively. The free surface temperature of the protective liquid is required to be 25 K less than the specified minimum flashpoint for the protective liquid. Sealed enclosures are required to be fitted with a pressure-relief device, and non-sealed enclosures with an expansion device which incorporates a mechanism for automatic tripping of the electrical supply on detection of gas evolution from the protective liquid as a result of a fault within the enclosure. The trip mechanism may only be manually reset.

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Powder filling Ex q or EEx q The explosion protection concept powder filling is not widely used and typical applications are, for example, capacitors in Increased Safety EEx ‘edq’ lighting fittings, and telecommunications equipment in some European countries. Standards

BS EN50 017 Powder filling ‘q’

BS 5501: Part 4 Powder filling ‘q’

IEC 79-5: Part 5 Sand-filled apparatus

BS 5345: Part 9 Installation and maintenance requirements for electrical apparatus with type of protection ‘q’ sand filled apparatus

Definition The definition for this type of protection is: ‘A type of protection in which the enclosure of electrical apparatus is filled with a material in a finely granulated state so that, in the intended conditions of service, any arc occurring within the enclosure of an electrical apparatus will not ignite the surrounding atmosphere. No ignition shall be caused either by flame or by excessive temperature of the surfaces of the enclosure’.

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Zone of Use: 1 & 2 Principle The filling, which may be quartz, or another material which complies with the requirements of relevant standards, achieves safety by what is known as “suppression of flame propagation”. It is inevitable that a flammable gas or vapour may permeate the granules and reach the parts producing arcs/sparks or hot surfaces. The quantity of gas or vapour, however, will be too small to support an explosion within the inert powder. The depth of granules is influenced by the level and duration of the of the arc current produced by the components within the filling material, and tests specified in the construction standard enable a safe correlation between these two parameters to be established. This method of protection is suitable for use in all group II gases or vapours. Construction The enclosure, which holds the filling material, is required to withstand, for one minute, an overpressure of 0.5 mBar (0.05 kPa) without permanent deflection of the walls in any direction by more than 0.5 mm, and maintain a minimum level of ingress protection to IP54. The size of granules for the filling material, usually quartz, must be within the range 250 µm - 1.6 mm. The relative weight of water which can permeate the filling material must not be in excess of 0.1%. Clearly, this method of protection is unsuitable where moving parts are involved since the filling material must be free of voids.

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Encapsulation Ex m or EEx m The method of protection, encapsulation, is used mainly for smaller items of equipment such as solenoid coils and electronic components. Standards

BS EN50 028 Encapsulation ‘m’

BS 5501: Part 8 Encapsulation ‘m’

IEC 79-18: Part 18 Encapsulated apparatus

Definition The definition for this type of protection is: ‘A type of protection in which the parts which could ignite an explosive atmosphere by either sparking or heating are enclosed in a compound in such a way that this explosive atmosphere cannot be ignited’.

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Zone of Use: 1 & 2 Principle With this type of protection, the encapsulant, typically a thermo-setting compound, establishes a complete barrier between any surrounding flammable gas or vapour and the source of ignition within the compound. Construction The construction standards state that the encapsulant must be free of voids and, therefore, this method of protection is unsuitable where components have exposed moving parts. Very small components which have enclosed moving parts, e.g. a reed relay, may be protected by encapsulation. The minimum depth of encapsulant above the components of, say, a printed circuit board is 3 mm, and must be able to withstand a 7 J impact test. The encapsulant depth may be reduced to 1 mm for very small apparatus where the free surface area is not in excess of 2 cm2, but this relaxation requires the use of additional protection since the apparatus will be unlikely to withstand the 7 J impact test. This difficulty may be overcome, for example, by installing the apparatus in an Increased Safety type ‘e’ enclosure which meets the impact test requirements. Such encapsulated apparatus may be subject to “special installation conditions” indicated by a suffix ‘X’ at the end of the certification number, or the apparatus may only have “component certification” indicated by a suffix ‘U’ at the end of the certification number on the apparatus.

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Special protection Ex s Apparatus which has not quite met the requirements of a particular construction standard will have been additionally certified under the BASEEFA Standard ‘Special Protection Ex s’ provided it had been established that, after close scrutiny of the design and testing of the apparatus, it was capable of operating safely in the hazard for which it was designed. Special Protection is not included in the BS EN50 series of harmonised construction standards, or in the installation, inspection and maintenance series of standards, BS EN60079-14 and BS EN60079- 17 respectively. Standards

SFA 3009 Special protection

BS 5345: Part 8 Installation and maintenance requirements for electrical apparatus with type of protection ‘s’ special protection

Zones of Use: 0, 1 & 2 Principle The constructional requirements of this standard, in terms of test and acceptance criteria, were intended to be unspecific in order to allow a broad range of designs to be considered for certification. Because apparatus may be of unorthodox design, the experience of test-house staff plays an important part in contriving appropriate tests and acceptance criteria. Special protection is not an easy option for obtaining certification for apparatus not quite meeting the requirements of a given standard, nor is this type of protection inferior to other more popular methods of explosion protection. Indeed the tests on apparatus presented for certification under Special Protection are likely to be more onerous than the tests for other types of explosion protection. A hand torch is a typical example of apparatus certified under Special Protection. Thorough testing will have established that the construction is robust enough to withstand a specified impact without causing, for example, a short-circuit of the battery, and breakage of the bulb, its holder and the glass cover are unlikely. A further requirement is that opening of the torch, i.e. to replace the battery, is only possible with the aid of a special tool, which is required to be kept in a non-hazardous area. Another known example of apparatus certified under Special Protection Ex s is a 6.6 kV poly-phase cage induction pump motor in which the method of explosion protection is basically dependent on

- 8 -

the interior of the motor being completely filled with water. Any free space within the motor is occupied by water, and hence, the entry of a flammable gas is prevented. Clearly, it is imperative that the interior of the motor remains completely full of water at all times, and this is ensured by a header tank to compensate for expansion due to thermo-cycling. The motor, which drives a pump, is intended for use in Zone 1 .

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Unit 9: Combined (hybrid) methods of protection Objectives: On completion of this unit, ‘Combined (hybrid) methods of protection’, you should know: a) the advantages of combining two or more methods of protection in apparatus; b) the installation requirements according to BS EN 60079-14; c) the inspection requirements according to BS EN 60079-17.

- 1 -

Combined (hybrid) methods of protection Electrical equipment may be manufactured with more than one method of explosion protection. Equipment of this type has combined methods of protection but may also be known as a hybrid. Such an approach combines the best features of each type of protection into one piece of equipment for both economic and practical purposes.

A traditional push-button station for use in an hazardous location comprises a flameproof EEx d enclosure, in which a standard industrial switch is fitted. An alternative to this arrangement is an Increased Safety EEx e enclosure with a small flameproof EEx d component certified switch fitted inside. Because the switch produces sparks in normal operation, clearly it has to be flameproof to comply with the Increased Safety concept of protection. Such equipment will be marked EEx ed or EEx de.

The advantages of the hybrid arrangement discussed over the traditional flameproof method are:

(a) lower cost and weight; (b) glanding arrangements are simplified; (c) minimum ingress protection IP54 but may be as high as IP66.

- 2 -

Standards Hybrid apparatus may be constructed using any combination of the various methods of explosion protection and, therefore, the apparatus will be marked with the symbolic letters and construction standard numbers relative to the types of explosion protection used. Probably the most commonly used combination involves ‘d’ and ‘e’ type apparatus, and so the table below shows these standards only. The full list of standards can be found in Unit 2. Hybrid apparatus must also be installed and maintained in accordance with relevant standards.

BS EN50 018 Flameproof enclosure ‘d’

BS 5501: Part 5: 1977 Flameproof enclosure ‘d’

BS EN50 019 Increased safety enclosure ‘e’

BS 5501: Part 6: Increased safety enclosure ‘e’

BS EN60079-14 Electrical apparatus for explosive gas atmospheres: Part 14. Electrical installations in hazardous areas (other than mines)

BS EN60079-17 Electrical apparatus for explosive gas atmospheres: Part 17. Inspection and maintenance of electrical installations in hazardous areas (other than mines)

Motors - EEx de

- 3 -

Motors - EEx de Manufacturers also produce electric motors in which there are combined methods of protection. The main body of the motor will be flameproof EEx d and the terminal box increased safety EEx e. An alternative terminal plate is fitted to a motor of this type to accommodate special terminals which are screwed into the terminal plate. These are hybrid terminals, i.e. they employ both flameproof EEx d and increased safety EEx e concepts in their construction.

EEx de motor terminal box To achieve the required level of ingress protection, gaskets are fitted between the terminal box and it’s cover, between the terminal plate and box, and between the gland plate and terminal box. On no account, however, should a gasket be fitted between the terminal plate and the frame of the motor as this joint is a flamepath. It must be emphasised that, on some motors, the increased safety terminal box looks very much like a flameproof box in terms of it’s construction. This likeness means that there is a possibility that the gaskets may be removed by personnel unaware of this concept and, therefore, it is important that certification labels are studied before any work is carried out. Removal of the gaskets in attempt to return the box to it’s assumed status, i.e. flameproof, would be an unauthorised modification which would invalidate the certification.

- 4 -

EEx de - sample certification label

- 5 -

Lighting fittings - EEx edq The lighting fitting illustrated below employs three protection concepts, i.e. increased safety type ‘e’, flameproof type ‘d’ and powder filling type ‘q’. This type of fitting is widely used in the petro-chemical industry.

The constructional features are:

1) flameproof lampholders; 2) increased safety choke designed not to overheat if lamp fails; 3) temperature rating based on internal and external surface temperatures; 4) enclosure sealing providing high ingress protection; 5) increased safety enclosure including glands designed to withstand specified impact.

In this lighting fitting, the circuits include capacitors which are protected by a method of protection, powder filling type ‘q’. Switches will be of flameproof type ‘d’ construction and terminals will be increased safety type ‘e’.

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EEx e m ib An enclosure may have an encapsulated component inside. A typical example is a telephone for use in a hazardous location. The casing of the telephone would use increased safety type ‘e’ protection, most of the internal circuits would be intrinsically safe, type ‘i’, but part of the circuitry would operate at a higher voltage and therefore encapsulation type ‘m’ would be used to protect that part of the circuit. Terminals would be increased safety type ‘e’.

Eex ‘e’ Enclosure

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EEx pde Enclosures which employ the protection concept, pressurisation type ‘p’, may have internal apparatus which have to remain energised in the absence of overpressure. Such apparatus must be protected in accordance with the Zone in which the enclosure is located. A typical example is an anti-condensation heater within a pressurised machine which will be energised when the machine is idle. Apparatus outwith the machine, e.g. junction boxes, pressure sensors etc., will also have to be protected in accordance with the Zone. Note: Since anti-condensation heaters are normally ‘live’ when a machine is idle, notices warning of this

danger should be displayed.

EEx pi The part(s) of an IS system which are marked to indicate that they should be installed in a non-hazardous area may be used in an hazardous area if installed in, for example, a pressurised enclosure as illustrated below.

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Unit 10: Wiring systems Objectives: On completion of this unit, ‘Wiring systems’, you should know: a) appropriate cable types for use with explosion protected apparatus; b) the selection procedure for cable glands by consideration of ‘Zone’, ‘Gas Group’, ‘volume’, ‘method of

entry’, ‘cable construction’ and’ ‘internal components’ of flameproof enclosures; c) the correct assembly techniques for various types of cable glands; d) recognised practices for terminating single and multiple pair cables with or without screens; e) recognised practices for maintaining ingress protection, earth continuity and termination of unused

conductor cores and screens; f) earthing requirements in hazardous areas; g) the installation requirements according to BS EN 60079-14; h) the inspection requirements according to BS EN 60079-17.

- 1 -

Wiring Systems Electrical equipment in hazardous areas may be wired using cable having metallic or non-metallic sheath, or conduit. The use of cable is generally predominant and one reason is it’s ease of installation compared to conduit. With regard to conduit, one of it’s disadvantages, particularly on offshore installations, is it’s susceptibility to corrosion as a result of exposure to seaspray. Deterioration due to corrosion can occur relatively quickly and, as a consequence, can reduce the strength of the conduit. This is undesirable particularly where conduit is the method of entry to a flameproof enclosure because of the possible inability of the conduit to contain an internal explosion in the run of conduit between the enclosure and the sealing device. Furthermore, corroded conduit may not meet the impact resistance requirements essential for use with Increased Safety apparatus.

Standards

BS EN60079-14 Electrical apparatus for explosive gas atmospheres: Part 14. Electrical installations in hazardous areas (other than mines).

BS 6121: Part 5 Code of practice for selection, installation and inspection of cable glands used in electrical installations.

BS 5345 (superseded but remains current)

Code of practice for the selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres.

- 2 -

Cables BS EN60079-14, the replacement for the Code of Practice BS 5345, specifies the following requirements for cables in Zones 1 and 2. Fixed apparatus Cables manufactured from thermoplastic, thermosetting, elastomeric or mineral insulated insulating materials may be used in fixed wiring installations. Cables commonly used in the industry are of the EPR/CSP type. Mineral insulated metal sheathed (MIMS) cable is also suitable for use in hazardous areas, but it’s aluminium variation requires careful consideration before use. Aluminium conductors must only be connected to suitable terminals and have a cross-sectional area (c.s.a.) not less than 16 mm2. Examples of the various cable insulation types are given in the table below.

chlorosulphonated polyethylene CSP

cross-linked polyethylene XPLE

ethylene propylene rubber EPR

ethylene vinyl acetate EVA

natural rubber NR

polychloroprene PCP

Elastomeric

silicone rubber SR

polyethylene PE

polypropoline PP

Thermoplastic

polyvinyl chloride PVC

Portable and transportable apparatus Cables for portable and transportable electrical apparatus may be wired using:

(a) ordinary tough rubber sheathed flexible cables; (b) ordinary polychloroprene sheathed flexible cables; (c) heavy tough rubber sheathed flexible cables; (d) heavy polychloroprene sheath; (e) plastic insulated cables of equally robust construction to heavy tough rubber sheathed flexible

cables.

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Cables (continued) Elastomeric cables Elastomeric cables comprising EPR insulated conductors, CSP sheath, which are heat and oil resistant and flame retardant (HOFR). Operating temperature range 30 °C - 90 °C.

Cable specified as ‘low smoke and fume’ (LSF) has insulation which does not contain halogens, so that smoke and acid emission are minimised in the event of a fire.

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Cables (continued) Cables may also be selected with consideration to their fire resistant and/or flame retardant properties, and two standards are relevant in this respect. IEC 331 (Fire resistant): A cable manufactured in compliance with this standard will continue to

operate in a fire without disruption of essential circuits and emergency circuits.

IEC 332 (Flame retardant): A cable manufactured in compliance with this standard is self-extinguishing

and will not propagate the fire. Cold flow Certain materials used in the manufacture of cables are susceptible to a condition commonly known as ‘cold flow’, which could have a detrimental effect on the type of explosion protection concerned. This condition can occur at ambient temperature, is caused by entry devices which have compression seals and results in an indentation on that part of the cable acted on by the seal. Recent developments by cable gland manufacturers have resulted in new designs of cable glands which can reduce, if not eliminate, the effects of ‘cold flow’ by the use of seals which apply less pressure on the cable insulation but still maintain the integrity of the type of explosion protection in apparatus. Requirements for cables and glands Cable glands must be selected with due regard to the methods of explosion protection employed and also environmental conditions. The requirements for cable glands include:

(a) to firmly secure the cable entering the apparatus; (b) to maintain the ingress protection of the apparatus; (c) to maintain earth continuity between the apparatus and any armouring in the cable; (d) to ensure containment of an internal explosion in flameproof apparatus; (e) to maintain the integrity of ‘restricted breathing’ apparatus.

Jointing of cables In hazardous areas, cable runs should, ideally, be continuous and without interruption where possible. Joints may only be made using appropriate methods, for example, in an enclosure having a type of explosion protection suitable for the Zone, or by epoxy or compound filled devices, or heat shrink sleeving in accordance with the manufacturers instructions. Whichever method is used, the joints must be mechanically, electrically and environmentally appropriate. Conductor connections are required to be made by either compression connectors, secured screw connectors, welding or brazing. Soldering is permissible if the conductors being connected are held together by suitable mechanical means and then soldered.

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Glands for mineral insulated cables Glands for use with MICC (Mineral Insulated Copper Cable) or MIMS (Mineral Insulated Metal Sheath) cable for use in hazardous areas will be marked EEx d. This gland, however, may be used as a means of entry to Increased Safety apparatus providing an alternative EEx e seal is used. This seal is specially constructed to comply with the requirements for Increased Safety apparatus as illustrated by the diagrams below. Seal assemblies

EEx d type seal

EEx e type seal

The EEx e seal assembly must only be used with doublebond non-metallic black epoxy putty 1536. The Component Certificate for this seal will contain a ‘schedule for conditions of use’ which must be observed. Difficulty may be experienced in achieving the desired level of ingress protection with MICC/MIMS cable glands due to the very small shoulder on the gland body, and may be overcome by the use of hard plastic washers manufactured for this purpose.

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Selection of cable glands The correct selection of cables and glands, particularly for flameproof apparatus, is very important since there are a number of factors which can jeopardise the integrity of this type apparatus. As previously discussed the cable construction, ingress protection, earth continuity and secureness of the cable entering the apparatus must be maintained. An additional consideration is electrolytic action caused by contact between dissimilar metals, which results in increased corrosion and premature degradation of glands and cable entries. Flameproof apparatus, however, introduces other considerations which are as follows.

1) Is the enclosure direct or indirect entry. 2) Does the enclosure contain a source of ignition. 3) Gas group of the apparatus. 4) Zone in which the apparatus is installed; 5) Internal volume of the enclosure.

These considerations are addressed in the flowchart on page 8. Direct entry method (flameproof EEx d)

(a)

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Indirect entry methods

(b) Flameproof EEx d

(c) Flameproof I Increased Safety EEx de

Maintenance of ingress protection at cable gland entries The cable gland selected as the means of entry to an enclosure must suit the cable used and also maintain the ingress protection (IP) rating of the enclosure. With regard to, for example, type ‘e’ or type ‘n’ enclosures, the cable gland must maintain the minimum ingress protection IP54 and, where the enclosure wall or gland plate has a thickness less than 6 mm, a sealing washer (or thread sealant) will be required between the gland and the enclosure to maintain this level of ingress protection. Where the enclosure wall or gland plate has a thickness greater than 6 mm and the cable gland is installed via a threaded entry, the use of a sealing washer or thread sealant is not deemed necessary to maintain IP54 unless a greater ingress protection level is required.

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Cable gland selection for flameproof apparatus Cable glands may be selected by following the procedure recommended in BS EN60079-14 and replicated in the flowchart below.

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Conduit The use of conduit in hazardous areas requires particular care, especially when used with flameproof enclosures. In addition to maintaining the ingress protection (IP) rating of an enclosure - this applies to all types of protection - the integrity of the enclosure must be maintained, i.e. the conduit in the run between the enclosure wall and the conduit sealing device must also be able to withstand the force of an explosion within the enclosure so that the flames/hot gases are prevented from reaching the external atmosphere. Where two flameproof enclosures are connected by means of conduit, seals must be fitted to avoid pressure piling occurring during an internal explosion. Sealing devices are also used to prevent the migration of gases from one hazardous location to another. Although not entirely gas-tight, they will limit, to an acceptable level, the quantity of gas which will pass at normal atmospheric pressure. Where positive or negative pressures are likely, appropriate measures must be implemented. Appropriate installation practices must, therefore, be observed and this requires observation of the manufacturer’s installation specification and the recommendations given in BS EN60079-14. Selection of conduit Conduit used with explosion protected apparatus will be that recommended by the manufacturer and selected from either:

(a) screwed heavy duty steel, solid drawn or seam welded conduit manufactured in accordance with IEC 614-2-1; or

(b) flexible conduit of metal or composite material construction, for example metal conduit with a

plastic or elastomer jacket, of heavy or very heavy mechanical strength classification manufactured in accordance with IEC 614-2-5.

Conduit entering flameproof enclosures is required to be engaged by 5 full threads. Sealing of conduit Conduit seals are required to be fitted:

(a) where conduit leaves or enters a hazardous area; (b) within a distance of 450 mm from the wall of any enclosure which contains a source of ignition

in normal operation; (c) at all enclosures entered by conduit of 50 mm diameter or greater, which contain taps,

splices, joints or terminals.

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IS cable requirements

The requirements for IS cables will be specified in the system documentation. Cables need not be mechanically protected since the energy in an IS circuit is below that which is necessary to ignite a flammable mixture, even if a spark is produced at a break in the cables.

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IS cable requirements

1) Insulation between screen and SWA or braid; 2) Screen (optional) - normally earthed at one point only, which is usually the barrier earth; 3) Individual insulated conductors; 4) SWA or braid (optional) - normally earthed at each end, and at any intervening junction boxes

through the cable glands.

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IS cable requirements

1) Zener safety barrier; 2) Barrier mounting rail/earth bar; 3) SWA/braid connected (earthed) to enclosure via gland; 4) Screen connected to barrier mounting rail/earth bar; 5) Dedicated earth conductors connected to main earth point using either:

a) two separate 1.5 mm2 minimum conductors (BS EN60079-14), or b) a single copper conductor 4 mm2 minimum (BS 5345 Part 4 & BS EN60079-14)

Note: Longer runs may require conductors of larger cross-sectional area, e.g. 6mm2 or

10 mm2

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IS cable requirements

6) Junction box; 7) Cable glands; 8) Junction box bonded locally to structure; 9) Screen through connected; 10) Screen terminated but not isolated at field apparatus.

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Earthing and bonding The principal reasons for earthing and bonding in electrical installations are:

1) to eliminate the possibility of electric shock to personnel; 2) to enable protection devices to operate correctly so that the duration of fault currents are kept

to a minimum; 3) to equalise the voltage potential of normally non-current carrying metalwork; 4) to prevent electrostatic charge of process plant due to fluid movement.

In hazardous areas, the elimination of sources of ignition is very important and effective earthing and bonding will play an important role here. Electrical faults, if allowed to persist, can develop to a point where excessive surface temperatures and/or arcs/sparks are produced. BS EN60079-17 clause 4.7 states: ‘Care shall be taken to ensure that the earthing and potential equalisation bonding provisions in hazardous areas are maintained in good condition’ (see inspection schedules table 1, item B6; table 2, items B6 and B7 and table 3, item B3). Explanation of terms Electrical earthing or circuit protective conductors (CPC) Conductors installed to provide a low impedance path for the current which flows under fault conditions to the general mass of earth. Normally the CPC is connected directly to any associated metal work of the equipment. Electrical bonding Conductors installed to establish continuity between adjoining metal work and the armouring of separate cables to ensure that, under fault conditions, all metal work and cable armouring are maintained at the same potential. Exposed conductive parts Exposed conductive parts include the metal work of switchboards, enclosures, motor frames and transformer tanks. Extraneous conductive parts My metal work associated with the plant, for example pipe work which can be touched at the same time as a metal switch board cover or motor frame, will be deemed extraneous conductive parts.

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Types of systems

(a) TN-S The system has separate neutral and protective conductors throughout. (b) TT A system in which one point of the source of energy is directly earthed but

which is electrically independent of the electrodes used to earth the exposed conductive parts of the electrical installation.

(c) TN-C A system in which a single conductor serves as both neutral and protective

conductor throughout the system. (d) TN-C-S A system in which a single conductor serves as both neutral and protective

conductor in part of the system. (e) IT A system in which there is no direct connection between live parts and earth

but exposed conductive parts of the electrical installation are earthed. Classification of systems A system comprises an electrical supply to which an electrical installation is connected. The first letter indicates the supply earthing arrangements where:

1) T represents a system having one or more points of the supply directly connected to earth; 2) I represents a system in which the supply is not earthed, but may be earthed through a fault-

limiting impedance. The second letter indicates the installation earthing arrangements where:

3) T represents the exposed conductive parts of the installation connected directly to earth; 4) N represents the exposed conductive parts of the installation which are connected directly

to the earthed point of the supply. The third letter indicates the earthed supply conductor where:

5) S represents separate neutral and protective conductors; 6) C represents neutral and protective conductors combined in a single conductor.

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System earthing configurations (a) TN-S

Generally, this method will be used when the electrical supply is provided via underground cables having metal sheaths and armour. The consumer’s earth terminal will be connected to the supply authorities protective conductor, that is the metal sheath and armour of the underground cable, thereby establishing a continuous path back to the supply transformer star-point which is earthed.

(b) TT

Generally, this method will be used when the electrical supply is provided via overhead cables but with no earth terminal provided by the supply authority. The consumer may have to provide an earth electrode for connection of the circuit protective conductors. With this system, it is recommended that consumers use residual current devices because of the difficulty in obtaining an effective earth connection.

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(c) TN-C

In this system the same conductor, perhaps the outer conductor of a concentric cable, is used for both the neutral and circuit protective conductor (PEN conductor) throughout the system. It is typically used where the electrical supply is provided by a privately owned transformer or converter, i.e. where there is no electrical connection between the consumer and the supply authority, or where the supply is provided by a private generator.

(d) TN-C-S

The supply authorities installation will use a TN-C system where both the neutral and circuit protective conductor are served by a single (PEN) conductor. If the consumers installation, which is connected to the TN-C supply system, employs a TN-S system where the neutral and circuit protective conductors are separate, then the overall system is known as a TN-C-S system. The majority of new installations use this arrangement which is termed a PME system by the supply authorities.

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(e) IT

In this arrangement the system may have no connection to earth, or be connected to earth via a relatively high impedance, the ohmic value of which will depend on the level at which fault currents will be limited. Protection in this method is afforded by a relay which monitors any earth-leakage current as a result of an earth-fault. This will activate an audio or visual alarm, or disconnect the electrical supply.

Regulations and standards The requirements for earthing practice within the UK may be found in the following documents.

BS EN60079-14 Electrical apparatus for explosive gas atmospheres: Part 14 Electrical installations in hazardous areas (other than mines)

BS 5345 Code of practice for: Selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres (other than mining applications or explosive processing and manufacture)

BS 7671 (1991) IEE Wiring Regulations

IEE Recommendations for the Electrical and Electronic Equipment of Mobile and Fixed Offshore Installations

Electricity Supply Regulations 1988

Electricity at Work Regulations 1989

BS 7430 Code of Practice for Earthing

BS 6651 Protection of Structures against Lightning

BS 5958 Control of Undesirable Static Electricity

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Earthing systems in hazardous areas BS EN60079-14 specifies the conditions for the following earthing systems in hazardous areas. Type TN systems If a type TN earthing system is used, it shall be type TN-S (with separate neutral N and protective conductor PE) in the hazardous area, i.e. the neutral and the protective conductor shall not be connected together, or combined in a single conductor, in the hazardous area. At any point of the transition from type TN-C to type TN-S, the protective conductor shall be connected to the equipotential bonding system in the non-hazardous area. The monitoring of leakage between the neutral and PE conductors in the hazardous area is also recommended in the standard. Type TT system If a type TT earthing system (separate earth’s for power system and exposed conductive parts) is used in zone 1, then it shall be protected by a residual current device. This system may not be acceptable where the earth resistivity is high. Type IT system If a type IT earthing system (neutral isolated from earth or earthed through an impedance) is used, an insulation monitoring device shall be provided to indicate the first earth fault. With this system, there may be a requirement for local bonding which is also known as supplementary equipotential bonding. Further information may be obtained by reference to IEC 364-4-41. Potential equalisation In order to prevent different voltage potentials occurring in the metal work of plant in hazardous areas, potential equalisation will be necessary. This applies to TN, TT and IT systems where all exposed and extraneous conductive parts are required to be connected to the equipotential bonding system. The bonding system may comprise protective conductors, metal conduits, metal cable sheaths, steel wire armouring and metallic parts of structures, but not neutral conductors. The security of connections must be assured by non-loosening devices.

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Maximum Disconnection Times for TN Systems Table 41A

U0 (volts) t (seconds)

120 0.8

220 to 277 0.4

400 0.2

greater than 400 0.1

Where U0 = Nominal voltage Earthing conductor cross-sectional area Calculation of csa The cross-sectional area of earthing conductors may be calculated using the following formula from the 16th Edition of the I.E.E. Wiring Regulations, BS7671.

k

t2IS =

Where: S = nominal cross-sectional area of earthing conductor mm2

I = fault-current for a negligible impedance fault that will flow through the associated protective device (the current-limiting effect of circuit impedance’s and limiting capacity (I2t) of protective device will be taken into account. A further consideration is the increase in resistance of the conductors as a result of the temperature rise during clearance of the fault.

(A)

t = operating time of protective device when clearing the fault-current I.

(secs)

k = factor which takes into account resistivity, temperature coefficient and heat capacity of conductor material, and the appropriate initial and final temperatures.

Values for k are given in tables 54B, 54C, 54D, 54E, & 54F in the I.E.E. Wiring Regulations.

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CSA of CPC in relation to phase conductor Alternatively, the minimum cross-sectional area of the protective conductor, in relation to the cross-sectional area of the associated phase conductor, may be determined by consideration of table 54G shown below. Table 54G Minimum CSA of protective conductor in relation to the CSA of associated phase conductor.

Minimum CSA of corresponding protective conductor (Sp)

CSA of phase conductor If the protective conductor is

of the same material as the phase conductor

If the protective conductor is not the same material as the phase conductor

mm2

S ≤ 16

16 ≤ S ≤ 35

S > 35

mm2

S

16

S 2

mm2

k1S k2

k116 k2

k1S k22

Note: The values of ‘k’ in the above table are given in the IEE Wiring Regulations, BS 7671

as follows where:

k1 is the value of k for the phase conductor, selected from table 43A in Chapter 43 according to the materials of both conductor and insulation.

k2 is the value of k for the protective conductor, selected from Tables 54B, 54C, 54D, 54E or 54F

as applicable.

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Main equipotential bonding conductors The IEE Wiring Regulations, BS 7671, specify that the main equipotential bonding conductor in an installation, other than a PME system, is required to have a cross-sectional area not less than half the cross-sectional area specified for the installation earthing conductor and not less than 6 mm2. If a copper bonding conductor is used, the cross-sectional need not be greater than 25 mm2 or, where other metals are used, the cross-sectional area which provides equivalent conductance will apply. With regard to a PME system, Table 54H below details the requirements for the main equipotential bonding conductor in relation to the neutral conductor of the supply. Table 54H

Copper equivalent cross-sectional area of the supply neutral conductor

Minimum copper equivalent cross-sectional area of the main equipotential bonding conductor

35 mm2 or less 10 mm2

over 35 mm2 up to 50 mm2 16 mm2

over 50 mm2 up to 95 mm2 25 mm2

over 95 mm2 up to 150 mm2 35 mm2

over 150 mm2 50 mm2

Practical example with and without earth bonding The diagram on page 29 shows a simple installation comprising a motor, distribution transformer, fuses and connecting cable. The fuses are necessary to provide protection against short-circuits which may occur between phases or between phase and earth. Electrical faults such as these must be disconnected as quickly as possible to prevent further damage to equipment and, more importantly, to prevent injury to personnel. The speed at which a fuse ruptures is dependent not only on the type of fuse, but also the circuit parameters, e.g. the resistance of the fault path - the earth-loop impedance - and the fault current magnitude. The lower the earth-loop impedance, the higher the fault-current will be and the faster the fuse will rupture. The 16th edition of the I.E.E. Regulations specifies the requirements for earth-loop impedance. Normally the star-point of the distribution transformer secondary winding is connected to an earth mat buried in the soil, but this alone will not provide a low enough earth-loop impedance, and so an earthing bond is required between the motor frame and the star-point of the transformer secondary winding. Let us now investigate the situation with and without the bonding conductor between the

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main earth conductor and the motor frame when a fault occurs between one phase and earth within the motor. It will also be assumed that the motor is bolted securely to the bedplate but, due to dirt, rust or paint, the resistance between the feet of the motor and the bedplate is l Ω. Case 1: No earth connection between main earth conductor and motor frame.

Consider the circuit on page 29 which comprises a motor, transformer and inter-connecting cable. For simplicity only resistive values are used. Rg = the resistance of one phase of the generator; Rm = the resistance of one phase of the motor R = the resistance between the motor feet and bedplate; Vph = the phase voltage.

Voltage across motor frame and bedplate, V = VphxRRmRg

R++

V = 240x10.010.05

1++

V = 208 V Thus, anyone standing next to the motor and touching it’s frame would receive a severe shock particularly if the deck was wet. Case 2: Earth connection between main earth conductor and motor frame.

The above situation is avoided with appropriate earthing and bonding. If the bonding conductor is connected between the motor frame and the main earth, the resistance of 1 Ω between the motor feet and bedplate is shunted and the effective resistance at this point is significantly reduced. Similarly, a bonding conductor connected between the motor feet and bedplate would achieve a similar result.

It is, therefore, essential that earth conductors have sufficient cross-sectional area (csa) to carry prospective fault-currents, which can be very high but usually of short duration, until they are interrupted by the electrical protection. It has been demonstrated that a contact resistance of 1 Ω can result in the presence of dangerous voltage levels. In order to avoid this difficulty, the earth-loop impedance should be significantly lower than 0.1 Ω.

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Practical example with and without earth bonding (continued)

No earth bond connected between motor frame and main earth conductor

Earth bond connected between motor frame and main earth conductor

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Static electricity Static electricity is more than capable of igniting flammable materials and its presence in the petro-chemical industry represents a very high risk which must be countered by the application of appropriate measures. Recommendations for the control of static electricity may be found in the British Standard BS 5958: Code of Practice for the control of undesirable static electricity. The passage of oil, gases or dusts through process pipework and containment vessels causes an internal build-up of static charges, which emerge on the exterior of the pipes and tanks to establish potentials the magnitude of which can be many thousands of volts. This is unacceptable in hazardous locations and can be eliminated by ensuring that all pipes, tanks, etc., are solidly bonded together and bonded to the main earth. Bonding across pipe flanges and joints can also reduce the problem of corrosion caused by static charges. Static electrical charges can be reduced in many instances by:

1) slowing the flow rate of fluids through pipes; 2) adding compounds to liquids; 3) the use of pipes manufactured from materials with high carbon content.

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Static electricity (continued) BS 5958 recommendations for earthing resistances Type of installation Area classification Recommended

maximum resistance to earth

(Ω)

Comments

Main metal plant structure,

Zones 0, 1 & 2 10 Ωl Earthing normally inherent in the structure.

Large fixed metal plant items. Reaction vessels + powder silos etc.

Zones 0, 1 & 2 10 Ω Earthing normally inherent in the structure. Occasionally items may be mounted on non-conducting supports and special earthing connections may then be required.

Metal pipelines. Zones 0, 1 & 2 10 Ω Earthing normally inherent in the structure. Special earthing connections may be required across joints if there is doubt that the 10 Ω criterion will be satisfied.

Transportable metal items: drums tanks etc.

Zones 0, 1 & 2 10 Ω Special earthing connections are normally required.

Metal plant with some non-conducting elements: Rotating shafts, stirrers etc.

Zones 0, 1 & 2 10 Ω In special cases a limit of 100/ 1Ω may be acceptable, but in general if 1MΩ criterion cannot be satisfied a special earthing connection should be used to obtain a resistance of less than 10 Ω to earth.

Higher resistivity non-conducting items with or without isolated metal components: e.g. bolts in a plastic pipeline

Zones 0, 1 & 2 10 Ω The general electrostatic ignition risk and the fire hazard normally preclude the use of such non-conducting materials unless it can be shown that significant charge accumulation will not occur. In the absence of charge accumulation, earthing is not required in Zone 2 areas

Items fabricated from conductive or antistatic materials

Zones 0, 1 & 2 1MΩ - 10MΩ

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Unit 11: Inspection & Maintenance to BS EN 60079-17 Objectives: On completion of this unit ‘Inspection & Maintenance’, you should know: a) the importance of appropriate and regular inspection and maintenance; b) the requirements of types of inspection ‘initial’, ‘periodic’ and ‘sample’; c) how to apply inspection schedules, Tables 1, 2 & 3 from BS EN60079-17 for ‘visual’, ‘close’ and

‘detailed’ grades of inspection..

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Inspection and Maintenance Introduction This unit is concerned with the inspection and maintenance of electrical apparatus used in hazardous locations in accordance with relevant standards. This is very important because, in addition to the risk of mechanical damage to apparatus, there is also the risk that degradation of the apparatus, due to environmental conditions and other factors, could affect the integrity of the apparatus and allow ignition of any flammable gas or vapour in an hazardous area. Inspection of equipment should be carried out on a regular basis to enable detection of potential faults early enough to prevent major breakdowns occurring, minimise downtime and loss of production, and also possible injury to personnel. A maintenance programme based on the results of inspection surveys can then be implemented which will allow continued reliability and safe operation of the equipment. Apparatus will only remain approved/certified if it is maintained in accordance with the recommendations provided by manufacturers and relevant standards. Standards

BS EN60079-17 Electrical apparatus for explosive gas atmospheres: Part 17. Inspection and maintenance of electrical installations in hazardous areas (other than mines).

BS 5345 (superseded but remains current)

Code of practice for the selection, installation and maintenance of electrical apparatus for use in potentially explosive atmospheres

Qualifications of Personnel It is essential that personnel involved in the selection, installation, inspection and maintenance of explosion protected apparatus in hazardous areas have a clear understanding of the various protection concepts, installation practices and regulations, and the general principles of area classification. Manufacturers have gone to great lengths to design and build apparatus in accordance with relevant standards and have it tested and certified by a third party test house to ensure the apparatus is safe for use in hazardous areas. All this effort will have been in vain if the technician in the field does not have the necessary knowledge to install and/or maintain apparatus in accordance with the manufacturers requirements, relevant standards and Codes of Practice. Personnel operating in this field must, therefore, have appropriate training, and thereafter, regular refresher training. Apparatus may be explosion protected at the time of leaving the manufacturers premises but, the way the apparatus is subsequently handled, selected, installed and maintained, will have an influence on whether the apparatus will be safe for use in an hazardous area and/or remain certified. Personnel need to be aware of, for example, the consequences of a broken foot on a flameproof motor. Increased Safety apparatus may have ‘special conditions of use’ and failure to observe these will

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reduce margins of safety and invalidate the certification. Furthermore, the incorrect selection of cable glands with regard to, for example, flameproof apparatus will affect the integrity of such apparatus. Principal causes of apparatus deterioration Table 4 of IEC 79-17 lists major factors which have a significant effect on the deterioration of equipment in hazardous locations. These factors are listed below.

1) Susceptibility to corrosion; 2) Exposure to chemicals or solvents; 3) Likelihood of accumulation of dust or dirt; 4) Likelihood of water ingress; 5) Exposure to excessive ambient temperatures; 6) Risk of mechanical damage; 7) Exposure to undue vibration; 8) Training and experience of personnel; 9) Likelihood of unauthorised modifications or adjustments; 10) Likelihood of inappropriate maintenance, for example not in accordance with manufacturer’s

recommendations. IEC Standards The International Electrotechnical Commission (IEC) Standards relative to explosion protected apparatus, numerically referenced in the series 79-xx, have been published since the late nineteen-sixties. These Standards, however, had not kept pace with the advances in technology and, as a consequence, had tended to lag behind the National and European Standards. Attempts were implemented to remedy this situation by the various committees within IEC to effect a complete revision of their Standards. There is also more co-operation between IEC and CENELEC so that eventually their respective Standards will fall into line with one another. With regard to the EU, explosion protected apparatus is normally constructed in accordance with national and harmonised standards. IEC standards, however, have tended not to be used for this purpose but, because of the trend towards global harmonisation of standards, in which the IEC has an important role, this situation is set to change. To fuel this change there have been instances where manufacturers have been requested by larger users of explosion protected apparatus to have such apparatus constructed and certified to the IEC Standards. The certification of such apparatus has created difficulties for the manufacturers and, in one instance, has led to the manufacturer issuing a ‘self declaration’ for apparatus they have manufactured to a particular IEC Standard.

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An IEC Standard which has become more widely accepted is IEC 79-17 - it is now a British Standard, BS EN60079-l7. This Standard comprises a series of Tables for the inspection of the various methods of explosion protection. Table 1 is an inspection schedule which lists the areas to be inspected for the types of apparatus Ex d, Ex e and Ex N. Table 2 and Table 3 are schedules for the inspection of IS apparatus and Pressurised Ex p apparatus respectively. These Tables are illustrated at the end of this section. For each type of explosion protection, three grades of inspection are specified which are ‘visual’, ‘close’ and ‘detailed’ and defined as follows: Visual: An inspection which identifies, without the use of access equipment or tools, those defects,

e.g. missing bolts, which will be apparent to the eye. Close: An inspection which encompasses those aspects covered by a Visual Inspection and, in

addition, identifies those defects, e.g. loose bolts, which will be apparent only by the use of access equipment, e.g. step ladders ( where necessary), and tools. Close inspections do not normally require the enclosure to be opened, or the equipment to be de-energised.

Detailed: An inspection which encompasses those aspects covered by a Close Inspection and, in

addition, identifies those defects, e.g. loose termination’s, which will only be apparent by opening the enclosure, and/or using, where necessary, tools and test equipment.

Inspection schedules are, therefore, a means by which electrical installations may be systematically assessed for correct installation and also the effects of environmental conditions such as water, ambient temperature, vibration etc. Documentation Prior to the implementation of an inspection / maintenance programme it is essential that all necessary documentation is available. These will include hazardous area drawings of the plant, and a complete inventory of all hazardous area equipment installed in the plant including their location in the plant and up-to-date Records of all previous Inspections and Maintenance tasks carried out. It is also vitally important that the Certification Documents for each item of explosion protected apparatus are available so that, for example, clarification of any ‘Special Installation Conditions’ may be verified at a later date. The maintenance of comprehensive records is thus an essential requirement for the safe operation of electrical equipment in hazardous areas. Experience has shown that modifications to existing hazardous area equipment, and also the installation of additional hazardous area equipment, has occurred in hazardous area installations without these actions being recorded in the relevant documentation.

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Inspections types Three types of inspection are specified in BS EN60079-17. These are:

a) initial inspection; b) periodic inspection; c) sample inspection.

An installation, including its systems and apparatus, should be subjected to an ‘initial inspection’ before being brought into service to establish that the types of protection selected, and their method of installation are suitable. The grade of inspection shall be ‘detailed’ in accordance with Tables 1, 2 and 3 of BS EN60079-17. Thereafter, ‘periodic inspections’ should be implemented to verify that the installation is being maintained in an appropriate condition for continued use in the hazardous area. The grade of inspection for ‘periodic inspections’ may be ‘visual’ or ‘close’ and should be carried out at regular intervals, the frequency of which will be influenced by the environmental conditions. Depending on the outcome of a ‘visual/close inspection’, it may be necessary to carry out a further ‘detailed inspection’. Experience gained in similar situations with regard to apparatus, plants and environments may be used to establish the inspection programme. Factors having an influence on the frequency and grade of ‘periodic inspections’ are:

a) type of apparatus; b) manufacturers recommendations; c) environmental conditions; d) Zone of use; e) results of previous inspections

It is recommended that, however, the interval between ‘periodic inspections’ does not exceed three years. Interim ‘sample inspections’ may be implemented to either support or modify the frequency of ‘periodic inspections’ and may be of a grade ‘Visual’ or ‘Close’. The flowchart overleaf illustrates how a typical maintenance programme may be established and how the various grades of inspection, i.e. ‘visual’, ‘close’ or ‘detailed’, may be applied during the various types of inspection, i.e. ‘initial’, ‘periodic’ or ‘sample’. Consideration is also given to frequency of periodic inspections. Note: * I.C. I.C. appearing in the flowchart below infers that electrical equipment contains components

which are ignition capable in normal operation. Typical components are switches, contactors, relays etc. which produce ignition capable arcs or sparks at their contacts, and, for example, resistors which may produce excessive surface temperatures.

Inspection Schedules The inspection schedules illustrated in Tables 1, 2 and 3 relate to the methods of protection types ‘d’, ‘e’, ‘n’; ‘i’ and ‘p’ respectively.

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Typical inspection procedure for periodic inspections

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BS EN60079-17: Table 1: Inspection Schedule for Ex‘d’, Ex‘e’, and Ex‘n’ Installations (D = Detailed, C = Close, V = Visual)

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BS EN60079-17: Table 3: Inspection Schedule for Ex ‘p’ Installations (pressurised or continuous dilution)

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BS EN60079-17: Table 2: Inspection Schedule for Ex ‘i’ Installations

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Unit 12: Sources of ignition Objectives: On completion of this unit, ‘Sources of ignition’, you should know: a) typical everyday sources of ignition in the workplace; b) lesser known sources of ignition;

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Sources of ignition Electrical sparks Electrical sparks are caused primarily by the opening and closing of contacts, for example, electrical switches, contactors and relays. To ignite a flammable mixture consisting of hydrogen and air requires only 20 µJ, the energy produced as a result of a break of 0.1 mS duration in a circuit carrying 20 mA at 10 V. From this perspective, it is clear that for devices such as these to operate safely in an hazardous area requires them to be installed in, for example, a flameproof enclosure. The voltage level has an influence on how incendive a spark will be. Flammable gases and vapours are more readily ignited at high voltages than low voltages, and is basically why IS circuits are seldom designed for use above 30 V. The use of electrical test instruments, typically voltmeters and insulation resistance testers etc., are a potential source of electrical sparks. These instruments should only be used under controlled circumstances, i.e. under the control of a work permit and tests to ensure gas free conditions. Hot surfaces The flow of current through, for example, the windings of an electric motor invariably produces heat which will raise the surface temperature of the motor. If the motor is excessively overloaded and the thermal overload device in the starter is incorrectly set, the surface temperature of the motor may well exceed it’s T-rating. Overheating can also be caused by blockage of the cooling fan intake, damaged cooling fan, or collapse of a bearing due to lack of lubrication. The latter can dramatically raise the surface temperature locally to a ‘blue heat’ state which equates to a temperature around 430 °C which is more than capable of igniting a flammable gas or vapour. Other sources of heat are process pipes and machinery, combustion engine manifolds and exhaust pipes, and light bulbs. Batteries Batteries whatever their size are a potential source of ignition as they will produce incendive sparks if their terminals are short-circuited. Current of the order 1000 A can be generated if the terminals of automotive batteries are short-circuited. There is also the added complication that during charging of lead-acid batteries, hydrogen and oxygen are released. This requires well ventilated battery rooms. The certification of portable instruments may only allow their use in hazardous areas if powered by low-power batteries. High-power batteries must not be used unless permitted by the manufacturer. Replacement of batteries must only be carried out in a non-hazardous area.

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Friction The abrasive wheels of portable grinding machines are more than capable of producing incendive sparks, and hot surfaces locally at the point of contact by the abrasive wheel. Drilling using portable tools can also generate heat between the drill bit and the workpiece. Power tools, of course, must not be used in hazardous areas, unless used under strictly controlled conditions, because they are themselves sources of ignition. Static electricity Static electricity is normally caused by two insulating materials rubbing together. The loosely held electrons in the atoms of one material are detached and transferred to the other material, so that the material which loses electrons becomes positively charged, and the other material which gains electrons becomes negatively charged. This condition may remain for some time because the materials are insulators and do not offer a conductive return path for the electrons. Nylon clothing removed from the body can generate enough static electricity to ignite a flammable gas or vapour, and there are instances on record of this occurring. Plastic explosion protected enclosures normally carry the warning that they should be cleaned using a damp cloth to avoid generation of static electricity. The movement of fluids can also generate electrostatic charges, and up to 5000 V can be generated at the nozzle of an aerosol canister. Similarly, 10000 V or more can be generated at the nozzle of high-pressure steam cleaning equipment. Bonding and earthing of aircraft during refuelling prevents the build up of electrostatic charges which might otherwise cause the aviation fuel vapour to ignite. Lightning Lightning is a type of static electricity caused by the movement of clouds. Air between clouds, or between clouds and earth, acts as an insulator allowing the charges to build up, and the result is that very high voltages are generated. Once the voltage reaches a critical point, breakdown of the air occurs and the energy is released suddenly in the form of a lighting strike. Lightning strikes will be readily discharged to earth by the normal metal construction of an installation, but flammable gases or vapours can be ignited by lightning. Impact The combination of rusty iron or steel, aluminium and impact between the two is a likely source of ignition, known as thermite action, which can produce sparks capable of igniting a flammable gas or vapour. The use of aluminium ladders in hazardous areas should therefore be avoided. The use of aluminium paint in hazardous areas also requires caution.

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Pyrophoric reaction Hydrogen sulphide (H2S), or other sulphide compounds passing through iron pipes, reacts with the iron of the pipe to produce iron sulphide. Iron sulphide when exposed to air very quickly oxidises and will reach temperatures capable of igniting a flammable gas or vapour. This phenomenon is known as pyrophoric reaction and can be prevented by soaking the iron sulphide with water or prevent its contact with air. Radio frequency The increase in the use of mobile telephones, which operate at high frequencies, has caused some concern. Such concern was expressed by a major oil company in 1993 about the risk of using mobile telephones in petrol stations. Petrol stations have Zone 1 areas around the pumps due to the presence of petrol vapour, and the energy transmitted by a mobile phone, if used in these areas, could be picked up by metalwork in the area which, acting as an aerial, could produce a spark of sufficient energy to ignite the petrol vapour. Other sources of radio frequency are of course radio and television transmitters and radar installations. With regard to radar installations, concern was expressed about the possible ignition of flammable gases at the St. Fergus Gas Terminal in the North East of Scotland by radar transmissions from the nearby radar installation at Crimond. Vibration Vibration is undesirable since it causes premature deterioration of equipment if allowed to persist. Typical examples are increased wear in bearings, loosening of electrical connections, etc. Vibration has also been known to cause metal fatigue of the copper sheath and conductors of MICC cable due to work hardening.

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Unit 13: Induction to Competence Validation Testing Objectives: On completion of this unit ‘Induction to Competence Validation Testing’, you should know: a) the procedures and competence standards for EX01: the Preparation and Installation of Ex d, Ex e

and Ex N apparatus; b) the procedures and competence standards for EX02: the Inspection & Maintenance of Ex d, Ex e and

Ex N apparatus; c) the procedures and competence standards for EX03: the Preparation and Installation of an

Ex i system and associated apparatus; d) the procedures and competence standards for EX04: the Inspection & Maintenance of an Ex i system

and associated apparatus.

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Induction to competence validation testing You are required to achieve the following competencies for each Unit of Assessment Competence Validation Tests EX01, EX02, EX03 and EX04

Off-site preparation Implement, with reference to Assessment Workpacks, all off-site procedures to include selection of materials, apparatus, equipment and tools. Safe working practices to be adhered to at all times and include the mandatory use of Personal Protective Equipment in the designated areas.

Permit-to-work and safe electrical isolation

Complete relevant section of permit-to-work in accordance with procedures detailed in Unit 14.

On-site preparation Implement, with reference to Assessment Workpacks, all

on-site procedures to achieve the competence standard required for Installation, Inspection and Maintenance relative to Units EX01, EX02, EX03 and EX04.

EX01 Preparation and Installation of EEx d, e & N Apparatus Section A Preparation and Safe Isolation of the electrical circuit

Correctly locate electrical supply source and safely isolate installation under the control of a permit-to-work system

Section B Installation comprising Ex d, Ex e and Ex N apparatus

a) Inspect suitability of pre-fixed apparatus, cables and glands.

b) Install appropriate cables and glands in a manner which will maintain the integrity of the pre-fixed apparatus.

c) Carry out appropriate electrical (instrument) tests after ensuring appropriate safeguards are implemented.

d) Fit apparatus covers and live-test installation.

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EX02 Inspection & Maintenance of EEx d, e & N Apparatus Sections A, B & C Inspection of apparatus and environment

With reference to Table 1 of BS EN60079-17:

a) Identify and record five Visual faults.

b) Identify and record five Close faults.

c) Identify and record five Detailed faults.

d) Compile a report of the faults found and specify the remedial action necessary to return the installation to specification.

Section D Safe Isolation of the electrical circuit. (Prior to Sections C & E)

Correctly locate electrical supply source and safely isolate installation under the control of a permit-to-work system.

Section E Maintenance of apparatus

a) From a list of fifteen faults, locate and correct each of the specified faults to restore integrity of apparatus to operational condition.

b) Compile a report of actions carried out.

EX03 Preparation and Installation of EEx i Apparatus Section A Installation comprising Ex i apparatus

a) Inspect suitability of pre-fixed apparatus, cables and glands.

b) Install appropriate cables, glands and safety interfaces in a manner which will maintain apparatus integrity.

c) Carry out appropriate electrical (instrument) tests after ensuring appropriate safeguards are implemented.

d) Fit apparatus covers and live-test installation.

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EX04 Inspection & Maintenance of EEx i Apparatus Sections A, B & C Inspection of apparatus and environment

With reference to Table 2 of BS EN60079-17:

a) Identify and record five Visual faults.

b) Identify and record five Close faults.

c) Identify and record five Detailed faults.

d) Compile a report of the faults found and specify the remedial action necessary to return the installation to specification.

Section D Safe Isolation of the electrical circuit. (Prior to Sections C & E)

Correctly locate electrical supply source and safely isolate installation under the control of a permit-to-work system.

Section E Maintenance of apparatus a) From a list of twelve faults, locate and correct each of the

specified faults to restore integrity of apparatus to operational condition.

b) Compile a report of actions carried out.

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BS EN60079-17: Table 1: Inspection Schedule for Ex‘d’, Ex‘e’, and Ex‘n’ Installations (D = Detailed, C = Close, V = Visual)

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BS EN60079-17: Table 2: Inspection Schedule for Ex ‘i’ Installations

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Unit 14: Permit to Work System and Safe Isolation Objectives: On completion of this unit, ‘Permit to Work and Safe Isolation’, you should know: a) the procedure to complete a ‘permit to work’ to enable safe completion of tests EX01, EX02, EX03

and EX04; b) the procedures to identify, from drawings, the location of protection and control devices for tests

EX01, EX02, EX03 and EX04; c) the procedure to safely isolate and secure isolation of electrical circuits and apparatus for tests EX01,

EX02, EX03 and EX04.

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Permit-to-work and safe isolation Candidates attending the 5-day CompEx course are required to carry out four practical assessments in the simulated hazardous areas. During these assessments, candidates must demonstrate their ability to work safely by ensuring that all precautions are taken to prevent ignition of a flammable gas which, for the purpose of the assessments, it is assumed may be present at any time. Work permit In order to ensure that safety is maintained, candidates must operate within the control of a work permit - a sample is shown overleaf - which must be requested from the Assessor/Authorised Person. In association with the work permit, a gas-free certificate must be endorsed by the Assessor/Authorised Person at all instances when, for example, a particular action is likely to produce a source of ignition. Such situations occur when electrical test instruments and/or portable electric tools are used. Procedures for CVT’s EX01, EX02, EX03 & EX04 Isolation

Candidates are required to: a) Request a ‘work permit’. b) Complete parts 1, 2 and 3 of the ‘Work Permit’ and obtain ‘Authorisation to Isolate’ the

electrical circuit for the respective workbay.

Note: PPE in the designated areas is mandatory c) Identify the ‘point of isolation’ plant reference No. and apparatus ID for ‘zero voltage

testing’. d) Isolate the electrical circuit and secure using two locks (candidate + assessor’s locks) and

warning label. e) Request ‘gas-free certificate’. f) Select appropriate test instrument, carry out ‘zero voltage test’ and confirm result on part 3

of ‘work permit’.

(Note: instrument must be proved before and after use) g) Obtain approval to proceed with work (part 4 of work permit). Note: Step ‘d’ does not apply to EX03 since isolation is achieved by switching off and

removal of keys in the fire and gas panel.

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Final instrument testing

h) Request endorsement of ‘gas-free certificate’ prior to final instrument testing (part 5 of work permit).

i) Prepare installation for live testing and clear worksite (part 6 of work permit).

De-isolation

j) Obtain authorisation to de-isolate (part 6 of work permit). k) Confirm de-isolation is complete (part 6 of work permit). l) Obtain cancellation of work permit (part 7 of work permit).

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Permit To Work 1. Work details

EX01 EX02 EX03 EX04 Date: Job description _______________________________________________________________ 2. Safety requirements - Personal Protective Equipment - (Tick items)

Coveralls Footwear Eye protection Hard hat Gloves

3. Isolation

Gas free test required Yes No

Gas free certificate endorsed Yes

Authorisation to isolate ___________________________ (Authorised person)

Equipment has been isolated at; __________________________________________

and checked for zero voltage at __________________________________________

Isolation complete & zero voltage confirmed by candidate _________________________________ (Candidate)

4. Authorisation / acceptance I hereby authorise _________________ to proceed with work ___________________

(candidate) (Authorised person) I understand and accept responsibility for the work ___________________ (Candidate) 5. Final instrument testing (EX01 & EX03 only)

Gas free test required Yes No

Gas free certificate endorsed Yes

6. Clearance / de-isolation

I hereby declare the above work has been completed and work site cleared ______________________ (Candidate)

I hereby authorise de-isolation _______________________(Authorised person)

De-isolation complete _______________________ (Candidate) 7. Cancellation All work detailed above is completed and ‘Permit-to-Work’ is cancelled________________ (Authorised person)

Gas Test Certificate This certificate confirms that the workbay nominated below in the Ex Training Facility has been

tested and deemed to be free of flammable gases for only the specific task(s) authorised below.

Work details (tick box)

EX01 EX02 EX03 EX04

Location

Workbay No.

Authorised by _____________________ (signed) Date __________

Action to be authorised Authorised person (signed)

Zero voltage test (isolation)

Use of portable heat gun

Final ‘instrument’ circuit testing

Appendix 1: Data for flammable materials

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Data for flammable materials BS 5345 : Part 1 : 1989 Section five

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Data for flammable materials (continued) BS 5345 : Part 1 : 1989 Section five

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Data for flammable materials (continued) BS 5345 : Part 1 : 1989 Section five

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Data for flammable materials (continued) BS 5345 : Part 1 : 1989 Section five

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Data for flammable materials (continued) BS 5345 : Part 1 : 1989 Section five

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Data for flammable materials (continued) BS 5345 : Part 1 : 1989 Section five

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Data for flammable materials (continued) BS 5345 : Part 1 : 1989 Section five

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Appendix 2: Self assessment project

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Self assessment project 1) Complete the following table:

Flammable material

Ignition Temperature

°C

Temperature Class

Apparatus Group

Butane

Hydrogen

Methane

Propane

2) The external surface temperature of an Ex d enclosure is 260 °C based on an ambient temperature of

40 °C. What T-rating will be marked on the enclosure and will it be suitable for use in Cyclohexane? 3) BS EN60079-14 specifies conditions which require the use of a barrier gland. Consider the

statements in the table below and indicate by answering ‘true’ or ‘false’ when a barrier gland is or is not required.

Situation True False

a) A barrier gland is always required when an enclosure contains a source of ignition.

b) A barrier gland is always required when a IIC gas is the hazard.

c) A barrier gland is always required when an enclosure has a volume greater than 2 litres, contains a source of ignition and is installed in Zone 1.

d) A barrier gland is always required when enclosures are installed in Zone 1.

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4) With reference to BS EN60079-17, Table 1: Inspection Schedule for Ex d, Ex e and Ex n apparatus, familiarise yourself with the types of faults that should be eliminated for Apparatus, Installation and Environment. Summarise your findings.

5) With reference to BS EN60079-17, Table 2: Inspection Schedule for Ex i apparatus, study the types

of faults to be eliminated for Apparatus, Installation and Environment. Summarise your findings.

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6) A terminal box has a certification label with the following information:

a) EEx e b) EN50019 c) II d) T6 e) Certificate No. 82289X f) BASEEFA g) Serial No. 90415/94 h) Max. current density amperes per sq.mm = 5 A i) Amperes per pole - 12.5 A

State what each of the above represent.

a) b) c) d) e) f) g) h) i)

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7) The following information was specified on the certification of an item of electrical apparatus.

a) EEx d b) Zone 1 & Zone 2 c) II d) BS 550l Part 5 EN50018 e) Ex 83A1216U f) IP65 The above information represents: a) b) c) d) e) f)

8) A junction box has the following information on its certification plate.

a) BS 4683 Part 3 b) Ex N II T6 c) Ex 83053X d) Max. volts 600 VAC e) Ser. No. 2859 f) Terminal rating: Type MK 6/6 Qty 1. Max. A31, 440 Max. V What does the above information represent? a) b) c) d) e) f)

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9) The certification label of a terminal box has the following information.

a) Type TB10 b) BS 4683 Part 4 c) Ex e II T6 d) BASEEFA Ex 77160/B e) Enclosure factor: 416 f) Max. circuit voltage: 415 at 400 A The above information represents: a) b) c) d) e) f)

10) A junction box certification label has the following information.

a) EEx e II T6 b) Cert. No. 84B3299X c) BS 5501 Part 6 d) Load limit 600 A The above information represents: a) b) c) d)

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11) The following information was marked on a junction box.

a) EJB.Tl0-01059 b) INIEX 90C 103.834 c) EEx d IIB T6 d) EN50018 e) IP66 What does the above information represent? a) b) c) d) e)

12) A mercury vapour bulkhead luminaire has the following information on its certification label.

a) BASEEFA No. Ex 76084/B b) Ex N II T5 c) Ta 40 °C Max. d) BS 4533-2-1 Restricted breathing e) 240 Volts, 50 Hz f) 70 W H.P Mercury g) Type VL 14/70 h) IP67 What does the above information represent? a) b) c) d) e f) g) h)

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13) The certification label of a bulkhead luminaire provides the following information.

a) Ex d b) Zone 1 & Zone 2 c) Groups II & III d) BS 229 e) FLP 3745 f) T5 max to BS 4683 Part l g) 250 V, 60 W What does the above information represent? a) b) c) d) e) f) g)

14) A fluorescent luminaire certification label provides the following information.

a) PTB b) EEx edq c) II T4 d) EN50019 (VDE 0l70/0171) e) No.89B4321X What does the above information represent? a) b) c) d) e)

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15) An Increased Safety motor certification label gives the following information.

a) Ex e b) tE T3 - 10 S c) tE T2 - 10.5 S d) tE T1 - 10.5 S e) IA/IN = 8.42 f) IEC79-7 g) BS4683/4 h) BASEEFA Ex No.79149X What does the above information represent? a) b) c) d) e) f) g) h)

16) An explosion protected item of apparatus is marked with following information.

a) Ex s II T6 b) BASEEFA 77224 c) BS 4683 Part 4 d) 0.l Ampere - up to l2 V Explain the terms: a) b) c) d)

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17) A zener barrier interface device is marked with the following information.

a) Ex [ia] IIC b) BASEEFA Ex 74442/B/S c) SFA 3012 What does the above information represent? a) b) c)

18) An item of explosion protection is marked with the following information.

a) EEx ia b) IIC c) T4 d) BASEEFA Ex 90C2345 e) U max in = 28.5 V f) I max in = 30 mA g) W max in = 1.3 W h) L eq = 0 i) C eq = 0.l5 µF What does the above information represent? a) b) c) d) e) f) g) h) i)

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19) A shunt-diode safety barrier is marked as follows.

a) 28 V, 300 Ω b) BASEEFA Ex832452 c) [EEx ia] IIC d) 250 Um e) 28 Uz f) I max out: 93 mA What does the above information represent? a) b) c) d) e) f)

20) The following information was specified on a Galvanic interface.

a) [EEx ia] IIC b) BASEEFA Ex86B2 133 c) Max. ambient temp. 60°C d) Um: 250V e) U Max out: ≤ 28 V f) I Max. out: ≤ 93 mA g) d.c. isolator: 4/20 mA

Explain what each of the above represent. a) b) c) d) e) f)

g)

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Appendix 3: Supplement to gland selection procedure

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Supplementary notes for the selection of flameproof cable glands The selection procedure for flameproof cable glands is given in Unit 10 of this manual, in the Code of Practice BS 5345: Part 3: 1989, and also in BS EN60079-14 which supersedes BS 5345. BS 5345 for the time being, however, remains current. Gland selection considerations Q - What is a barrier gland, stopper box or sealing chamber? A - Devices which utilise compound to occupy the spaces between the individual insulated

conductors of a cable to prevent the passage of the hot gasses produced by an explosion within a flameproof enclosure.

Q - When should a suitable sealing device or barrier gland be used? A - A sealing device or barrier gland will be used where:

1) A cable is not thermoplastic, thermosetting or elastomeric, is not substantially compact and circular and/or does not have extruded bedding. Cables which are free of these difficulties will be manufactured to BS 5308, BS 5467, BS 6116, BS 6346 or BS 6883.

2) Cables meet the constructional requirements in the standards except that they are

manufactured from a material which is susceptible to “cold flow”. 3) The gas group is IIC and the enclosure, which is located in Zone 1 or Zone 2,

contains components which are a source of ignition. 4) The gas group is IIA or IIB and the enclosure, which is located in Zone 1 and contains

components that are a source of ignition, has a volume greater than 2 litres.

Note: If a flameproof enclosure contains non-sparking components, typically terminals, then a barrier gland may not be necessary, but this should be verified by consultation with the manufacturer.

Q - What is the condition referred to as “cold flow” with regard to cable insulation and by

what means may this difficulty be overcome? A - The pressure applied on the cable insulation by the seals of a gland when it has been

tightened in accordance with the manufacturers instructions produces an indentation on the cable insulation. The insulation effectively “flows” away from the point of pressure to leave an ineffective seal which could allow the free passage of hot gasses produced by an explosion within the enclosure.

Attention to the manufacturer’s cable specification during the selection process may reveal if “cold flow” is a possibility, otherwise consultation with the manufacturer will be necessary. If “cold flow” cannot be avoided, appropriate glands which accommodate this difficulty should be selected.

Questionnaire - selection of cable glands With reference to diagrams (a), (b) and (c) overleaf specify the type of cable glands necessary to maintain the integrity of the flameproof enclosures for the conditions detailed in the four questions below.

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Note: Assume for each question that filled cables manufactured to recognised standards are used and that sparking components are installed except where enclosures have separate terminal chambers.

1) For an enclosure of the type illustrated in figure (a), gas group IIB, with a volume greater than 2 litres

and installed in Zone 2.

Type of gland required? _______________________________ 2) For an enclosure of the type illustrated in figure (b), gas group IIB, with a volume greater than 2 litres

and installed in Zone 1.

Type of gland required? _______________________________ 3) For an enclosure of the type illustrated in figure (b), gas group IIC, with a volume less than 2 litres

and installed in Zone 2.

Type of gland required? _______________________________ 4) For an enclosure of the type illustrated in figure (c), gas group IIC, with a volume greater than 2 litres

and installed in Zone 2.

Type of gland required? _______________________________

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Enclosure entry methods

a) Indirect entry EEx d enclosure

b) Direct entry EEx d enclosure

c) EEx d enclosure with indirect entry EEx e terminal chamber

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