31.25 kbit/s Intrinsically Safe Systems Application Guide · 31.25 kbit/s Intrinsically Safe...

Fieldbus Foundation. . . d e d i c a t e d t o a s i n g l e i n t e r n a t i o n a l f i e l d b u s

31.25 kbit/s IntrinsicallySafe SystemsApplication Guide

FOUNDATION™ fieldbus

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FOUNDATION Fieldbus Application Guide31.25 kbit/s Intrinsically Safe Systems

AG-163 Revision 1.0

This application guide has been prepared to aid understanding of the application considerationsof the FOUNDATION fieldbus.

The booklet begins with a brief overview of 31.25 kbit/s intrinsically safe systems and theirimportant parameters, followed by an outline of the specifications of devices and otherrelated equipment.

The main portion of the booklet provides guidance on certifying equipment for use in thesesystems, and offers advice on sound design and installation practices.

I sincerely hope that this information proves useful to you. Please contact the Fieldbus Foundationif you need additional information about this exciting new technology.

David A. GlanzerQuality Director

For additional information please contact:

Fieldbus Foundation9390 Research Boulevard

Suite II-250Austin, TX 78759

USA

Voice: 512 794 8890Fax: 512 794 8893

Visit our World Wide Web Site:http://www.fieldbus.org

DISCLAIMER OF WARRANTIES

This document is provided on an “as is” basis and may be subject to future additions, modifications, or corrections. The FieldbusFoundation hereby disclaims all warranties of any kind, express or implied, including any warranty of merchantability or fitness fora particular purpose, for this document. In no event will the Fieldbus Foundation be responsible for any loss or damage arisingout of or resulting from any defect, error or omission in this document or from anyone’s use of or reliance on this document.

Preface

© 1996 Fieldbus Foundation, Austin, Texas. All rights reserved.

1


1.0 INTRODUCTION AND SCOPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3

2.0 REFERENCE DOCUMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .4

3.0 GLOSSARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5

4.0 PRINCIPLES OF INTRINSIC SAFETY APPLIED TO FOUNDATION FIELDBUS . . . . . . . . . . . . . . . . . . . . .7

4.1 Classifying Hazardous Areas and Flammable Atmospheres . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7

4.2 Ignition Curves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9

5.0 CERTIFYING EQUIPMENT FOR INSTALLATION IN HAZARDOUS AREAS . . . . . . . . . . . . . . . . . . . . .12

5.1 Bus Powered Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .13

5.2 Separately Powered Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

5.3 Fieldbus Terminators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

6.0 CERTIFYING ASSOCIATED EQUIPMENT FOR INSTALLATION IN THE SAFE AREA . . . . . . . . . . . . .16

6.1 Fieldbus Power Supplies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .18

6.2 I.S. Barriers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .19

6.3 I.S. Galvanic Isolators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .21

6.4 Requirements for Other Safe Area Equipment Connected to the Fieldbus . . . . . . . . . . . . . . . .22

7.0 ANALYZING THE SAFETY OF A FOUNDATION I.S. FIELDBUS SYSTEM . . . . . . . . . . . . . . . . . . . . . . . .23

8.0 SYSTEM CABLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .27

Table of Contents

2


Introduction

1.0 INTRODUCTION AND SCOPE

This application note relates only to FOUNDATION

fieldbus equipment and systems operating at a31.25 kbit/s data rate and conforming to the specifi-cations and recommended parameters in theFOUNDATION 31.25 kbit/s Physical Layer ProfileSpecification, FF-816 Revision 1.0 May 13,1996.The advice given is not intended to apply to otherfieldbus systems operating to different device specifications or data rates, and could lead to apotentially unsafe system if used in that way.

The intention of this application note is fourfold:

• Explain the basic concepts of intrinsically safe systems, and their important parameters;

• Outline the specifications of devices and other relatedequipment for use on FOUNDATION 31.25 kbit/s intrinsically safe fieldbus systems in hazardous areas;

• Provide guidance on certifying equipment for use in these systems;

• Offer advice to installers and users of these systems on good design and installation practice.

3

2.0 REFERENCE DOCUMENTS

Fieldbus Standards

IEC 1158-2 : 1993 Fieldbus Standard for Use in IndustrialControl Systems.

Part 2 : Physical Layer Specification and Service Definition.

ISA-S50.02-1992 Fieldbus Standard for Use in IndustrialControl Systems.

Part 2: Physical Layer Specification and Service Definition.

ISA dS50.02, Part 2 Draft Standard Fieldbus Standard forUse in Industrial Control Systems.

Part 2: Physical Layer Specification and Service Definition,Amendment to Clause 22 (Formerly Clauses 11 and 24).

FOUNDATION Specification

FOUNDATION Specification : 31.25 kbit/s Physical LayerProfile

DOCUMENT FF-816 REVISION: 1.0 DATE: May 13, 1996

Intrinsic Safety Standards

IEC 79-4 : 1975 (Second edition) Electrical Apparatusfor Explosive Gas Atmospheres.

Part 4 : Method of Test for Ignition Temperature.

IEC 79-11 : 1991 (Third edition) Electrical Apparatus forExplosive Gas Atmospheres.

Part 11 : Intrinsic Safety “i”.

CENELEC EN 50014 : 1992 Electrical Apparatus forPotentially Explosive Atmospheres - GeneralRequirements.

CENELEC EN 50 020 : 1977 (First edition) ElectricalApparatus for Potentially Explosive Atmospheres -Intrinsic Safety “i”.

CENELEC EN 50020 : 1994 (Second edition) ElectricalApparatus for Potentially Explosive Atmospheres -Intrinsic Safety “i”.

CENELEC EN 50 039 : 1982 Electrical Apparatus forPotentially Explosive Atmospheres - Specification forIntrinsically Safe Electrical Systems “i”.

Factory Mutual Approval Standard. Class Number 3610,October 1988. Intrinsically Safe Apparatus andAssociated Apparatus for Use in Class I, II and III,Division 1 Hazardous (Classified) Locations.

Codes of Installation Practice

IEC Publication 79-14 : 1984 (First edition) - IEC Report.Electrical Apparatus for Explosive Gas Atmospheres.

Part 14 : Electrical Installations in Explosive Gas Atmospheres (other than mines).

British Standard BS 5345 : Part 4 : 1977. Code ofPractice for Selection, Installation and Maintenance ofElectrical Apparatus for Use in Potentially ExplosiveAtmospheres (other than mining applications or explo-sive processing and manufacture).

Part 4 : Installation and Maintenance Requirements for Electrical Apparatus with Type of Protection “i”. Intrinsically Safe Electrical Apparatus and Systems.

ISA RP12.6, Recommended Practice, 1987. Installationof Intrinsically Safe Systems for Hazardous (Classified)Locations.


1


4

Reference Documents


Introduction

5

3.0 GLOSSARY

For the purpose of this Application Note the followingdefinitions (taken largely from CENELEC standards EN 50014 and EN 50020) apply:

associated apparatus Electrical apparatus whichcontains both intrinsically safe circuits and non-intrin-sically safe circuits and is constructed so that thenon-intrinsically safe circuits cannot adversely affectthe intrinsically safe circuits.

NOTE – Associated apparatus may be either:

a) electrical apparatus which has an alternative type of protection for use in the appropriate explosive gas atmosphere or:

b) electrical apparatus not so protected and which therefore shall not be used within an explosive gas atmosphere: for example, a recorder which is not itself in an explosive gas atmosphere, but is connected to a thermocouple situated within an explosive atmosphere where only the recorder input circuit is intrinsically safe.

electrical apparatus Assembly of electrical compo-nents, electrical circuits or parts of electrical circuits nor-mally contained in a single enclosure.

explosive gas atmosphere A mixture with air, underatmospheric conditions, of flammable substances in theform of gas vapor or mist in which after ignition com-bustion spreads throughout the unconsumed mixture.

fault Any defect of any component, separation, insula-tion or connection between components, not defined asinfallible, upon which the intrinsic safety of a circuitdepends.

fuse rating (In) The current rating of a fuse according toits manufacturer's specification.

ignition temperature of an explosive gas atmosphereThe lowest temperature of a heated surface at which,under specified conditions according to IEC 79-4, theignition of a flammable substance in the form of a gas orvapor mixture with air will occur.

infallible component or infallible assembly of com-ponents Component or assembly that is not likely tobecome defective, in service or in storage, in such amanner as to invalidate intrinsic safety.

infallible separation or insulation Separation or insu-lation between electrically conductive parts that is notconsidered as becoming short circuited in service orstorage and therefore will not be considered to fail forthe purposes of fault analysis and the application of thespark test apparatus.

intrinsically safe apparatus Electrical apparatus inwhich all the circuits are intrinsically safe circuits.

intrinsically safe circuit Circuit in which any spark orany thermal effect produced in the conditions specifiedin this standard, which include normal operation andspecified fault conditions, is not capable of causing igni-tion of a given explosive gas atmosphere.

maximum external capacitance (Co) Maximumcapacitance in an intrinsically safe circuit that can beconnected to the connection facilities of the apparatuswithout invalidating intrinsic safety.

maximum external inductance (Lo) Maximum value ofinductance in an intrinsically safe circuit that can beconnected to the connection facilities of the apparatuswithout invalidating intrinsic safety.

maximum external inductance to resistance ratio(Lo/Ro) Ratio of inductance (Lo) to resistance (Ro) of anyexternal circuit that can be connected to the connectionfacilities of the electrical apparatus without invalidatingintrinsic safety.

maximum input current (Ii) Maximum current (peaka.c. or d.c.) that can be applied to the connection facili-ties for intrinsically safe circuits without invalidatingintrinsic safety.

maximum input power (Pi) Maximum input power inan intrinsically safe circuit that can be dissipated withinan apparatus when it is connected to an external sourcewithout invalidating intrinsic safety.

maximum input voltage (Ui) Maximum voltage (peaka.c. or d.c.) that can be applied to the connection facili-ties for intrinsically safe circuits without invalidatingintrinsic safety.

maximum internal capacitance (Ci) Total equivalentinternal capacitance of the apparatus which is consid-ered as appearing across the connection facilities of theapparatus.

maximum internal inductance (Li) Total equivalentinternal inductance of the apparatus which is consid-ered as appearing at the connection facilities of theapparatus.

Glossary


6

Glossary

maximum internal inductance to resistance ratio(Li/Ri) Ratio of inductance (Li) to resistance (Ri) which isconsidered as appearing at the external connectionfacilities of the electrical apparatus.

maximum output current (Io) Maximum current (peaka.c. or d.c.) in an intrinsically safe circuit that can betaken from the connection facilities of the apparatus.

maximum output power (Po) Maximum electricalpower in an intrinsically safe circuit that can be takenfrom the apparatus.

maximum output voltage (Uo) Maximum output volt-age (peak a.c. or d.c.) in an intrinsically safe circuit thatcan appear under open circuit conditions at the connec-tion facilities of the apparatus at any applied voltage upto the maximum voltage, including Um and Ui .

NOTE – where there is more than one applied voltage,the maximum output voltage is that occurring under the most onerous combination of applied voltages.

maximum r.m.s. a.c. or d.c. voltage (Um) Maximumvoltage that can be applied to the non-intrinsically safeconnection facilities of associated apparatus withoutinvalidating intrinsic safety.

NOTE – the value of Um may be different at different sets of connection facilities.

maximum surface temperature The highest tempera-ture which is attained in service under the most adverseconditions (but within the recognized tolerances) by anypart or surface of an electrical apparatus, which wouldbe able to produce an ignition of the surrounding explo-sive atmosphere.

minimum igniting current (MIC) Minimum current inresistive or inductive circuits that causes the ignition ofthe explosive test mixture in the spark test apparatus.

minimum igniting voltage (MIV ) Minimum voltage ofcapacitive circuits that causes the ignition of the explo-sive test mixture in the spark test apparatus.

normal operation Operation of intrinsically safe appa-ratus or associated apparatus such that it conformselectrically and mechanically with the design specifica-tion produced by its manufacturer.

potentially explosive atmosphere An atmospherewhich could become explosive (the danger is a potentialone).

rated value A quantity value assigned generally by themanufacturer, for a specified operating condition of acomponent, device or apparatus.

simple apparatus An electrical component or combi-nation of components of simple construction with welldefined electrical parameters which is compatible withthe intrinsic safety of the circuit in which it is used.

type of protection The specific measures applied toelectrical apparatus to avoid ignition of a surroundingexplosive atmosphere.

4.0 PRINCIPLES OF INTRINSIC SAFETYAPPLIED TO FOUNDATION FIELDBUS

Intrinsic Safety (I.S.) is a method of ensuring the safetyof electrical equipment where flammable materials arepresent. The areas at risk are known as HazardousAreas and the materials that are commonly involvedinclude crude oil and its derivatives, alcohols, naturaland synthetic process gases, metal dusts, carbon dust,flour, starch, grain, fiber and flyings. To protect bothplant and personnel, precautions must be taken toensure that these atmospheres cannot be ignited.

There are several ways this can be achieved: by enclos-ing the electrical equipment in a heavy, robustFlameproof enclosure designed to contain any explo-sion which occurs; by preventing access for the flam-mable atmosphere to the equipment using techniquessuch as sand or oil filling; by encapsulating the equip-ment in an epoxy resin; or by pressurizing the equip-ment so that the flammable atmosphere cannot enter. It is obvious that application of any of these techniquesresults in increased equipment size and weight, anddoes not allow live working when any degree of hazardis present. Some of the techniques also make theequipment completely impossible to service or repair.

I.S. assumes a different approach. The flammableatmosphere is allowed to come in contact with the elec-trical equipment without introducing a potential hazard.This is possible because the system (including both theI.S. apparatus in the hazardous area and the safe areaassociated apparatus directly connected to it) has beendesigned to be incapable of causing an ignition in thatatmosphere, even with faults applied either to it or theinterconnecting cables. The electrical energy available inhazardous area circuits is restricted to a level such thatany sparks or hot surfaces which occur as a result ofelectrical faults are too weak to cause ignition. Thisapproach allows measurements to be made on theequipment (using suitably approved test equipment) andadjustments to be carried out during live operation.

Similarly, equipment can be removed or replaced whilethe system is operating. I.S. can be applied only to lowvoltage, low power equipment (up to a few Watts) but isthe favored approach for instrumentation, where itsoperational benefits are significant. It is also a techniquewhere, unlike most of the alternatives mentioned, thereis a large degree of international standardization. Thismakes it possible for the same equipment and systemsto be certified and installed in most areas of the world.

4.1 Classifying Hazardous Areas and FlammableAtmospheres

Hazardous areas are classified by their degree of hazard(Area Classification) and the gases that are present (GasGroup). Area classification categorizes areas accordingto the probability that an explosive atmosphere is pre-sent, and this dictates whether or not a particular explo-sion protection technique can be used. I.S. is internation-ally recognized as the technique offering the highestdegree of safety and in most countries is specified as theonly one allowed for use in the highest degree of hazard.

Unfortunately there are two different approaches to areaclassification. Most countries have adopted the IEC 79Zone classifications, while the USA and Canada have aDivision classification system. The European CENELECstandards follow the IEC approach. The two systemsare summarized in Table 1, where it can be seen thatthe North American Division 1 encompasses the IECZones 0 and 1, while Division 2 is almost identical toIEC Zone 2. This equivalence is not exact and anydesigner, installer or user of equipment should makehimself familiar with the detailed requirements of thestandards and process. The 1996 US National ElectricalCode recognizes the IEC Zone classification, openingthe way for further adoption of IEC designations in theUSA. It is expected that equivalent changes to theCanadian Electrical Code will follow soon.

The position relating to dust hazards is less standard-ized and general advice cannot easily be given here.Potential users are advised to consult authorities in thecountry where the equipment will be installed.


7

Principles

Table 1 – Area Classifications

IEC & CENELECZone 0: Explosive gas-air mixture continously present,or present for long periods.

Zone 1: Explosive gas-air mixture is likely to occur innormal operation.

Zone 2: Explosive gas-air mixture not likely to occurand, if it occurs, it will exist only for a short time.

USA & CanadaDivision 1: Hazardous concentrations of flammablegases or vapors - or combustible dusts in suspension -continuously, intermittently or periodically presentunder normal operating conditions.

Division 2: Volatile flammable liquids or flammablegases present but normally confined within closed containers or systems from which they can escapeonly under abnormal operating or fault conditions.Combustible dusts not normally in suspension norlikely to be thrown into suspension.


8

Principles

Table 2 – I.S. Equipment Categories

Table 3 – Gas Classifications

The IEC standard defines two categories of I.S. system,dependent upon the number of component or otherfaults (1 or 2) that can be present while the equipment

remains safe; the North American standards define onlya single category of equipment that is safe with up totwo faults introduced, as indicated in Table 2.

Gases are classified according to the spark energyneeded to ignite them. All I.S. equipment is designedand certified as being safe for a particular group ofgases, although in practice most I.S. systems aredesigned to be safe with all the gas groups normally

encountered - while a few employ more power in partic-ular defined environments. Again, there is a differentclassification of gases between IEC and North America,as shown in Table 3.

IEC & CENELECEx ia : Explosion protection maintained with up to twocomponent or other faults. I.S. apparatus may belocated in, and associated apparatus may beconnected into Zone 0, 1 and 2 hazardous areas(Germany requires galvanic isolation and a systemcertificate for Zone 0).

Ex ib : Explosion protection maintained with up to onecomponent or other fault. I.S. apparatus may belocated in, and associated apparatus may beconnected into Zone 1 and 2 hazardous areas.

USA & CanadaOne category only : Safety maintained with up to twocomponent or other faults. I.S. apparatus may belocated in, and associated apparatus may beconnected into Division 1 and 2 hazardous locations.

IEC & CENELECFlammable gases, vapors and mists are classifiedaccording to the spark energy required to ignite the mosteasily ignitable mixture with air. Apparatus is groupedaccording to the gases that it may be used with.

Surface industriesGroup IIC : acetylene moreGroup IIC : hydrogen easilyGroup IIB : ethylene ignitedGroup IIC : propane

DustsUnder consideration

Mining industryGroup 1 : methane (firedamp)

USA & CanadaFlammable gases, vapors and mists and ignitabledusts, fibers and flyings are classified according to thespark energy required to ignite the most easily ignitablemixture with air.

Surface industriesClass I, Group A : acetylene moreClass I, Group B : hydrogen easilyClass I, Group C : ethylene ignitedClass I, Group D : propane

Class II, Group E : metal dustClass II, Group F : carbon dustClass II, Group G : flour, starch, grain

Class III : fibers and flyings

Mining industryUnclassified : methane (firedamp)


9

Principles

Table 4 – Temperature Classification

The Temperature Classification of a piece of hazardousarea equipment is defined by the highest surface tem-perature reached within any part of it when a specifiedamount of power is supplied to it under fault conditions(at an ambient temperature of 40° C unless otherwisestated). Fortunately there is international agreement ontemperature classifications, and these are given in Table4. There is no correlation between the spark energyrequired to ignite a gas mixture and its susceptibility toignition by hot surfaces. For example, hydrogen is easilyignited by spark energy (19 mJ) but requires a surfacetemperature in excess of 560° C to produce ignition. Allgases (with the exception of carbon disulphide) are cov-

ered by a T4 temperature classification, and this is thenormal design level. Fortunately, within I.S. there areinternational agreements between certification bodiesthat allow a T4 classification to be assigned to mostcomponents within equipment without testing. Theseagreements only apply if the supplied power from theassociated safe area apparatus under fault conditions,does not exceed 1.3 W at 40° C (equivalent figures are1.2 W at 60° C and 1.0 W at 80° C ambient). Althoughthis agreement excludes very small components, it cangreatly simplify and shorten the task of certifying I.S.equipment.

Hazardous area apparatus is classified according to the maximumsurface temperature produced under fault conditions at an ambienttemperature of 40° C, or as otherwise specified.

T1

T2

T3

T4

T5

T6

450° C

300° C

200° C

135° C

100° C

85° C

4.2 Ignition CurvesThe energy required to ignite the most easily ignitablemixture of a given gas with air is called the MinimumIgnition Energy of that gas. The current required to sus-tain ignition varies with the voltage level in the circuit.Curves of permitted voltage and current for each gasgroup in a purely resistive circuit are published asIgnition Curves in each of the intrinsic safety standards.They result from experiments over several years and theresults are agreed upon internationally. Typical curves(taken from the CENELEC standard EN 50 020) areshown in Figure 1. A 1.5 factor of safety has to beapplied to the currents shown in these curves when they

are applied in practice. Although the curves cover a con-siderable range of voltages and currents, the choice ofoperating region for an I.S. fieldbus is limited. Thischoice is determined by several factors:• A low open circuit operating voltage to allow a

large permitted system capacitance in the mosthazardous gas group, IIC (North AmericanGroups A & B);

• A short circuit current low enough to allow a usablesystem inductance under similar conditions;

• A peak output power from the safe area equipmentthat will minimize the design and certification prob-lems for equipment vendors.

Principles


10

Figure 1 – Ignition Curves

These factors define an operating region below about30 V, 300 mA and 1.2 W at 60° C, as shown in Figure 2(which includes the required 1.5 factor of safety on theignition curves). Capacitance is treated as a lumpedparameter and its permitted value reduces sharply asthe system voltage is increased. This proves to be astronger defining effect than the reduction of permittedinductance as the current increases. This is because theincreased inductance of long system cables is always

accompanied by an increased series resistance whichreduces its effect. Any item of I.S. associated apparatusnormally has maximum specified values for permittedcapacitance, inductance and inductance-to-resistance(L/R) ratio that may be safely connected to its I.S. termi-nals. Cables may be specified by their L/R ratio, as analternative to a simple inductance parameter, which canmake system inductance a less significant factor.

5A

2

200

100

50

20

1010V 20V 50V 100V

IIC

IIBIIAI

CENELEC standard EN 50020Curves apply to electrical apparatus withcadmium, zinc, magnesium or aluminiumpresent.

Voltage200V 500V

1

500mA

Cur

rent

Principles


11

Figure 2 – Practical Operating Region

300

200

100

00 10

Voltage (V)

Low permittedinductance

PermittedcurrentGroup IIC

PermittedcurrentGroup IIB

1.2Wmatchedpower

Lowpermittedcapacitance

Cur

rent

(m

A)

20 30

Certification

5.0 CERTIFYING EQUIPMENT FORINSTALLATION IN HAZARDOUS AREAS

Equipment of various different types has been certifiedfor hazardous area use over many years. The addedfactor that fieldbus introduces is the need for equipmentfrom different manufacturers to be certified in a compat-ible way for connection onto a common bus. This isessential to the success of fieldbus in hazardous areaapplications, otherwise a common communication stan-dard will not be translated into installed, multi-vendorhazardous area systems. Previously each manufacturerhas specified the safety parameters of their hazardousarea products (together with their own associated safearea equipment or those of a specialist manufacturer) ina way that suited them, without reference to other man-ufacturers or competitors.

This approach is not sufficient for fieldbus. For this reason,the FOUNDATION 31.25 kbit/s Physical Layer ProfileSpecification has established some recommended certifi-cation parameters for communicating devices and acces-sory items. If followed, these will lead to equipment fromdifferent manufacturers being able to connect together onFOUNDATION fieldbus in a demonstrably safe manner.

This profile specification, derived from the IEC/ISAFieldbus Physical Layer Standard IEC 1158-2 : 1993(ISA standard ISA - S50.02 - 1992 has identical Englishtext) Clause 11 and draft Clause 22, defines eight pro-files for communicating fieldbus device types. Of these,four are specified as suitable for connection to an I.S.fieldbus within a hazardous area. These profile types,with their FOUNDATION type designations, are:

Type 111 – standard-power signaling, bus powered, I.S.Type 112 – standard-power signaling, separately

powered, I.S.Type 121 – low-power signaling, bus powered, I.S. Type 122 – low-power signaling, separately powered, I.S.

These types are distinguished from the remaining onesas being suited for hazardous area use by their propertyof introducing no electrical energy onto the fieldbus dur-

ing either reception or transmission of signals. This isbecause their Medium Attachment Unit (MAU) is speci-fied as current sinking at all times; at no time duringoperation does it supply current to the fieldbus.

The remaining device profile types either employ trans-former coupling of signals onto the fieldbus, in which caseenergy is supplied to the bus during half of each transmit-ted bit time, or do not include the internal energy limitingunder fault conditions needed to obtain an I.S. certification.

The I.S. approach is inherently a system concept. Allsources of possible energy into the flammable atmos-phere must be considered and limited. It would be possi-ble for a particular system including transformer coupleddevices to be certified, but not in any generalized way orin an acceptable timescale. The FOUNDATION PhysicalLayer Profile Specification therefore specifies devicesand accessory equipment for use on an I.S. fieldbuswhere the hazardous area equipment is supplied by onesingle source of power (from an I.S. power supply orthrough an I.S. interface of some type - see Section 6).

To achieve compatible device certifications, the PhysicalLayer Profile Specification recommends minimum inputvoltage, current and power levels with which hazardousarea devices should be certified to operate. These are list-ed in Table 5. An "ia" certification (IEC designation, seeTable 2) is recommended since this meets the require-ments for international acceptance and ensures that sys-tems can be analyzed without the need for repeatedtesting at each certification authority. Provided manufac-turers certify devices to comply with at least these mini-mum levels, it should be possible safely and legally toconnect certified devices from different manufacturers tothe same fieldbus.

Also included in Table 5 is a recommended T4 devicetemperature classification. This will be required for oper-ation with the majority of applications. The significanceof the recommended device maximum residual capaci-tance and inductance parameters, also shown in Table5, may not be immediately apparent, but these areexplained more fully in Section 8.


12

Table 5 – Recommended I.S. Parameters For Hazardous Area Devices

ParameterDevice approval voltageDevice approval currentDevice input powerDevice residual capacitanceDevice residual inductanceI.S. classification

Recommended Value24 V min.250 mA min.1.2 W min.< 5 nF< 20 mHEx ia, IIC (gas groups A & B), T4

Certification


13

Figure 3 – I.S. Fieldbus with Bus-Powered Devices

Individual manufacturers can choose to certify theirequipment at figures higher than the minimum levelsshown. There are good reasons to do this for particularapplications, but they should be aware that if the field-bus is powered in a way to take advantage of theincreased specifications (usually by increasing the levelsof short circuit current and maximum transferred powerfrom the safe area) then only equipment certified to thehigher levels can be connected to that bus. Also, themanufacturer should not under-estimate the difficulty ofmaintaining a T4 temperature classification withincreased power levels supplied to the device, or thetime needed to achieve device certification.

Any combination of device profile Types 111, 112, 121and 122 can be connected to a powered I.S. fieldbus.The FOUNDATION Profile Specification defines a fieldbuscapable of operating with both standard-power andlow-power signaling devices present (due to the chosencharacteristics of power supply and terminators – seeSection 4 for more details) and any combination ofthese four types of bus powered and separately pow-ered units.

Any of the device profiles specified for hazardous opera-tion can, as an alternative, be connected to a non-I.S.segment of a fieldbus, or to a totally safe area fieldbus,operating to FOUNDATION specifications. The only

requirement is that the bus must be powered (even forseparately powered Types 112 and 122 devices).

NOTE – As with existing analog installations, care mustbe exercised in the installation and use of I.S. apparatusin a hazardous area if it has previously been connectedinto a non-I.S. system.

5.1 Bus Powered Devices (Profile Types 111 and 121)

These are devices whose only source of electricalpower is the fieldbus itself. A typical system might be asshown in Figure 3. The device MAU performs the dualfunction of regulating power from the fieldbus to thedevice and signaling onto the bus.

For the manufacturer, the major design challenges are tominimize the current drawn by each device – since thetotal current supplied to an I.S. fieldbus is limited (seeSection 4) – and ensuring that only very low levels ofunsuppressed capacitance and inductance can bemeasured external to the device. Recent emphasis onthe immunity aspects of Electromagnetic Compatibility(EMC) makes the fitting of transient suppression com-ponents in the input of each device almost essential, butthese present a problem in minimizing the residual reac-tive elements from an I.S. viewpoint.

Safe Area

I.S. Interface

Hazardous Area

T Th

P

P Fieldbus power supply (Type 131)TerminatorDevice (any profile Type)I.S. certified bus-powered device (Type 111 or 121)I.S. certified terminator

=====

TDDb

Th

D Db Db Db

Certification


14

5.2 Separately Powered Devices (Profile Types 112 and 122)

These device types have one or more sources of electri-cal power in addition to the power supplied from thefieldbus itself. An example of such a device is an analyz-er where the application circuitry requires higher levelsof power than can be obtained from an I.S. fieldbus, ora battery powered hand-held unit. These types of equip-ment, and their power supplies, would be designed touse another form of explosion protection, but the field-bus communication element may still be required toconnect to an I.S. fieldbus. An example system isshown in Figure 4.

The MAU within the device is, in this case, required toprovide power only to the fieldbus communications cir-cuit itself. This is one application where the use of low-power signaling (device Type 122) has an immediateadvantage in reducing the current drawn from the field-bus, but a standard-power signaling Type 112 devicecan be used as an alternative. In either case, the MAUrequirements completely mirror those of the corre-sponding bus powered device profile, and the certifica-tion parameters (Table 5) and operating characteristicsare independent of the powering arrangements.

The major design requirement for I.S. fieldbus devicesincluding separate sources of power is to ensure thecomplete separation of these sources from the fieldbusitself. This will normally involve some form of galvanicisolation to I.S. standards, with the communication sig-nals being transferred across suitable opto-couplers ortransformers. For connection to the FOUNDATION profileI.S. fieldbus this complete separation of power sourcesis an essential requirement.

5.3 Fieldbus Terminators

The FOUNDATION Physical Layer Profile (Section 12.4)defines the recommended I.S. parameters for a termina-tor certified for installation in a hazardous area, as listedin Table 6. All FOUNDATION fieldbus segments require twobus terminators to be present. For an I.S. fieldbus, atleast one of these will be mounted in the hazardousarea (see Figures 3 and 4). Any terminator designed forpossible installation in a hazardous area must be formal-ly certified since it includes a resistive-capacitive circuitwhich cannot be evaluated from standardized referencecurves. The minimum I.S. parameters recommendedin the FOUNDATION Physical Layer Profile Specificationare compatible with those of devices and other equip-ment and, provided these are complied with, will allow

Figure 4 – I.S. Fieldbus with Separately-Powered Devices

Safe Area

I.S. Interface

Hazardous Area

T Th

P D

SeparatePowerSource

Battery Powered

P Fieldbus power supply (Type 131)

TerminatorDevice (any profile Type)I.S. certified separately-powered device (Type 112 or 122)

NOTE: A Type 112 or 122 device will use I.S. in combination with other explosion protection techniques.The external power source or internal battery must be galvanically isolated from the fieldbus MAU.

I.S. certified terminator

=====

TDDs

Th

Certification


15

Table 6 – Recommended I.S. Parameters for Hazardous Area Mounted Terminator

hazardous area fieldbus systems to be constructed.Some certification authorities may assign an equivalentunprotected capacitance to a particular manufacturer’sterminator. In this case, the allocated capacitance valuewill be marked on the unit and should be taken intoaccount when assessing available cable parameters, asoutlined in Section 8.

The FOUNDATION terminator profile is derived from draftClause 22 of the IEC/ISA Fieldbus standard, for reasonsexplained in Section 6.1. Bus terminators with otherimpedance characteristics should not be used onFOUNDATION fieldbus systems, as the transient responseof the system could be adversely affected when differ-ent device profile types are added to the fieldbus.

ParameterMountingGas groupInput voltageInput currentInput power

ValueZone 0 (US Div. 1)IIC (US Groups A & B)24 V min.250 mA min.1.2 W min.

Certification


16

6.0 CERTIFYING ASSOCIATED EQUIPMENT FORINSTALLATION IN THE SAFE AREA

It is clear from the previous section that the only sourceof power to a FOUNDATION hazardous area fieldbus isthat from the I.S. power source installed in the safe area.All safe area mounted equipment connected directly towiring or equipment in the hazardous area that is part ofan I.S. system must be formally certified compatible

with its intended application. Such equipment is classedas Associated Apparatus. In the fieldbus case the powercomes from an I.S. power supply which can be a sepa-rate item of equipment, may be included in an I.S. gal-vanic isolator, or can be formed by a general purposefieldbus power supply used in conjunction with an I.S.barrier. The FOUNDATION Physical Layer ProfileSpecifications for these items include recommended I.S.parameters, as listed in Table 7.

The 24 V maximum output voltage specification in Table7 is determined by two main factors:• The requirement in the IEC/ISA fieldbus standard to

operate with 1900m of a defined (Type "A") cable having 24 Ω /km DC resistance (each conductor). This introduces a 92 Ω cable loop resistance on a 1900m long fieldbus. When this resistance is considered together with

a) the minimum operating voltage specification for field devices (9 V);

b) the rapidly reduced permitted current level allowed by the ignition curves as the voltage rises, and

c) the required restriction of transferred power level (1.2 W max.), a supply voltage below 24 V maximizes the current available to fieldbus devices.

• The rapid decrease in permitted system capacitance as the system voltage increases much above 20 V (This is normally only a practical problem in the most hazardous gas group and is discussed more fully in Section 8).

A plot of the maximum current available to field devicesas the certified voltage of the I.S. power source varies isshown in Figure 5. This confirms that there is no advan-tage to be gained from operating an I.S. fieldbus at volt-ages above 24 V.

The recommended maximum short circuit current andtransferred power specifications from the safe areapower source are chosen to be compatible with theminimum current and power levels recommended in theI.S. device profiles listed in Table 5.

Table 7 – Recommended I.S. Parameters for Fieldbus I.S. Power Sources

ParameterLocation of hazardous area apparatusGas groupOpen circuit output voltageShort circuit output currentMatched output power

ValueZone 0 (US Div. 1)IIC (US Groups A & B)24 V max.250 mA max.1.2 W max.

Certification


17

The current available to hazardous area devices on anI.S. fieldbus also varies with the length of the hazardousarea bus. This is simply due to an increasing DC loss inthe cable resistance as the bus length increases. Figure6 shows a typical characteristic, based on an assump-tion that the I.S. interface is designed for about the opti-mum voltage indicated in Figure 5 and that all the haz-ardous area devices are connected at the far end of thefieldbus.

NOTE – Figures 5 and 6 as based on the followingassumptions:

1. Type “A” cable characteristics, from the IEC/ISA Physical Layer fieldbus standard. This cable is nominal 0.8 mm

2(#18 AWG). Results will be different

with other wire sizes.

2. An I.S. fieldbus with a power supply voltage 3 V below the shown certification voltage. This figure is made up of a 2 V difference between certification and operatingvoltages, plus an additional 1 V peak signal excursion on the bus (which can occur when one bus terminatoris removed).

3. All devices located at the far end of the bus and operating with a minimum device voltage of 9 V.

4. Figure 6 assumes I.S. certification parameters of 22 V, 214 mA and a 19 V power supply voltage.

A practical system may comprise equipment with oper-ating characteristics different from these assumptions.

Figure 5 – Variation Of Available Device Current With I.S. Source Voltage

Bus length = 0

Bus length = 950 m (3117 ft)

Bus length = 1900 m (6234 ft)

I.S. certification voltage (V)

Max

. cur

rent

for

bus-

pow

ered

dev

ices

(mA

)120

100

80

60

40

20

5 10 15 20 25 30


18

Certification

Apparatus connected to the safe area terminals of asso-ciated apparatus does not need to be certified, but mustcomply with certain requirements outlined in Section 6.4.

6.1 Fieldbus Power Supplies

The FOUNDATION Physical Layer Profile Specification(Section 12.1) includes a Type 133 I.S. power supply. Thisunit is specified, including recommended I.S. parametersas given in Table 7, such that it can be connected directlyto an I.S. fieldbus circuit in a hazardous area. In such anapplication it is likely to be associated with an I.S. galvan-ic isolator (carrying communications between safe areaand hazardous area mounted devices, see Section 6.3)designed and certified as providing negligible additionalpower to the hazardous area system.

An alternative application is where the whole fieldbus iscontained in the hazardous area, with no communicationto or from safe area devices. The power supply and onebus terminator are then the only items mounted outsidethe hazardous area. This type of application does notoccur very often.

A Type 133 fieldbus power supply suitable for direct con-nection to the hazardous area wiring of an I.S. systemwill be marked with its safety parameters. If these are notclear and understood, or if there is any doubt regardingthe safety characteristics of an item of fieldbus equip-ment that is part of an I.S. system, then full data shouldbe requested from the manufacturer or his authorizedlocal representative.

NOTE – Only I.S. power supplies with linear outputcharacteristics are discussed here, but the use of resis-tively limited power supplies having a non-linear outputcharacteristic is not precluded.

The impedance characteristic of all the fieldbus powersupplies defined in the FOUNDATION Physical Layer ProfileSpecification is chosen to conform with the requirementsof draft Clause 22 of the IEC/ISA Fieldbus standard,rather than the characteristic defined by Clause 11 ofthat standard. This is important since it allows devicesemploying both standard-power and low-power signalingto be present on the same system in any combinationwithout degrading signal waveforms. This is possible

Figure 6 – Variation Of Available Device Current With Bus Length

Bus length (m)

100

80

60

40

20

0

0 500 1000 1500 2000

(2000) (4000) (6000) (ft)

Max

. cur

rent

for

bus-

pow

ered

dev

ices

(mA

)


19

Certification

because the FOUNDATION fieldbus network as specifiedhas a flat frequency response characteristic over a widefrequency range, achieved by tailoring the inductive reac-tance of the power supply to offset the capacitive reac-tance of the specified terminator. Before the advent oflow-power signaling devices this did not appear veryimportant, but these devices introduce current changeson the network at both bit and frame data rates and aflat characteristic is necessary to guarantee acceptablecommunication along the bus under all conditions.

The impedance of fieldbus cables and resistive elementsin I.S. interface equipment (and even component toler-ances in the fieldbus power supplies and terminators)degrade the flatness of this overall system frequencycharacteristic somewhat, but the specified test condi-tions in the draft Clause 22 of the standard ensureacceptable limits are maintained. Power supply charac-teristics other than those specified in the Physical LayerProfile Specification should not be used on FOUNDATION

I.S. fieldbus networks.

When considering the current required by devices on ahazardous area I.S. fieldbus compared with that availablefrom the power supply it is important to remember thatthere are two classes of device in this respect:

a) Devices whose average current draw remains constant between receiving and transmitting (These may be a Type 111, 112, 121 or 122 device);

b) Devices whose current draw increases when they aretransmitting (These will be a Type 121 or 122 device).

When considering the first class of device it is sufficientto sum the current taken by all devices and compare thiswith the available current in the particular system config-uration. This is calculated from the power supply voltage,the device minimum operating voltage (9 V), and all theresistive elements present in the particular system (powersupply, cables and I.S. interface).

With the second class of device the quiescent current(during reception) of each device is again summed, butan additional current equal to the highest current differ-ence between receiving and transmitting of any device inthe system must then be added to this figure.

In either case it may be considered prudent to allow forthe additional current taken during device start-up or by,for example, a portable bus analyzer that may berequired on the system occasionally during operation. Ifthis margin is not available reliable bus communicationcould be affected during these periods.

6.2 I.S. Barriers

The FOUNDATION Physical Layer Profile Specification(Section 12.2) includes an I.S. barrier. Such barriers areintended for mounting in the safe area only (unless theircertification expressly allows installation in a hazardousareas using additional protection techniques). A typicalI.S. barrier comprises a simple network of shunt-connect-ed zener diodes, series current limiting resistors and pro-tection fuses. The barrier performs its function by divert-ing excessive voltage or current surges or overloads safe-ly to the system earth (or ground in North American par-lance) before they can cause ignition of the hazardousatmosphere. The existence of a properly rated, highintegrity connection from the barrier to earth is therefore aprimary safety requirement when using a barrier-protect-ed fieldbus in a hazardous area. Most national installationcodes require that the earth cable has a DC resistance of<1 Ω between the barrier earth terminal and the systemsafety earth (normally the connection to the neutral pointof the power distribution system). An I.S. barrier intendedfor operation with a safety earth must never be operatedwithout the safety earth properly installed and connected.

The topic of earthing requirements for I.S. systems is toolarge to cover in any depth here. FOUNDATION I.S. fieldbussystems must obey the same well established generalearthing principles as other I.S. systems. If installershave any doubt about what the system requires they areadvised, before installing it, to consult published materialfrom specialist I.S. suppliers (or speak directly to them)regarding specific installation requirements for their prod-ucts. Some general points to take note of are:

• The selected safety earth connection point should be free from the possibility of invasion by other large fault currents (such as those from a large electric motor, for example);

• The safety earth connection should not carry any significant current during normal operation (except, forinstance, the noise current present in cable screens);

NOTE – The IEC/ISA Fieldbus Physical Layer Standardprecludes the use of the cable screen as a power conductor.

• The safety earth cable should be clearly identified, and checked regularly for mechanical damage and integrity;

• For testing purposes, the fitting of two safety earth cables in parallel is recommended by some authorities. This facilitates the periodic measurement of resistanceto earth (by disconnecting one cable and measuring the loop resistance).


20

Certification

The IEC/ISA Physical Layer Standard requires that thefieldbus is operated in a balanced mode with respect toearth. Barriers to the FOUNDATION I.S. barrier profile mustmaintain this balanced operation. This will normallyrequire a dual channel I.S. barrier with equal positive andnegative channel voltages. Usually it will be suppliedfrom a FOUNDATION Type 131 power supply, as illustratedin Figure 7. For correct fieldbus operation, the outputvoltage of the power supply must match the specifiedWorking Voltage of the I.S. barrier.

This working voltage will normally be lower than the cer-tified output voltage (safety voltage) of the barrier – byperhaps as much as 2 V. In addition to this, an allowanceof 1 V needs to be made for the maximum excursion ofthe signal voltage on the fieldbus (which occurs if onebus terminator is removed). If an attempt is made tooperate the system with a power supply voltage ratingwhich results in voltages greater than its working voltagebeing applied to the barrier, fieldbus communications willbe disrupted (by turning on of the internal barrier diodes).This may eventually result in permanent damage to thebarrier by blowing its internal protection fuses. The valuesshown on Figure 7 are simply examples on a typical sys-tem. Other measured values may be equally valid with aparticular manufacturer's equipment.

The I.S. barrier represents a significant lumped imped-ance in each fieldbus line (perhaps up to 80 Ω , depend-ing on its characteristics). The IEC/ISA Physical LayerStandard (Clauses 11.7.5 and 22.7.5) requires a termina-tor to be placed at both ends of the fieldbus trunk cable.An impedance discontinuity, such as that introduced byan I.S. barrier, constitutes one end of the fieldbus trunk.The IEC/ISA Physical Layer Standard (Clauses 11.8.2and 22.8.2) specifies a maximum cable length of 100 m(328 ft) between an I.S. barrier and the nearest termina-tor. Wiring from the barrier to any equipment in the safearea should therefore be treated as a spur cable, andlimited in length by the recommendations in Annex B ofthe IEC Physical Layer Standard (printed as Annex C inISA-S50.02-1992).

Some barrier manufacturers may therefore choose, foruser convenience, to include a fieldbus terminator withinthe I.S. barrier. Users should check whether a particularI.S. barrier is specified as having an integral terminator, orrequires an external terminator suitably certified for con-nection to hazardous area circuits.

Figure 7 – Fieldbus With Earthed I.S. Barrier

Safe Area Hazardous AreaFieldbusPowerSupply

(Type 131)

Hazardous Area Devices(Type 111, 112, 121 or 122)

Safe Area Devices(Any Type) System

SafetyEarth

(Ground)

I.S. Barrier(±11V, 51Ωchannels) Terminator

(I.S. certified)

19V

+

-

- -

- - -

+

+ + +

+

≥9V Th

DhD

Cable shield

(Terminatorincluded within

I.S. Barrier)

PowerSource

Note: Device Types 114 and 124 donot require a powered bus.


21

Certification

6.3 I.S. Galvanic Isolators

The FOUNDATION Physical Layer Profile Specification(Section 12.3) includes an I.S. galvanic isolator. This typeof unit, in contrast to an I.S. barrier, maintains safety byhaving high integrity isolation that prevents the transmis-sion of electrical surges or overloads from the safe to thehazardous area. Transmission of electrical power andsignals through the isolator is achieved using speciallydesigned components such as transformers and opto-couplers with guaranteed minimum segregation and iso-lation characteristics. Voltage and current limiting is thenapplied before signals are transmitted to the hazardousarea circuit, defining the isolator hazardous area outputcharacteristic as in the case of an I.S. barrier. When usingan I.S. galvanic isolator a safety earth connection to theunit is therefore not required (in contrast to the I.S. barrierequivalent), since surge currents are no longer diverted toearth. It is, however, still necessary to provide a systemearth for the proper termination of cable shields. I.S. gal-vanic isolators are intended for safe area mounting only(unless their certification expressly allows installation in ahazardous area using additional protection techniques).

One advantage of the I.S. galvanic isolator over the I.S.barrier is that it has a hazardous area circuit which is fullyisolated from earth, making it easier to ensure the fieldbus

is operated in a balanced mode with respect to earth. The active circuitry employed in the galvanic isolator alsomakes it possible for the isolator to present a reducedeffective impedance in each fieldbus line (compared withthe I.S. barrier) at the signaling frequencies. Manufacturerswho implement this reduction technique within their I.S.galvanic isolator products will probably choose not toinclude a bus terminator within the unit, since the bus seg-ments on both sides of the isolator may be considered asa single continuous bus. However, it is recommended in allcases to check the manufacturer’s documentation regard-ing placement of bus terminators.

An I.S. galvanic isolator to the FOUNDATION Physical LayerProfile Specification is likely to include the equivalent of aType 133 fieldbus power supply. An example system isshown in Figure 8. Alternatively, it may be specified tooperate in conjunction with an external fieldbus powersupply of this same type. In either case, the characteris-tics of the galvanic isolator and power supply elementsmust be considered together when assessing the overallsafety parameters represented by the combination.Where these are separate items, both units will bemarked with their certified safety parameters.

Figure 8 – Fieldbus With I.S. Galvanic Isolator

Terminator

Hazardous Area Devices(Type 111, 112, 121 or 122)

Safe AreaDevices

(Any Type)

FieldbusPowerSupply

(If required)

SystemEarth

(Ground)

I.S. Galvanic Isolator(22V, 102Ω output) Terminator

(I.S. certified)

19V ≥9V

++

- -

-(-)

(-) (-)

(+) (-) (+)

(+) (+)

+

Th

DhDP

T

Cable shield

Safe Area Hazardous Area


22

Certification

6.4 Requirements For Other Safe Area Equipment Connected To The Fieldbus

By employing a suitably certified I.S. barrier or galvanicisolator, most of the restrictions are removed from equip-ment mounted in the safe area and connected to thesafe area terminals of these devices. However, one sig-nificant requirement remains: no supply or source of gen-erated voltage over 250 V RMS AC or DC may be pre-sent in any of the equipment connected to the safe areafieldbus. Most of this equipment will employ low voltagecircuits and have no problems in complying. However, apersonal computer (used as part of a fieldbus communi-cations analyzer, for instance) or a DCS visual display

system often contain high voltage circuits driving theCRT display. These technically invalidate the I.S. certifi-cation of the barrier or isolator unit (because the protec-tion fuses in them can be bypassed by arcing from highinput voltages) but, for personnel safety reasons, thesetypes of equipment normally include a large separationbetween their high voltage circuits and the fieldbus orother user-accessible ports. Where there is any doubtthat this is the case, the problem can normally be over-come by interposing a surge protection device betweenthe equipment containing the high voltage circuitry andthe associated safe area apparatus of the I.S. fieldbus.


23

Safety

7.0 ANALYZING THE SAFETY OF A FOUNDATION I.S.FIELDBUS SYSTEM

Requirements for formal third-party certification of aplanned fieldbus system before installation vary betweendifferent countries. In the UK there is no legal require-ment to obtain this certification, whereas in some otherEuropean countries it is required under some conditions.In France and Germany, for example, a system certifica-tion is specifically required when the hazard includesZone 0. In Japan, at the present time, a systems certifi-cate is a mandatory requirement.

North America has pioneered the acceptance of anEntity Concept. This is a system that assigns safetyparameters to both power sourcing and receiving equip-ment in an I.S. system, allowing competent users to ana-lyze the suitability and safety of any proposed system bycomparing assigned device parameters. An extension ofthis approach is very suited to the analysis of a multi-vendor fieldbus system using equipment complying withthe FOUNDATION Physical Layer Profile Specification. Thiscould be a safety evaluation by the system designer, orfor presentation of the results in an easily analyzed formto a third-party certification body. Although system certi-fication is not a legal requirement in many countries, asoutlined above, many major users still require it as anadditional safeguard that the system installed is safe tooperate. Where this system certification is needed,authorities will require each item of equipment to becompletely defined.

The entity concept approach compares directly the para-meters of equipment providing energy to the I.S. fieldbussystem with those that receive it. With compatible equip-ment, certified to the recommended I.S. parameters con-tained in the FOUNDATION Physical Layer Profile Specifi-cation, adequate safety can quite easily be demonstrated.This must always be done prior to installation, whether ornot a third-party system certification is required.

The recommended procedure consists of listing detailsfor each piece of apparatus (both associated safe areaapparatus and hazardous area mounted items) in astructured format as shown in Figure 9. Relevant infor-mation for each piece of equipment includes the follow-ing items:

For all items of equipment:Details of manufacturerManufacturer's type referenceCertification bodyCertification standardCertificate or file reference numberCertification category : Ex ia or Ex ib (Class 1)Permitted gas group(s) : IIA, IIB, IIC (Groups A, B, C, D)

For intrinsically safe, hazardous area apparatus:Maximum certified input voltage, Ui (Vmax)Maximum certified input current, Ii (Imax)Maximum certified input power, Pi (Pmax)Temperature classification, TResidual device capacitance, Ci

Residual device inductance, Li

For associated safe area apparatus:Maximum certified open circuit output voltage, Uo

(Voc, see Note 2)Maximum certified short circuit output current, Io

(Isc, see Note 2)Maximum transferred output power, Po (Pm )Maximum permitted capacitance, Co (Ca)Maximum permitted inductance, Lo (La)Maximum permitted inductance to resistance ratio,

Lo/Ro (L/R)

NOTES

1. The symbols normally used for these parametersare shown here (and in figures 9 through 11) inIEC terminology. Where the equivalent NorthAmerican symbol is different it is shown in paren-theses after the IEC symbol.

2. When these entity parameters result from the combi-nation of several I.S. sources they are designated asVt and It, rather than Voc and Isc, in North Americanterminology.

The relevant information is required for each item presentin the planned system. If there is any doubt about theparameters for a piece of equipment these should beobtained in writing from the manufacturer or his autho-rized local representative. Where compliance with anyspecial conditions is required as part of the equipmentcertification these should be particularly noted. When allthe information has been gathered, the table format ofFigure 9 can be completed in detail, as illustrated inFigure 10. This has been completed for equipment certi-fied to IEC categories, but can easily be translated toNorth American categories where equipment is certifiedto these standards. It is generally not possible to mixequipment certified to different (not fully compatible)standards in a single I.S. system.

NOTE – The data shown in Figure 10 is for illustrationpurposes only and does not indicate anything about theexistence or suitability of the particular equipment listed.


24

Safety

For a safe system, the following conditions must applyfor each piece of apparatus on the intended fieldbus net-work:

Ui ≥ Uo (Vmax ≥ Voc)Ii ≥ Io (Imax ≥ Isc)Pi ≥ Po (Pmax ≥ Pm)

These conditions are clearly met in the example systemshown in Figure 10.

Figure 9– Recommended Table For System Safety Analysis

Intrinsic Safety Approval Standard:

Associated Apparatus (Safe Area)

I.S. Apparatus (Hazardous Area)

Man. TypeRef.

Certn.Auth.

Cert. No.

Certn.Cat.

GasGroup

Uo(Voc)

Io(Isc)

Po(Pm)

Co(Ca)

Lo(La)

Lo/Ro(L/R)

Man. TypeRef.

Certn.Auth.

Cert. No.

Certn.Cat.

GasGroup

Temp.Class

Ui(Vmax)

Ii(Imax)

Pi(Pmax)

Ci Li

Complete System

Certn.Auth.

Cert. No.

Certn.Cat.

GasGroup

Temp.Class

Uo(Voc)

Io(Isc)

Po(Pm)

Max. Cfor

Cable

Max. Lfor

Cable,or

Max.L/Rfor

Cable


25

Safety

The overall system takes the lowest certification categoryand gas group of any of the apparatus present in thesystem. In Figure 10 the overall system is Ex ia IIC, as allthe individual pieces of equipment listed have been certified to the recommended FOUNDATION I.S. parame-ters. However, if any of the items of equipment had beencertified as category ib, or for use in less hazardous gas

groups IIB or IIA then the overall system certificationwould be limited to these categories. The overall tem-perature classification of the system is defined by theseveral pieces of equipment as T4 at up to 60° C ambi-ent temperature.

The interpretation of capacitance and inductance figureswill be dealt with in Section 8.

Figure 10– Example of A System Safety Analysis

Intrinsic Safety Approval Standard: CENELEC EN 50 020 (First Edition, 1977)

Associated Apparatus (Safe Area)

I.S. (Hazardous Area)

Man.

ABCLtd.

TypeRef.

B791

Certn.Auth.

BAS

Cert. No.

BAS27

Certn.Cat.

ia

GasGroup

IIC

Uo(Voc)

22.0 V

Io(Isc)

214 mA

Po(Pm)

1.19 W

Co(Ca)

165 nF(IIC)

1.14mF(IIB)

Lo(La)

0.35 mH(IIC)

1.04 mH(IIB)

Lo/Ro(L/R)

31 mH/Ω(IIC)

93 mH/Ω(IIB)

Man.

ABCLtd.

DE Inc.

FGHGmbH

JJ Inc.

KL Ltd.

TypeRef.

FBT1

DP 04

TT9

IP956

Valvetype 7P

Certn.Auth.

BAS

LCIE

PTB

PTB

BAS

Cert. No.

BAS45

L9506

PTB92

PTB97

BAS72

Certn.Cat.

ia

ia

ia

ia

ia

GasGroup

IIC

IIC

IIC

IIC

IIC

Temp.ClassT4 at60° CAmb.T4 at60° CAmb.T4 at60° CAmb.T4 at60° CAmb.T4 at60° CAmb.

Ui(Vmax)

30.0 V

25.0 V

24.0 V

24.0 V

25.0 V

Ii(Imax)

300 mA

300 mA

350 mA

300 mA

250 mA

Pi(Pmax)

1.2 W

1.2 W

1.6 W

1.2 W

1.2 W

Ci

0

2 nF

5 nF

5 nF

3 nF

Li

0

10 mH

10 mH

20 mH

10 mH

Complete System

Certn.Auth.

Cert. No.

Certn.Cat.

ia

GasGroup

IIC

Temp.Class

T4 at60° CAmb.

Uo(Voc)

22.0 V

Io(Isc)

214 mA

Po(Pm)

1.19 W

Max. Cfor

Cable

150 nF(IIC)

1.12 mF(IIB)

Max. Lfor

Cable,or

0.30 mH(IIC)

0.99 mH(IIB)

Max.L/Rfor

Cable

31 mH/Ω(IIC)

93 mH/Ω(IIB)


26

Safety

Figure 11– Example Of A System Control Drawing

A completed form such as Figure 10, together with ageneric diagram of the system layout (known as aControl Drawing in North America) similar to that shownin Figure 11, and supporting documentation for each

piece of equipment, should be sufficient to enable sys-tem certification to be obtained efficiently from any rec-ognized I.S. certification body.

Safe AreaSafe AreaApparatus I.S. Interface

Terminator

Device Device

Device ParametersInterface Parameters

Uo (Voc)

Io (Isc)

Po (Pm)

≤

≤

≤

Ui (Vmax)

Ii (Imax)

Pi (Pmax)

Hazardous Area


27

Cabling

8.0 SYSTEM CABLES

The structured table of system equipment with its safetyparameters, as shown in Figures 9 and 10, also allowsthe maximum values of permitted capacitance andinductance in the hazardous area fieldbus cables to bedetermined easily. These parameters are important sincethey represent sources of stored energy present in thehazardous area that could be released by open circuitingor short circuiting the hazardous area field cables. Thecertification of an item of associated safe area I.S. appa-ratus will define maximum permitted values for thecapacitance and inductance that may be connected toits hazardous area terminals. These permitted values aredifferent for each gas group, the lowest ones being forIIC (North American Groups A & B) gases.

The maximum capacitance allowed for cables is calcu-lated by subtracting the sum of all residual device capac-itances from the capacitance permitted for the safe areaassociated apparatus. What remains is available for thecables. Similarly, the maximum inductance allowed forcables is calculated by subtracting the sum of all residualdevice inductances from the inductance permitted forthe safe area associated apparatus. For the examplesystem shown in Figure 10, the permitted cable capaci-tance is calculated for IIC gases as:

Ccable max. = 165 nF - (2 + 5 + 5 + 3) nF = 150 nF

and the permitted inductance as:

Lcable max. = 350 mH - (10 + 10 + 20 + 10) mH = 300 mH

With some types of cable this capacitance value couldrestrict the length of fieldbus permitted in the hazardousarea.

Most practical restrictions arising from the maximum per-mitted inductance parameter can be overcome by usingthe allowed alternative L/R ratio parameter (mentioned inSection 4.2). A maximum permitted L/R ratio for cablesconnected to the hazardous area terminals of a piece ofsafe area associated apparatus will normally be assignedby the authority that certifies it. This parameter can becomplied for cables as an alternative to meeting the per-mitted inductance figure. For the example IIC system inFigure 10 this means cables having an L/R ratio of up to31 mH/Ω may be used – which covers most cable con-structions that are normally used in I.S. systems. Thisovercomes any problem with the low allowed inductancefigure.

Cable parameters are really only of concern in IIC (NorthAmerican Groups A & B) applications. The greatlyincreased permitted maximum cable capacitance andinductance values, shown in Figure 10, for operation in aIIB (North American Group C) gas atmosphere are highenough not to impose any practical limitation on an I.S.fieldbus system. Corresponding values for a IIA (NorthAmerican Group D) gas atmosphere are even higher.Work is underway within the IEC committee on I.S. tosimplify further the assessment of cable parameters inI.S. systems, but it is likely to be several years before anygeneral changes are introduced

Where it is necessary to determine the capacitance andinductance of an already installed cable, a first approachshould be to contact the particular cable manufacturerfor specified values corresponding to the installed cabletype. Often these are not readily available but if they areit provides the easiest solution. If they cannot beobtained this way then the parameters of a known lengthof the installed cable can be measured using an LCRbridge. The highest capacitance value from measure-ments made core-to-core and core(s)-to-shield shouldbe used. Alternatively, typical values can be used. Mostof the types of cables normally used to interconnectinstrument systems have L/R ratios below 25 mH/Ω andcapacitances below 200 pF/m.

The maximum allowable length of a hazardous area field-bus is determined by taking the calculated maximumpermitted cable capacitance for the system and gasgroup (from Figure 10) and dividing this by the measuredor estimated capacitance per unit length of the cablebeing used. For example, with a cable having a capaci-tance value of 140 pF/m the maximum length of fieldbusin a IIC hazardous area would be limited to 1071 m (3515 ft) - this is derived from 150 nF ÷ 140 pF/m. Usingthe same cable in a IIB hazardous area, the permittedlength increases to 8035 m (26,363 ft).

Most national installation codes require segregationbetween wires carrying I.S. circuits and other cableswithin equipment racks or cubicles. Normal good prac-tice is to run the two cable types in separate, clearlylabeled cable ducts. Field wiring within a particular multi-core cable will also normally be required to carry either allI.S. circuits or all non-I.S. circuits. The two types cannotusually be mixed within a single cable. Practice variesbetween different countries on whether multicore cablesof these two types may be carried within a commoncable duct or conduit, and users are advised to check onlocal practice before installation. It is also normallyrequired to run I.S. circuits through separate field junctionboxes from non-I.S. wiring.

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