The “Loop-the Loops” of Intrinsically safe electrical systems

SANS 60079-25

SANS 60079-11

Danie Visser

Expert!:

An expert is a person with extensive knowledge or ability based on research, experience, or occupation and in a particular area of study.

Experts are called in for advice on their respective subject, but they do not always agree on the particulars of a field of study.

What is an “Intrinsically safe electrical system?

• SANS 60079-25 Clause 3.1.1:

Intrinsically safe electrical system:

assembly of interconnected items of electrical apparatus, described in a descriptive system document, in which the circuits or parts of circuits, intended to be used in an explosive atmosphere, are intrinsically safe circuits

What is an “Intrinsically safe electrical system?

• Reworded:

Intrinsically safe electrical system:

Approved assembly of interconnected IA certified intrinsically safe equipment with clearly defined safety parameters, described in a descriptive system document, and intended to be used in an explosive atmosphere.

One step backWhat is Intrinsic safety?A technique restricting the level of energy in circuits to below the ignition energy of the gas. It achieves this even under prescribed faults in the electronics. Surface temperatures of components are also considered.

• Spark ignition (Io & Ii)

• Capacitive (Co, Ci & Cc)

• Inductive (Lo, Li & Lc)

• Thermal (Po & Pi)• Surface and service

Temperatures

NO

I

G

N

I

T

I

O

N

IS Requirements

• Resistive Assessment – Spark - Supply

• Capacitive Assessment – Spark – Potential Energy

• Inductive Assessment - Spark – Potential Energy

• Thermal Assessment – Power & Surface Temp

• Separation & segregation

• Physical / Construction

Spark Ignition

Capacitive

Inductive

Thermal• Group I : 150°C

• Group II : T1 – T6

Intrinsically safe Certified equipmentPSU Man (Pty) Ltd

12V IS PSU

Ex [ia] I Ma

Um = 220V

Uo = 12V

Io = 3A

Co = 10µF

Lo = 40 µH

Lo/Ro = 38µH/Ω

The Gas Man (Pty) Ltd

Fixed Gas Sensor

Ex ia I Ma

Supply Comms

Ui = 12V Ui = 12V

Ii = 1A Ii = 250mA

Ci = 10µF Ci = 10 µF

Li = 1 mH Li = 1mH

What is an “Intrinsically safe electrical system?

• Reworded:

Intrinsically safe electrical system:

Approved assembly of interconnected IA certified intrinsically safe equipment with clearly defined safety parameters, described in a descriptive system document, intended to be used in an explosive atmosphere.

Why is an Intrinsically safe electrical systems approval important?

• All the same rules that apply for individual IS equipment…applies for a system.

• Using only IS equipment in a system doesn’t imply that the system is intrinsically safe

• Intrinsic safety is the only Ex technique that considers the “faults” introduced by the cables between equipment

Descriptive System Document (How?)

• A descriptive system document shall be created for all IS systems. The descriptive system document shall provide an adequate analysis of the safety achieved by the system.

The minimum requirements are as follows:

• block diagram of the system listing all the items of apparatus within the system including simple apparatus and the interconnecting wiring.

• Gas Group, EPL, Temperature classification and ambient temperature requirements.

• the requirements and permitted parameters of the interconnecting wiring

• details of the earthing and bonding points of the systems

• where applicable, the justification of the assessment of apparatus as simple apparatus in accordance with SANS 60079-11

• where the intrinsically safe circuit contains several pieces of intrinsically safe apparatus the analysis of the summation of their parameters shall be available. This shall include all simple apparatus;

• a unique identification of the descriptive system document shall be created and the system designer shall sign and date the document.

Block diagram

Equipment identification and specificationsSAFE AREA DEVICE / TRANSMITTER

1 Manufacturer EATON ELECTRIC LIMITED

2 Model No. Channels MTL7787P+

3 Description ZENER BARRIER

4 Area Classification Zone 0

5 Environment Temperature Group I/IIC/IIIC 150°C / T4 / T135°C

6 Uo [V] 26.6Certificate of Approval

BAS01ATEX7217

7 Io [mA] 100 IA certificate MASC MS/17-3333

8 Po [mW] 650

9 Co [nF] 100 NOTES

10 Lo [mH] 3.5

11 Ro [Ω] -

12 L/R [mH/Ω] 33

13 Tamb [°C ] (-20°C ≤ Ta ≤ +60°C)

Equipment identification and specificationsFIELD DEVICE

1 Manufacturer TYCO SAFETY PRODUCTS

2 Model No. Channels FV411F

3 Description FLAME DETECTOR

4 Area Classification Zone 0

5 Environment Temperature Group I/IIC/IIIC 150°C / T4 / T135°C

6 Ui [V] 28Certificate of Approval

ITS12ATEX17586X

7 Ii [mA] 100 IA certificate MASC MS/18-1234X

8 Pi [mW] 652

9 Ci [nF] 0 NOTES

10 Li [mH] 0

11 Ri [Ω]

12 L/R [mH/Ω]

13 Tamb [°C ] -40°C ≤ Ta ≤ +80°C

Cheques and balances

CHECKS /CALCULATIONS

Uo ≤ Ui ?

Io ≤ Ii ?

Po ≤ Pi ?

Co ≥ Ci + Ccable ?

Lo ≥ Li + Lcable ?

Gas Group, EPL, Temperature classification and ambient temperature requirements

• Hazardous Area Classification – Not Equipment rating.

Area Classification: Zone 0

Subgroup: IIC

Temp: (-20°C to 40°C) T4

Cheques and balances

CHECKS /CALCULATIONS

Uo ≤ Ui Io ≤ Ii Po ≤ Pi ZONE SubGroup Temp Class

26.6V ≤ 28V 0.1A ≤ 0.1A 0.65W ≤ 0.652W 0 IIC T4

TRUE TRUE TRUE TRUE TRUE TRUE

PASS PASS PASS PASS PASS PASS

The requirements and permitted parameters of the interconnecting wiring

CABLE A

PUR Cable (As per IECEx BAS 08.0034X)

Capacitance [nF/km] 53.00

Inductance [mH/km] 0.72

Resistance [Ω/km] 36

Max Cable Length 1500

The requirements and permitted parameters of the interconnecting wiring

Maximum Length for Cable (only Capacitance considered)

= (Co - Ci ) / Ccable

= (100 nF - 0 nF) / 53 nF/Km

= 1886.8 m

Maximum Length for Cable (only Inductance considered)

= (Lo - Li ) / Lcable

= (3.5 mH - 0 mH) / 0.72 mH/Km

= 4861.1 m

TOTAL CABLE NOT TO EXCEED Suitable to 1886.8 m

Details of the earthing and bonding points of the systems

• Indicated on Block Diagram

• Special notes on the descriptive document i.e:• Earthing (screen) – Individual screens and or overall

screens shall be made continuous with isolated earth bar. In all cases screen systems are to be earthed at the barrier earth bar only (HQE)

• Earthing (spare cores) – Spare cores must be terminated and earthed in the control room only

• Where IS and Non-IS share a cabinet, 50mm separation shall be maintained at all times.

Where applicable, the justification of the assessment of apparatus as simple apparatus

• What is a simple device?

a)Passive components, e.g. switches, junction boxes, resistors and simple semi-conductor devices.

b)Sources of stored energy with well-defined parameters, e.g. capacitors and inductors whose values shall be considered when determining the overall safety of the system.

c)Sources of generated energy, e.g. thermocouples and photocells, which do not generate more than 1,5 volts, 100 mA and 25 mW. (Any inductance or capacitance present in these sources of energy shall be considered as in b.)

• How simple is “simple apparatus” really?

1.Simple Apparatus – No voltage and / or current limiting and / or suppression devices.

2.Simple Apparatus shall not contain boost circuitry

3.500 VRMS for 1 minute test between earth and the intrinsic safety circuit. E.g. the switch contacts and the metallic housing (connected to earth).

4.Terminals shall conform to creepage and clearance requirements

5.Enclosures (when located in the hazardous area): Static / plastic enclosures require a warning label Light metals Shall conform to at least IP20

6.When simple apparatus is located in the hazardous area it shall be temperature classified.

What do I need to prepare a loop?

• A Loop drawing (descriptive system document)

• Valid IA certificates of all IS equipment

• Cable specifications (sample)

• Justification of the assessment of apparatus as simple apparatus

Questions