257: Exi (Intrinsic Safety) Calculation for fire alarm system

Design, product selection and installation of fire alarm system in explosion-hazard areas must comply with the local national safety regulations. Proof of the intrinsic safety (I.S.) of the electrical circuits needs to be provided for projects with EEx i specification.

Intrinsic Safety Circuit:

An intrinsic safety circuit, contains three below sections:

1=> Barrier:

Isolated barrier is a device which used as an interface to transfer DC signals between non-intrinsically safe circuits and intrinsically safe circuits. Barrier restricts electrical power to prevent the sparks and ignition of a gas mixture. Therefore, stored energy (inductances and capacitances) must remain limited so that cannot generate an ignition spark.

• Input: From safe area (non-hazardous area)
• Output: To hazardous area

For example:

• Shunt Zener Diode Barrier (SB2/SB3), STAHL
• Current Driver/Repeater (KFD0-CS-Ex1.51P), PEPPERL+FUCHS

2=> Transmitter Device => I.S Field device

I.S devices must be rated for below items:

• hazardous location
• gas group
• temperature class
• ambient temperature

In fire alarm system common detection and alarm equipment which are used in Exi zones:

• Exi Smoke Detectors
• Exi Heat Detectors
• Exi Combined Smoke & Heat Detectors
• Exi Flame Detectors
• Exi Sounders (Horns)
• Exi Flashers (Beacons)
• Exi Combined Sounder-Flashers (Horn-Beacons)
• End of lines: For conventional (non-addressable) devices, end of line elements also must be selected according to explosion hazard regulation.

3=> Cables (between Barrier and Transmitter)

Intrinsic Safety Parameters:

For Exi calculation according to ATEX / IECEx the following parameters for each element, barrier and cables must be extracted from data sheets and instruction manuals:

Field devices (I.S Elements):

• Ui (V): Voltage (=> data sheet)
• Ii (mA): Current (=> data sheet)
• Pi (mW): Power (=> data sheet)

=> Maximum energy which field device can handle

Isolated barriers:

• Uo (V): Voltage (=> data sheet)
• Io (mA): Current (=> data sheet)
• Po (mW): Power (=> data sheet)

Cables:

• Lc (mH): Inductance (=> data sheet) => if it was not available: 1 mH/km
• Cc (nF): Capacitance (=> data sheet) => if it was not available: 200 nF/km
• Length(m): between source of energy and barrier (=> site)

Please note that according to NEC, Parameter indexes are as below:

Vmax, Imax, Pi, Voc, Isc and Po

Exi Calculation steps:

If we consider the indexes as below:

• o: Output, Barrier
• i: Input, I.S. elements on the site
• c: Cables (After the barrier)

Voltage, Current and Power of output (Barrier) must be less than or equal to input (I.S. Elements):

• Uo (V) ≤ Ui (V)
• Io (mA) ≤ Ii (mA)
• Po (mW) ≤ Pi (mW)

Energy storage (Induction and Capacitance) of the barrier must be more than total of I.S. elements and cables:

• Lo (mH) ≥ ∑Li + Lc (mH)
• Co (nF) ≥ ∑Ci + Cc (nF)

If Ci = 0 or < 1% of Co or Li = 0 or < 1% of Lo   =>   Use maximum values of Co and Lo of the barrier

Otherwise, you have to follow as below:

1. Value of mixed circuit: Co/Ca and Lo/La => barrier documentation
2. 50% of Co/Ca and 50% of Lo/La

Longest cable length from Cc ((nF/km)):

Then finally we need to find the maximum allowed length of the cable:

Maximum allowed length of the cable = ((Co - Ci) / Cc) x 1000

Related electrical standards:

• Local Standards
• IEC/EN 60079-11
• IEC/EN 60079-14
• IEC/EN 60079-25
• EN50014 (IEC60079-0)
• EN50020 (IEC60079-11)
• ANSI/ISA-RP 12.06.01