37.Intrinsically safe circuit

An intrinsically safe circuit is one that is designed for a power so low that any spark or thermal effect produced by it is incapable of igniting the surrounding flammable gas or vapour.

Intrinsically safe circuit is made to 2 standards.
Exi(a) - For higher standards that required that safety is maintained up to two faults.This can be fitted in any hazardous area.

Exi(b) - Safe up to one fault. Can not fitted in any hazardous area.
This method of protection is suitable for electrical supplies at less than 30 volts and 50 mA.

Exi protection used in low power instrumentation, alarm and communication circuits.
An electrically safe barrier also fitted in the circuit to limit the voltage and current under fault conditions. It must be fitted outside the hazardous area.

The zener diode characteristic shows that when connected with reverse bias it has an approximately constant voltage across it irrespective of the size of current flow. In normal operation the instrumentation circuit has a supply voltage lower than the voltage rating of the zener diodes so no current flows through them.

When an accidental high voltage appears at the input to the barrier, the diodes conduct to limits the maximum voltage appearing on the hazardous area wiring. While the zeners are conducting, the current level is designed to blow the fuse which now isolates the circuit to maintain safety in the hazardous area.

In the event of a short-circuit on the hazardous area wiring or equipment, the in-line resistors within the barrier will limit the size of fault current while the fuse blows. 

Two or three zener-resistor combinations are used within a barrier to provide back-up voltage anchors while the fuse is blowing

Power and intrinsically safe cable runs should be separately identified, i.e. by labels or by using cables with a distinctive colour (typically blue for Exi)