Intrinsic Safety (i)
A circuit or part of a circuit is intrinsically safe, when any spark or thermal effect produced (that is, by breaking or closing the circuit) or accidentally, by short-circuit or earth fault is incapable, of causing ignition of a gas or vapour. They work by restricting the electrical energy available in hazardous area circuits to a level at which any sparks or hot spots caused by circuit faults are too weak to cause ignition.
The only method to achieve IS loop is by using Safety Barriers.
This can be classified into 2 types: Active Barriers = In this case line resistance is negligible and is dominant these days.
Passive Barriers = In this case, the barriers also impose resistance to the tune of 200 ohms (app) on the loop and this factor needs to be considered while selecting the instrument.
Intrinsic safety certification is classified as Ex(ia) Applicable for individual inst. Ex(ib) Applicable for complete system/Loop.
Ex(ib) shall include all components connected with loop like cables, local indicator, transmitter etc.
Intrinsic Safety Principles
This is based on principle of restricting the electrical energy available in hazardous area circuits, such that any sparks or hot surfaces that may occur as a result of electrical faults are too weak to cause ignition. The useful power is about 1W, which is sufficient for most modern Instrumentation.
This is the only technique accepted for Zone 0 areas. It is also safe for personnel, since the voltages are low and it allows the field equipment to be maintained and calibrated ‘live’ without the need for power isolation