A circuit or part of a circuit is intrinsically-safe when any spark or any thermal effect produced in test conditions is incapable of causing ignition of the prescribed gas atmosphere.
An intrinsically safe circuit is one that is designed for a power so low that any spark or thermal effect produced by it whether there is a fault or not, is incapable of igniting the surrounding gas or vapour.
It follows that intrinsically safe equipment is used in such circuits and is designed on the same basis, i.e., of being unable to produce a spark with enough power to ignite the specific flammable vapour or gas involved. The intrinsic safety technique requires not only that a system be designed for operation with very low power, but also that it is made invulnerable to high external energies and other effects.
Intrinsically safe equipment is currently made to two standards of safety –
- Ex i(a):-is the symbol for the higher standard, which requires that safety is maintained with upto two faults. This type of equipment can be fitted in any hazardous area.
- Ex i(b):- apparatus made to this specification is safe with upto one fault. Ex i(b) products are not used in most hazardous areas.
Manufacturers state that this method of protection is suitable for electrical supplies at less than 30 volts and 50 mA. It is extensively used for instrumentation and some control functions.