CompEx `i` - Intrinsic Safety
CompEx type of protection `i` is such that the energy available in the hazardous area is limited below that which could ignite a flammable gas.
Intrinsically safe equipment is electrical equipment in which all the circuits are intrinsically safe ones.
Associated equipment is electrical equipment which consists of both intrinsically safe and non-intrinsically safe circuits. By design and construction the non-intrinsically safe circuits are limited to non-hazardous areas; the intrinsically safe circuits inside a hazardous area must not be adversely affected in any way.
Clearances
There are some important values to remember:
- Minimum clearance of 50 mm between the terminals of IS and non-IS circuits.
- Minimum 3 mm clearance to earth terminals.
- Minimum 6 mm clearance between IS circuits terminals.
Glands
Glands on cables containing IS (intrinsically safe) circuits must be appropriate to the EPS of the equipment containing the IS circuits.
For example if a cable containing IS circuits is entering an Ex `d` type enclosure, then Ex `d` (or Ex `de`) type gland must be used.
Safety barriers
Safety barrier is the electric equipment installed in non-hazardous area and is designed to limit the energy available for the intrinsically safe circuits located inside the hazardous area.
There are two basic types of safety barriers available: the shunt diode barrier (Zener barrier) and the galvanically isolated barrier.
Zener Barrier
Zener barriers are devices that contain at least two Zener diodes, a resistor and a fuse, in order to limit the available energy entering the intrinsically safe circuit inside a hazardous area. The energy limitation is provided by current and voltage limitation. The Zener diodes breakdown voltage is not exceeded in normal operation. If this voltage is exceeded due to a fault in non hazardous area, the diodes start to conduct, causing the fuse to blow, thus preventing the transfer of unacceptable high energy into the hazardous area.
Zener barriers must have a dedicated earth connection which is called the intrinsically safe earth. The connection must be done with at least 4 mm2 conductor and should ideally have less than 0.1 ohm resistance.
Galvanically Isolated Barriers
Galvanically isolated barriers allow the transference of energy without any directly wired interconnection.
This is usually achieved with the use of a transformer where the primary coil is connected to the source located in the safe area, and the secondary coil is providing energy to the intrinsically safe circuits inside the hazardous area.
Galvanically isolated barrier does not require an earth connection.
