Intrinsic safety standards require — for instruments in hazardous locations — that certain points of the system be grounded, and others be isolated from ground.
Grounding intrinsically safe circuits reduces the probability of generating excessive energy levels. Isolation from ground prevents the possibility of having two grounded points with different potentials, and the possible circulation of a high current.
IS standards also require that only one point be grounded, and the rest of the circuit isolated from ground (500 VAC minimum). This is done with a conductor connected to the reference ground system and isolated from any other plant grounds.
The grounding requirement, maintenance, and testing can be considerably reduced by using isolated barriers — that don’t require a maintained IS ground.
The circuits are floating ground systems, and are usually equipped with three transformers (on input, output, and power). Below, you can see a comparison of wiring methods using isolated barriers.
Conventional Passive IS Zener Barriers
A good ground connection must be provided and maintained.
Field devices must be isolated from ground.
Voltage drop across the barriers can make some applications difficult.
Improper connection or voltage surges could blow the fuse.
Poor common mode rejection values.
Active (Powered) IS Isolation Barriers
Ground connection not required.
Field devices can be grounded.
Full voltage is available to field devices.
Reverse polarity protected and surge arrestors incorporated.
Tolerates high common mode voltage
Signal conditioning and circuit protection are combined.
Simple installation with elimination of ground loops.