One Way Barrier
The security barrier ensures that any failure in the secure area (control room) can not provide enough energy to ignite the gaseous atmosphere in the hazardous area. However, the system will not be intrinsically safe unless all the equipment in the circuit is also intrinsically safe.
A typical one-way safety barrier is shown:
The barrier consists of a series resistor and a fuse plus a Zener diode to ground. The series resistance limits the current to approximately 100 mA from a 28 V supply when the dangerous terminals are shorted. The Zener diode operates in the 30 V region and the fuse has a power of around 30 mA.
The barrier thus ensures that either too much current or too much voltage will blow the fuse. This will keep any dangerous energy levels away from the hazardous area.
Two Way Barrier
The one-way barrier is not suitable for 4-20mA loops which are floating (neither side connected to earth). A two-way barrier is used with a floating supply.
This barrier has additional zeners and resistances. The resistance R is actually 3 diodes in series. When they are working normally, they act as a resistance and allow the return current to pass. If a fault occurs, a dangerous reverse current may flow. R then acts as a diode to stop the reverse current.
It is important that the grounding is good, that is, less than IΩ to ground. All ground wires must also be grounded at the same point. If the earth is not at the same point, then currents can circulate on the land line. If this happens, it is possible that a failure in another equipment destroys all the system’s barriers.
The diagram below shows a typical Zener barrier earthing system: