Page 194 - Electrical Safety of Low Voltage Systems
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Extra-Low-V oltage Systems 177
FIGURE 10.8 Ground faults in SELV control circuit.
Contact 2 loses its control capability and even if it is opened the coil
stays energized and so might the machine.
Disconnection of supply can be achieved by turning the first fault
into a short circuit by using PELV systems (Fig. 10.9).
The first fault toward the enclosure determines a short circuit
caused by the earthing connection of the safety insulating trans-
former’s pole. This provokes the tripping of the protective device,
typically a fuse, which clears the fault and puts the machine in
safety.
FIGURE 10.9 Ground faults in control circuits supplied by PELV systems.
