Would an earth leakage sensor in the lineside circuit detect bare wires inside the relay room?
Such systems tend to operate at very much lower voltages and fitting earth leakage sensors onto every circuit would double the size of the wiring, if not more.
Power in a relay room is usually divided up into internal and external busbars fed from separate transformer isolated supply units. Typically the internal supply is not provided with earth leakage detection, but the external busbar is, and any circuit that exits the building is connected (unsurprisingly!) to the external supply.
There is no earth leakage device for each individual circuit. By monitoring the power supply at the feed end all the circuits connected to that busbar are monitored as a result. In UK, modern signalling line circuits are earth free on both legs and dual pole switched. Leakage detectors are thus designed to pick up current flowing between earth and either leg of the circuit.
Circuitry wholly in the relay room is considered to be protected adequately from mechanical and rodent damage and connection to the internal power supply means there is no earth leakage monitoring. Also single pole switching is allowed in wiring up the required logic functions for internal circuits.
