Old Electrical Equipment - Chilton ELCB

Obsolete earth leakage circuit breaker.

Chilton earth leakage circuit breaker Rear panel of chilton earth leakage circuit breaker These were installed on TT supplies prior to 1981. Since this type of supply relies on an earth rod, the earth loop impedance is typically too high to operate normal circuit breakers or fuses in the event of a live-earth fault.

These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over 50 volts.

The images here show the front and rear of the device. The rear panel includes wiring instructions.

These devices are no longer used, as they have a significant flaw. If the fault is between live and a circuit earth, they will disconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal water pipe), they will NOT disconnect, as the voltage on the circuit earth will not change.

Even if the fault is between live and a circuit earth, parallel earth paths created via gas and water pipes can result in the ELCB being bypassed. Most of the fault current will flow via the gas or water pipes, since a single earth stake will inevitably have a much higher impedance than hundreds of metres of metal service pipes buried in the ground.

Additional images

Text on the front of a Chilton earth leakage circuit breaker Chilton ELCB terminals These obsolete ELCBs have a very large yellow test button on the front. This one also states 'voltage operated' on the front, compared to modern RCDs which are current operated.

The smaller image shows the connection terminals. The black strip along the bottom is a rubber gasket, which seals around the wiring to prevent objects such as fingers coming into contact with the live terminals.