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.
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.