Earth-leakage circuit breaker

Lore & Background

Early ELCBs, first introduced about sixty years ago, were voltage operated devices (VO-ELCBs), detecting a voltage rise between installation metalwork and an external electrode. These were later replaced by current sensing devices about forty years ago. For some time afterwards both forms were referred to as ELCBs which brought some confusion. To address this the IEC introduced the term residual current device (RCD).

Reader's Guide

ELCBs are specialized latching relays that disconnect power when earth leakage is detected. Voltage-operated types detect a rise in potential between protected metalwork and a remote Earth reference electrode, operating at around 50 volts. Current-sensing RCDs use a current transformer to detect imbalance between line and neutral currents. Voltage-operated ELCBs are less sensitive to fault conditions and have fewer nuisance trips, but require an additional third wire and can be disabled by a wire break or additional Earth connections. Current-sensing devices are now standard, though the term ELCB is sometimes incorrectly used to refer only to voltage-operated types. Both types are prone to nuisance trips from lightning, neighboring Earth rods, or accumulated burden currents. ELCBs should be tested periodically as mechanical failures can occur.

Did You Know?

• Voltage-operated ELCBs detect a rise in potential of around 50 volts to open a main breaker.
• A wire break in the fault to load section or earth to ground section will disable a voltage-operated ELCB.
• ELCBs are prone to nuisance trips from nearby high current lightning strikes or close Earth rods of neighboring buildings.
• Some ELCBs do not respond to rectified fault current, and older VO-ELCBs are more likely to have fault current waveforms they will not respond to.