ResidualCurrent

Residual current refers to the portion of electrical current that does not return to its source through the intended path, typically because some of the current leaks to earth or travels through an unintended path. In a normal circuit the current in the live conductor equals the current in the neutral conductor. Any leakage to earth creates a residual current, which can pose electric shock or fire hazards if not addressed.

Residual current is detected by protective devices known as residual current devices (RCDs) or residual current circuit breakers (RCCBs). These devices monitor the imbalance between the live and neutral conductors, usually by passing both conductors through a toroidal current transformer. When the measured residual current exceeds a predefined threshold, the device trips and interrupts the circuit within milliseconds. Some units include overcurrent protection and are called RCBOs (residual current circuit-breakers with overcurrent protection).

Common operating thresholds include 30 milliamperes (mA) in many domestic installations for protection against electric shock, with lower thresholds used in particularly damp or wet areas. Higher thresholds may be employed where nuisance tripping must be minimized or where protection requirements differ. In North America, a related concept is the ground-fault circuit interrupter (GFCI), which provides a similar function.

Applications of residual current protection are widespread in buildings, bathrooms, kitchens, outdoor outlets, and portable devices, where they reduce the risk of electric shock and fire due to insulation faults. They are typically mandated by electrical safety codes and standards in many jurisdictions. Limitations include that they do not detect all fault types (such as perfectly balanced leakage) and can nuisance-trip under transient conditions, so proper coordination with other protection is important.