residual-current device
An automatic circuit breaker that rapidly disconnects an electric circuit when the imbalance of the currents going through it exceeds a specific value.
RCD: the safety switch that catches dangerous current leaks
A residual-current device, or RCD, is a protection mechanism that sits in an electrical circuit and monitors the current flowing in and out. If more current leaves the circuit than returns to it, the RCD detects this imbalance, typically within 30 milliseconds, and trips open to cut power. This imbalance signals that current is escaping somewhere it should not be, usually through a person or conductive path to ground, and the RCD's job is to stop that current before it causes serious harm or fire.
RCDs work by comparing the current in the live conductor against the current in the neutral return path. A transformer coil monitors both conductors; if they carry equal current, the magnetic fields cancel and nothing happens. Any leakage current creates an imbalance that induces a signal in the trip coil, releasing a solenoid that opens the contacts. The sensitivity is set by the manufacturer, most commonly to trip at 30 milliamps (mA) for general socket circuits, though 100 mA and 300 mA versions exist for industrial use where nuisance tripping matters less.
The device comes in several forms. Socket-mounted RCDs plug directly into a wall outlet and protect anything plugged into them; these are portable and useful for temporary installations or high-risk work. Fixed RCDs are wired into a distribution board and protect entire circuits or groups of circuits downstream. Some RCDs are combined with miniature circuit breakers into a single unit called an RCBO, which handles both overload protection and residual-current protection in one casing.
Limitations and failure modes
RCDs do not protect against all electrical hazards. They are ineffective against phase-to-phase faults, where current leaks between live conductors without touching ground, because both conductors see the same change. They also cannot protect someone who completes a circuit between the live conductor and ground with very low resistance, because that high current will cause the RCD to trip only after significant energy has been delivered. Nuisance tripping remains a practical problem: older RCDs trip easily in damp conditions or in circuits with capacitive or inductive loads that naturally develop small leakage currents during normal operation.
Testing is critical because a failed RCD offers no protection. A small button labeled TEST on the device allows manual tripping; pressing it should open the circuit immediately. Monthly testing is standard practice in safety-critical installations. RCDs themselves can degrade, especially in harsh environments with temperature swings or moisture, and should be replaced if they no longer respond to the test button or trip nuisance and erratically.