A residual current device (RCD) is a safety switch in an electrical installation that instantly cuts off the power supply when it detects current leaking to earth. It continuously monitors the balance between the live and neutral conductors and trips when a difference (residual current) is detected. This protects people from electric shock and reduces the risk of electrical fires caused by insulation faults.
Application
An RCD is installed in the consumer unit (breaker panel) and monitors all downstream circuits. In the UK, BS 7671 (the IET Wiring Regulations) requires RCD protection with a sensitivity of 30 milliamps (mA) for socket outlets and circuits in bathrooms and outdoor areas. In the US, the equivalent device is called a ground fault circuit interrupter (GFCI), required by the National Electrical Code for kitchens, bathrooms, and exterior outlets.
The operating principle relies on a current transformer that surrounds both the live and neutral conductors. Under normal conditions, the current flowing through the live wire is exactly equal to the current returning through the neutral wire. When current leaks to earth through a fault or a person, a difference arises. The transformer detects this imbalance and triggers a relay that disconnects the circuit within milliseconds.
There are several types of RCDs. A fixed RCD protects entire circuits from the consumer unit. A socket-outlet RCD replaces a standard socket and protects only that single point. An RCD combined with a miniature circuit breaker (RCBO) provides both earth leakage and overcurrent protection in one unit.
Regular testing is essential. Pressing the test button on the RCD should cause it to trip immediately. Electrical safety guidelines recommend testing at least every six months. An RCD that fails to trip during testing must be replaced without delay.
Related terms
- Circuit breaker
- Consumer unit
- Earthing
- Earthing network
- Wiring
Learn more about building terms in our knowledge base at fredsdiyplans.com.