RCD (Residual Current Device)
An electrical safety device that monitors the flow of current and instantly cuts the power if it detects a fault that could cause electric shock or fire.
An RCD — Residual Current Device — is a life-saving electrical safety device fitted in your home’s electrical system (typically inside the consumer unit). It continuously monitors the electrical current flowing through a circuit and trips (switches off) almost instantly if it detects an imbalance, which indicates that current is leaking to earth — through a person, faulty wiring, or a damaged appliance.
How an RCD works
In a healthy circuit, the current flowing out through the live wire should exactly match the current returning through the neutral wire. If some current is “leaking” — for example, flowing through a person who has touched a live part — the RCD detects the imbalance and disconnects the circuit in under 30 milliseconds (0.03 seconds). This is fast enough to prevent a fatal electric shock in most situations.
Types of RCD
- Fixed RCD — built into the consumer unit, protecting one or more circuits. The most common type in modern homes
- RCBO — a combined RCD + MCB (miniature circuit breaker) that protects a single circuit against both earth faults and overloads. More expensive but provides independent protection per circuit
- Socket-outlet RCD — a special socket with a built-in RCD, used where fixed protection is not available (e.g., older homes without a modern consumer unit)
- Plug-in RCD — a portable device that plugs into a standard socket, providing RCD protection for a single appliance. Commonly used with outdoor power tools and garden equipment
Why RCDs matter in a renovation
- Building regulations — current electrical regulations require RCD protection on most circuits in a home. If your renovation involves any electrical work, the circuits affected must have RCD protection
- Bathroom and kitchen safety — circuits serving bathrooms, kitchens, and any outdoor sockets must have RCD protection due to the increased risk from water
- Consumer unit upgrades — if your existing consumer unit lacks RCD protection, a renovation is the ideal time to upgrade it to a modern unit with full RCD or RCBO coverage
RCD vs. fuse vs. MCB
| Device | Protects against | Speed |
|---|---|---|
| Fuse / MCB | Overload and short circuit | Moderate (seconds) |
| RCD | Earth fault (leaking current) | Very fast (< 30 ms) |
| RCBO | Both overload and earth fault | Very fast |
An RCD does not protect against overloads or short circuits — that is the job of the fuse or MCB. A modern consumer unit uses both types of protection together.
Practical tip
Test your RCDs regularly by pressing the “Test” button on the device (usually marked with a “T”). If it does not trip when tested, call a qualified electrician. During a renovation, ensure your contractor’s electrician specifies RCD or RCBO protection for all new circuits in the scope of work.