An RCD — Residual Current Device — is a safety device that detects imbalances in the electrical current flowing through a circuit and cuts the power in a fraction of a second. RCDs are one of the most important electrical safety innovations of the past 50 years and are now required on most domestic circuits.
How Does an RCD Work?
In a healthy circuit, the current flowing out through the live conductor equals the current returning through the neutral conductor. If someone receives an electric shock, or a fault develops, current flows via an unintended path — the RCD detects this imbalance (as little as 30 milliamps) and disconnects the circuit in under 40 milliseconds — fast enough to prevent serious injury or death.
Types of RCD
RCD (Residual Current Device) — protects a group of circuits. If one circuit trips, all circuits on that RCD are disconnected.
RCBO (Residual Current Breaker with Overcurrent protection) — combines the function of an MCB and RCD in one unit. Each circuit has its own RCBO, so a fault on one circuit doesn't affect others. More expensive but the preferred modern option.
Socket RCD — built into individual sockets or plug adaptors. Can be used where a full RCBO board isn't practical. Common on outdoor extension leads.
What Protection Does Your Home Currently Have?
Older homes (pre-2000) may have no RCD protection at all, or just a single RCD covering all circuits — meaning a fault anywhere trips everything.
Modern homes typically have either:
- A dual-RCD board (circuits split across two RCDs)
- A full RCBO board (individual protection per circuit)
An RCBO board is now considered best practice as it provides maximum protection without the nuisance of losing multiple circuits when one trips.
Why Is RCD Protection Required?
Current regulations (BS 7671:2018) require RCD protection on:
- Socket outlets up to 32A (likely to be used outdoors or for portable equipment)
- Circuits in bathrooms, kitchens and outdoors
- Cables buried in walls at less than 50mm depth
- Mobile home and caravan supplies
How to Check Your RCD
Every RCD has a test button. Pressing it should cause the RCD to trip immediately — this verifies the mechanism is working. We recommend testing your RCDs quarterly.
To upgrade your consumer unit to include full RCBO protection, call Onyx Electrical Solutions on 07000 000000.