RCD Protection Explained: Why It Could Save Your Life

An RCD (Residual Current Device) is a life-saving electrical safety device that detects dangerous fault currents and disconnects the supply faster than a conventional circuit breaker. Where a standard MCB protects against overcurrent and short circuit, an RCD protects against the specific scenario that most commonly causes electric shock — a fault current that takes an unintended path to earth, potentially through a person.

RCDs work by continuously monitoring the current flowing in the live and neutral conductors. Under normal conditions these are equal. If current leaks to earth — for example, through a person who has touched a live conductor — there is an imbalance. The RCD detects this imbalance and disconnects the supply within 30–40 milliseconds. At a sensitivity of 30mA (milliamperes), this is fast enough to prevent cardiac arrest in most cases. A current of 50mA sustained for more than a second can cause cardiac arrest; the RCD trips before this threshold is reached.

BS 7671 has required RCD protection for most socket circuits and all bathroom and outdoor circuits since the 17th Edition (2008), and the requirement has been progressively extended since. A modern consumer unit in a domestic property should provide RCD protection to all circuits. The most common arrangement is a split-load board with two RCDs, each protecting half the circuits; increasingly, individual RCBOs (combined MCB and RCD) on every circuit are used, which prevents a single fault from taking out half the house.

If your consumer unit does not have RCD protection, you are relying on the slower and less sensitive protection of old-style MCBs or fuse wire. You can test your existing RCDs by pressing the test button on the device — it should trip immediately. This should be done every 3–6 months. If you are unsure whether your installation has RCD protection, contact DP Power Solutions for an assessment.