What Is RCD Protection and Why Does It Matter?

An RCD (residual current device) monitors the balance of current flowing in a circuit. When current leaks to earth, the device trips within 30 milliseconds, fast enough to prevent a fatal electric shock in most situations. That response time, 30 milliseconds at a 30mA leakage threshold, is the critical specification.

What an RCD actually does

In a healthy circuit, the current flowing out through the live conductor exactly equals the current returning through the neutral. When someone touches a live component and receives a shock, some of that current flows through them to earth rather than returning through the neutral. The imbalance is detected by the RCD, which trips within milliseconds.

Without an RCD, a shock current is only interrupted when enough current flows to trip a standard MCB. An MCB on a 2.5mm² socket circuit is rated at 32 amps. A current of 30 milliamps (0.03 amps) can be fatal. An MCB will not trip at 30mA. An RCD will.

MCB, RCD, and RCBO: the difference

An MCB (miniature circuit breaker) protects against overload and short circuit. It does not provide shock protection.

An RCD (residual current device) provides shock protection and protection against earth faults. Older installations used a single RCD to protect a group of circuits: if it tripped, multiple circuits lost power simultaneously.

An RCBO (residual current circuit breaker with overload protection) combines both functions in a single device. Modern consumer units use one RCBO per circuit. If a fault occurs, only that circuit is disconnected. This is the current standard under BS 7671 18th edition.

Why older properties often have no RCD protection

RCD protection for socket circuits became a requirement under BS 7671 in 1992, but widespread retrofitting only happened from the mid-2000s onwards. Properties built or rewired before that period often have no RCD protection on socket or bathroom circuits. During an EICR, the absence of RCD protection on these circuits is typically coded as C2 (potentially dangerous), meaning the overall result is unsatisfactory.

How to check your own consumer unit

Open the cover of your consumer unit (the fuse board). If all the devices are single-width MCBs with no separate RCD, your installation has no RCD protection. If there is a larger double-width switch on one side with a test button, you have a split-load board with partial RCD coverage. If each device is a double-width unit with its own test button, that is a full RCBO board and the standard most current installations use.

What the remedial work involves

Where RCD protection is absent or inadequate, the remedial work typically involves replacing the consumer unit with a modern RCBO board. This provides independent protection on every circuit and removes the C2 observations from the EICR. The job takes most of a day on a standard house and includes full testing and a new Electrical Installation Certificate.