Posted on 2026-10-05

Understanding RCDs and circuit breakers in Guildford

Modern consumer units in UK homes contain two types of protective device: residual current devices (RCDs) and miniature circuit breakers (MCBs). Both interrupt the power supply when a fault occurs, but they respond to different electrical conditions and protect against different hazards. Understanding how each works helps you recognise what a tripping device is signalling and how to respond safely. ## Circuit breakers A miniature circuit breaker protects the cables in a circuit against overcurrent — a situation where more current flows than the wiring is designed to carry. Each MCB is calibrated to a specific current threshold, matched to the capacity of the cables it protects. If the current through the circuit exceeds that threshold — because a faulty appliance draws too much power, or because too many high-demand devices are running on a single circuit — the breaker trips and cuts power. MCBs are designed to be reset. Once the cause of the overcurrent is identified and removed, the breaker can be switched back on. However, if an MCB trips repeatedly, this signals that something needs investigation. Persistent tripping can indicate a failing appliance, deteriorating cable insulation, or a wiring defect. Repeatedly resetting a breaker without identifying the cause is not a safe response to this pattern. ## Residual current devices An RCD operates on a different principle. It monitors the balance of current flowing through the live and neutral conductors of a circuit. In a healthy circuit these values are equal: all current flowing out through the live wire returns through the neutral. If some current takes an unintended path — through a person who has contacted a live part, or through damaged insulation that allows current to flow to earth — the balance is disrupted. The RCD detects this imbalance and disconnects the circuit within a fraction of a second, a speed that significantly reduces the risk of a serious electric shock. Under current wiring regulations, RCD protection is required for socket outlet circuits in domestic properties and for circuits supplying certain locations, including specified zones within bathrooms and shower rooms. In a modern consumer unit, individual RCBOs (residual current circuit breakers with overcurrent protection) combine both functions in a single device, giving each circuit its own independent protection. ## Testing your RCDs Consumer units fitted with RCDs include a test button on each device. Pressing the test button should cause the RCD to trip immediately. If it does not trip, or if it trips but cannot be reset, the device should be assessed by an electrician. Electrical Safety First advises that RCDs be tested periodically to confirm they operate correctly — their guidance on this is available at electricalsafetyfirst.org.uk. ## What to do when a device trips When an RCD trips, the practical first step is to unplug all appliances connected to the affected circuits before attempting to reset the device. Once the RCD is reset, reconnect appliances one at a time to identify whether a specific item is causing the fault. If the RCD trips with no appliances connected, the problem may lie in the fixed wiring itself, and a professional assessment is appropriate rather than continued resetting. When an MCB trips, check first whether the circuit has been overloaded — too many high-demand appliances running simultaneously on a single circuit is a common cause. Reducing the load and attempting to reset is a reasonable first step. If the MCB trips again immediately upon reset, leave it in the tripped position and contact an electrician. Forcing a breaker back on against an active fault condition risks damage to cables or appliances. ## When a consumer unit assessment is worthwhile A consumer unit that trips frequently, that still uses rewirable fuse wire in place of modern MCBs, or that lacks RCD protection on circuits where current regulations require it, merits a formal assessment. An electrician can carry out a condition check of the unit and the circuits it serves, and advise on whether an upgrade is appropriate. For detailed information on what this work involves — including what an assessment covers and what replacement entails — the [Fuse Board / Consumer Unit Replacement](/fuse-board-replacement) page sets out the process clearly. A periodic Electrical Installation Condition Report (EICR) is also a practical step for any property where the last formal inspection is unknown or out of date. This assessment checks whether protective devices are functioning correctly and whether the installation as a whole meets current standards. It is separate from the Building Regulations compliance document issued after notifiable work, and serves a different purpose: establishing the ongoing condition of an existing installation rather than certifying new work.

Signs that protective devices may need attention

Persistent or repeated tripping

If an MCB or RCD trips repeatedly under normal household load, this points to an underlying fault rather than a one-off overload. The device should not be reset continuously without identifying the cause, as doing so may allow a fault condition to persist and worsen.

Immediate re-trip on reset

If a breaker trips again the moment it is reset, an active fault condition is likely present. Leave the device in the tripped position rather than attempting further resets, and arrange for an electrician to assess the circuit.

RCD test button produces no response

The test button on an RCD should trigger an immediate disconnection when pressed. If pressing it produces no response, or the device cannot be reset after the test, the RCD itself should be assessed and may need replacement.

Responding safely when a device trips

Identify which device has tripped

Check the consumer unit to see which MCB or RCD is in the tripped position. Make a note of which circuits are affected — this helps narrow down which appliances or wiring areas may be involved and is useful information to have when speaking to an electrician.

Remove appliances and test methodically

For an RCD trip, unplug all appliances on the affected circuits before resetting the device. Reconnect them one at a time after reset to identify any item that triggers another trip. For an MCB trip, check whether the circuit was overloaded and reduce the load before attempting to reset.

Seek a professional assessment for persistent faults

If a device trips again immediately after reset, or trips repeatedly over time without a clear appliance cause, an electrician should assess the circuit and consumer unit. Do not attempt to bypass protective devices or hold a tripped breaker in the on position by force.

Published by Guildford Electrician