Posted on 2026-10-07

Understanding RCDs and circuit breakers in Poole

RCDs (residual current devices) and circuit breakers are the two main categories of protective device found inside a domestic consumer unit. Both disconnect a circuit when a fault occurs, but they operate on different principles and protect against different types of fault. Understanding the distinction helps homeowners respond appropriately when a device trips and know when to seek a professional assessment. A miniature circuit breaker (MCB) is a resettable switch that responds to excess current in the circuit it protects. If too many appliances run simultaneously on one circuit, or if a direct fault causes a surge of current, the MCB trips and disconnects. This protects the cable from overheating. Older consumer units used rewirable fuses or cartridge fuses for the same purpose, but unlike an MCB these cannot simply be reset and must be replaced or rewired after they operate. An MCB or fuse can disconnect a sufficiently high earth-fault current, but does not detect the small residual currents addressed by suitable RCD protection. These are different functions. An RCD monitors the balance of current flowing out along the live conductor and returning along the neutral conductor. In a healthy circuit, these two values are equal. If current is diverting to earth, because someone is in contact with a live conductor or because a fault has developed in fixed wiring or an appliance, the balance is disrupted. A suitable RCD disconnects when the applicable residual-current threshold is reached. It can reduce certain electric-shock risks, but does not prevent every shock or make contact with live parts safe. It does not replace overcurrent protection, sound wiring or safe use. Electrical Safety First guidance explains that an RCD provides a level of protection that an MCB alone cannot offer, because an MCB responds to current magnitude rather than earth leakage: see https://www.electricalsafetyfirst.org.uk/safety-advice/your-questions-answered/ for further detail. A residual current circuit breaker with overcurrent protection (RCBO) combines both functions in a single device. Modern consumer units frequently use an RCBO for each individual circuit, which can reduce the number of circuits affected by a fault compared with a shared RCD. The actual arrangement and any upstream device operation still matter. Repeated trips need investigation. The cause may be an appliance, fixed wiring, accumulated leakage or a faulty device; the trip alone does not identify it. Do not repeatedly reset protective devices or connect suspect equipment to other outlets to reproduce a fault. Stop using affected equipment and seek qualified assessment. Isolate only if controls are safely accessible; for fire or smoke suggesting one, get out, stay out and call 999. RCDs normally have a labelled test button. Pressing it operates a controlled internal fault to confirm the device responds correctly. The RCD should trip when the button is pressed. If it does not trip, the device may have failed internally and a qualified electrician should be contacted. The test button does not measure the actual operating time or establish that the whole installation is safe. Trips during normal use still need investigation; they do not identify a particular cause. If your consumer unit uses older technology such as rewirable fuses, or if a qualified electrician finds that the protective devices no longer operate within the required parameters, a consumer unit replacement may be the appropriate course of action. Replacing a consumer unit is notifiable work under Part P of the Building Regulations in England. It must be carried out by a registered competent person or notified to the local building control body, and an Electrical Installation Certificate should be issued on completion. This certificate is a separate document from any Building Regulations completion document issued by the local authority. See our [Fuse Board / Consumer Unit Replacement](/fuse-board-replacement) page for more information on what this type of work involves and what documentation you should expect to receive.

Signs your protective devices need attention

RCD trips repeatedly or will not reset

Repeated trips need investigation but do not identify a particular cause. Do not repeatedly reset devices or move suspect appliances between sockets to reproduce the fault. Stop using affected equipment and seek qualified assessment; isolate only if controls are safely accessible.

Test button produces no response

A labelled test button should disconnect the protected supply when used according to the manufacturer's instructions. A failed test needs qualified assessment. A successful button test does not measure operating time or prove the whole installation safe.

Consumer unit uses rewirable fuses or lacks RCD protection

Fuses and MCBs provide overcurrent protection; suitable RCDs provide a different residual-current function. An older board or shared RCD arrangement does not automatically require replacement. Actual condition, circuit requirements, compatibility and proposed work need assessment.

What an electrician checks when assessing your consumer unit

Visual and physical inspection

The electrician checks the condition of the consumer unit enclosure, cable connections, circuit labelling and the arrangement of protective devices. Signs of damage, overheating or unlabelled circuits are recorded as part of the assessment.

Functional testing of RCDs and MCBs

Each RCD is tested using a calibrated instrument to confirm it disconnects within the required time when an earth fault is simulated. The test button on each device is operated. Circuit breakers are checked to confirm they are appropriate for the circuits they protect. Results are recorded and compared against BS 7671 requirements.

Documentation and findings

Once testing is complete, the electrician explains the findings and advises on any remedial work. Where a consumer unit replacement is needed, the electrician will explain that this is notifiable under Part P and will describe what documentation, including an Electrical Installation Certificate, should be issued on completion.

Published by Poole Electricians