Your trip switch keeps tripping and you want to know why. This guide explains the difference between MCB and RCD tripping, covers every common cause, shows you how to find a faulty appliance, and tells you when to call an electrician.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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The trip switch keeps flipping off, plunging part or all of your house into darkness. You push it back up, it holds for a while, then trips again. This is one of the most common electrical problems in UK homes, and one of the most searched-for — because it is frustrating, disruptive, and can feel alarming.
The trip switch is doing its job. It is a safety device designed to disconnect the circuit when it detects a fault. The question is not "why is it tripping?" but "what fault is it detecting?" Understanding the answer starts with knowing which type of trip switch is tripping — because MCBs and RCDs trip for completely different reasons.
This guide explains the difference clearly, covers every common cause, shows you how to isolate a faulty appliance yourself, and tells you when you need a qualified electrician. If you are an electrician, the later sections cover systematic fault finding for tripping circuits.
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This is the single most important thing to understand about tripping. Look at your consumer unit (the box with the switches, usually near your front door or under the stairs). You will see two types of switch:
MCBs are the smaller switches, usually in a row. Each one protects a single circuit (e.g., "Kitchen sockets", "Upstairs lights", "Cooker"). They are typically rated at 6A, 10A, 16A, 20A, or 32A.
An MCB trips because of overcurrent. This means more current is flowing through the circuit than the MCB is rated for. There are two causes: overloading (too many appliances drawing too much current) or a short circuit (live conductor touching neutral or earth, causing a massive current surge). MCBs protect the cable from overheating and catching fire.
RCDs are the larger switches, usually one or two in the consumer unit. They often have a "T" or "Test" button on the front. Each RCD protects a group of circuits.
An RCD trips because of an earth fault. It continuously compares the current flowing out on the live conductor with the current returning on the neutral. If there is a difference (even as small as 30 milliamps), it means current is leaking to earth through an unintended path — possibly through a person. The RCD disconnects in milliseconds to prevent electric shock. Regulation 411.3.3 of BS 7671:2018+A4:2026 requires additional RCD protection (rated residual operating current not exceeding 30mA) for socket-outlets rated up to 32A. In dwellings there is no exception; in non-dwellings a documented risk assessment may justify omission.
Why this matters: If an MCB is tripping, the fault is overcurrent — look for overloaded circuits or short circuits. If the RCD is tripping, the fault is earth leakage — look for faulty appliances, damaged cables, or moisture ingress. If you have RCBOs (which combine both functions), the tripped device tells you both the circuit and the type of fault.
An overloaded circuit is the simplest and most common cause of MCB tripping. The total current drawn by all the appliances on one circuit exceeds the MCB rating, and the MCB trips to protect the cable.
This typically happens in kitchens, where high-power appliances are concentrated. A ring final circuit protected by a 32A MCB can supply up to 7.36kW — but a kettle (3kW), a toaster (2kW), a microwave (1.4kW), and a dishwasher (2.2kW) running simultaneously draw 8.6kW (37A), exceeding the circuit capacity.
The fix is straightforward: reduce the load on the circuit by staggering high-power appliance use (do not run the kettle and toaster at the same time), or have an electrician install additional circuits to distribute the load. In kitchens, a dedicated circuit for high-power appliances is often the best solution.
An earth fault occurs when current finds an unintended path to earth. This could be through damaged cable insulation allowing the live conductor to touch the metal conduit or back box, a faulty appliance with a breakdown in its internal insulation, or water creating a conductive path between live parts and earth.
Earth faults cause RCD tripping. The RCD detects the imbalance between live and neutral current (because some current is flowing to earth instead of returning via neutral) and disconnects the circuit. This is a critical safety function — the same earth fault that trips the RCD could deliver a lethal shock to a person if the RCD were not present.
Moisture is one of the most common causes of RCD tripping, and it often catches homeowners by surprise because it is intermittent and seasonal. Water is a conductor — when it gets into electrical enclosures, it creates a path for current to leak to earth, tripping the RCD.
If your RCD trips mainly during wet weather, after rain, or during cold snaps, moisture ingress is the most likely cause. An electrician can isolate individual circuits and use insulation resistance testing to identify which circuit has the moisture problem.
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If your RCD or MCB trips repeatedly, a faulty appliance is often the cause. You can identify which appliance is responsible using this simple isolation technique:
Go around the house and unplug every appliance from every socket on the affected circuit (or all circuits if the RCD has tripped). Physically unplug them — do not just switch off at the socket, as some faults occur even when the appliance is switched off but still plugged in.
With everything unplugged, reset the tripped MCB or RCD. If it holds in the "on" position, the fault is in one of the appliances you unplugged. If it still trips with everything unplugged, the fault is in the fixed wiring — call an electrician.
Plug in one appliance, switch it on, and wait a minute. If the trip holds, that appliance is fine — leave it plugged in and move to the next one. When you plug in the faulty appliance, the trip switch will go off again. That is your culprit.
Once identified, leave the faulty appliance unplugged. If it is under warranty, contact the manufacturer. If it is an older appliance (washing machine, dishwasher, tumble dryer), it may need repair or replacement. Do not continue using an appliance that trips the RCD — it has an earth fault that could cause a shock.
Sometimes the problem is not a single fault but the consumer unit itself. If your consumer unit is old or inadequate, it may contribute to tripping problems or fail to provide proper protection:
Amendment 4 of BS 7671 (2026) introduced Regulation 411.3.4, which requires that AC final circuits supplying luminaires in domestic premises shall be provided with additional protection by an RCD whose rated residual operating current does not exceed 30 mA. An older split-load consumer unit where lighting circuits are protected only by MCBs — with no RCD upstream — does not meet this requirement. If a lighting MCB is tripping repeatedly in an older board, this is also the point at which an upgrade to RCD- or RCBO-protected lighting circuits should be discussed with the homeowner.
Some tripping situations you can resolve yourself (overloaded circuit, faulty appliance identification). Others require an electrician:
When the electrician visits, they will use insulation resistance testing, earth fault loop impedance testing, and circuit-by-circuit isolation to identify the fault. They may recommend a full EICR if the installation has not been inspected recently, as repeated tripping can be symptomatic of wider installation problems.
Tripping call-outs require systematic diagnosis. Resist the urge to start changing components before you have identified the fault:
Confirm whether it is an MCB, RCD, or RCBO tripping. This determines your fault finding path — overcurrent (MCB) or earth fault (RCD). Check all devices in the consumer unit, not just the one the customer points to. A customer saying "the trip switch" may not know which device has tripped.
For RCD tripping: turn off all MCBs under the tripped RCD. Reset the RCD. Turn on MCBs one at a time. When the RCD trips, you have identified the faulty circuit. For that circuit, perform insulation resistance testing at 500V DC (L-E, N-E minimum 1 megohm). Low insulation resistance on L-E or N-E indicates the earth fault location (further testing or half-split isolation narrows it down).
Outdoor circuits (check all IP-rated enclosures for moisture), immersion heater (disconnect and test insulation resistance), cooker connection (check the terminal block behind the cooker for heat damage), and shower (check element insulation). For MCB tripping on ring circuits, check the ring is continuous — a broken ring concentrates load on one leg and can cause overcurrent tripping at lower-than-expected loads.
Record your test results and remedial actions. If the work involves replacing a consumer unit or adding circuits, a full Electrical Installation Certificate is required. For repairs to existing circuits, issue a Minor Works Certificate.
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