One circuit has gone off. The lights in one room, or the sockets in one area — but everything else in the house is fine. That is a single MCB trip, and it is a different situation from an RCD trip. Here is what it means, how to reset it safely, and how to tell whether it is a simple overload or a fault that needs an electrician.
MCB vs RCD — two different devices doing two different jobs
Before doing anything, it helps to understand what you are looking at. Your consumer unit (fuse box) contains two types of protective device, and they are not the same thing.
An MCB (Miniature Circuit Breaker) is the smaller switch assigned to a single circuit — Kitchen Sockets, Upstairs Lights, Downstairs Ring, and so on. Its job is to protect that circuit from overload (too much current being drawn) or from a short circuit (a direct fault in the wiring or an appliance). When it trips, only that one circuit is affected. Everything else keeps working.
An RCD (Residual Current Device) is the larger switch that typically sits over a group of MCBs. It monitors for earth faults — current escaping to earth in a way that could mean someone is receiving an electric shock. When an RCD trips, it usually cuts an entire side of the board. Multiple circuits go off at once.
One circuit off = MCB trip. Multiple circuits off at once = likely an RCD trip. The reset process and the cause are completely different. If multiple circuits have gone off together, see our article on RCD keeps tripping instead.
For a deeper look at how your consumer unit is laid out — and what the different types contain — see our guide to types of consumer unit.
Why a single MCB trips: two very different causes
A single MCB trips for one of two reasons. How you respond depends on which one applies.
Cause 1: Overload
The circuit was carrying more current than it is rated for. Too many appliances running at the same time on the same ring, or a single high-demand appliance pushed it over the limit. The MCB did exactly what it is designed to do — it disconnected the circuit before the wiring could overheat.
This is a normal, expected event. It does not mean anything is wrong with the wiring or the consumer unit. It means the circuit was overloaded at that moment.
Cause 2: Short circuit or wiring fault
A live conductor has come into direct contact with a neutral or earth conductor, either inside an appliance, at a socket or fitting, or in the wiring itself. This produces a very high fault current that trips the MCB immediately.
This is a different situation from overload. Something has actually gone wrong, and simply resetting the MCB will not fix it.
How to reset an MCB correctly
Open the consumer unit door. Look at the row of MCBs. The tripped one will usually have moved to a middle or OFF position while the others remain ON. It may also be visually different — some units have a coloured indicator window that changes when a breaker trips.
Check the label on the consumer unit to confirm which circuit has tripped — Kitchen, Upstairs Lights, Sockets, and so on. This tells you which part of the house is affected.
- Push the tripped MCB firmly to OFF.
- Then push it firmly to ON.
- If it holds, the circuit is restored. You are done.
- Watch whether it trips again in the next few minutes.
Think back to what was running on that circuit when it tripped. If it was a high-load scenario — several kitchen appliances at once, for example — that is almost certainly the cause. Spread the load differently going forward, and the MCB should not trip again under normal use.
If it trips immediately on reset — stop
If the MCB trips the moment you push it to ON, or within seconds, this is not an overload. An overload takes time to build up. An immediate trip means there is an active fault on the circuit right now — a short circuit, a damaged appliance, or a wiring problem.
Every time you reset an MCB into an active fault, you are forcing current through a fault path. This can cause damage to wiring, overheat connections, or create a fire risk. The MCB is doing its job by tripping — do not override it by resetting repeatedly. Diagnose the cause first.
Identifying what is on that circuit
Before you can work out the cause, you need to know what the tripped MCB feeds. Look at the label on the inside of the consumer unit door — most boards label each MCB with the circuit it protects. Common labels include: Kitchen Sockets, Ring Main, Upstairs Lights, Downstairs Lights, Immersion, Oven, Garage.
Once you know the circuit, walk around that part of the house. Which sockets have gone dead? Which lights are off? This tells you the scope of what is affected, and it helps you identify which appliances were on the circuit at the time of the trip.
The overload scenario in detail
The classic overload case is a kitchen ring. A kettle, microwave, and toaster running at the same time on the same circuit will often exceed the rated current for that ring. The MCB trips, the kitchen sockets go off, everything else in the house is fine.
Ask yourself: what was running on that circuit when it tripped? Were multiple high-draw appliances on at the same moment? If the answer is yes, overload is the almost certain cause.
The solution is straightforward: spread the load. Move one appliance to a socket on a different circuit (a socket in a different room, or one that feeds from a different MCB). Reset the MCB, and do not run those combinations at the same time going forward.
If the same MCB is tripping regularly under normal use — without running multiple high-draw appliances — it may be that the circuit is simply under-rated for how the room is being used, or that an appliance is beginning to draw more current than it should. An electrician can assess whether an additional circuit would be the right solution.
The fault scenario: how to diagnose it
If the MCB trips immediately on reset, or trips without any obvious overload, work through this process:
- Unplug everything on that circuit. Switch off any hardwired appliances (cooker, immersion, extractor fan) at their local isolator or switch.
- Reset the MCB. If it trips again with nothing connected, the fault is in the wiring or in a hardwired fitting — not in a plug-in appliance. Stop here and call an electrician.
- If it holds with everything unplugged, plug appliances back in one at a time, waiting a moment between each one. The appliance that causes the trip when you plug it in is the faulty one.
- Remove that appliance from use and have it checked or replaced. The MCB should now hold with everything else running normally.
If the MCB trips even with nothing plugged into the circuit, do not attempt to diagnose further yourself. This requires an electrician to carry out insulation resistance testing and a proper inspection of the wiring and any hardwired fittings. Call us or book our emergency electrician service if you need same-day attendance.
When an RCBO trips vs a plain MCB
If your consumer unit is a modern fully-protected board, you may have RCBOs rather than plain MCBs. An RCBO (Residual Current Breaker with Overcurrent protection) combines the functions of both devices in a single unit. Each circuit gets its own individual earth fault protection, rather than sharing an RCD with a group of circuits.
An RCBO looks slightly wider than a standard MCB, and it has a small test button (usually labelled T or Test) on the face of the unit. The trip behaviour looks similar to a plain MCB — only one circuit goes off — but the cause may be an earth fault rather than just an overload or short circuit. If you have RCBOs, the reset process is the same: push to OFF, then to ON. But if it trips immediately, the cause could be an earth fault on that circuit, not just an overloaded appliance.
For a full explanation of how RCBOs work and how they differ from RCDs and MCBs, see our article What is an RCBO?
If you are unsure whether your board has plain MCBs, RCBOs, or a mix of both, check whether any of the breakers have a small test button on the face. If they do, those circuits have RCBO protection. Our guide to types of consumer unit explains the different configurations in detail.
MCB tripping and you can’t find the cause?
If the MCB trips immediately on reset, or keeps tripping under normal use, there is a fault that needs diagnosing. Our team can attend, carry out the relevant testing, and give you a clear picture of what is wrong and what needs to be done.