How to fault-find a tripping MCB in a UK home

Helpful video reference. We use Artisan Electrics' "DIY Fault Finding - Home Electrics" as the video reference here. Artisan Electrics are a Cambridge-based electrical contracting company with over 300,000 YouTube subscribers. Their video walks through fault-finding a domestic circuit failure in a practical UK setting, demonstrating the unplug-and-test method that isolates faulty appliances from wiring faults.

1. MCB versus RCD: make sure you are dealing with the right device

Before starting, check you are actually looking at an MCB (Miniature Circuit Breaker) rather than an RCD (Residual Current Device). An MCB protects a single circuit and typically controls one set of sockets, lights or a fixed appliance. An RCD covers multiple circuits and trips when it detects earth leakage.

MCBs and RCDs trip for different reasons:

• MCB trips: too much current (overloaded circuit), or a short circuit (dead fault between live and neutral).
• RCD trips: current leaking to earth (a shock risk from a faulty appliance or damaged cable).

The diagnosis process differs for each. If your whole upstairs or downstairs has lost power, you are probably looking at an RCD trip. If one specific circuit (say, the kitchen sockets) has gone dead while everything else is fine, it is likely an MCB. Check the consumer unit to be sure.

2. Identify the tripped MCB and what it covers

At the consumer unit, look for the MCB in the middle position, sitting between ON and OFF. It will not sit cleanly in either position. In some makes the button or switch also turns a different colour (often orange or red) when tripped.

Open the consumer unit cover (you do not need to open the inner enclosure) and check the circuit schedule. This should be a table on the inside of the door listing which MCB covers which rooms or appliances. If there is no schedule, use the opportunity to make one later. It saves a lot of guesswork next time.

3. Unplug everything on that circuit

Before you attempt to reset the MCB, go round every socket on the tripped circuit and unplug everything. This includes items you may not think of as high-wattage: phone chargers, televisions and computers all draw current. For a kitchen ring, unplug the kettle, microwave, toaster, dishwasher (if plugged in) and anything else connected.

If the circuit covers a fixed appliance (like an oven) with its own MCB, the oven is the suspect appliance if its dedicated MCB has tripped. An oven cannot be unplugged in the usual sense, so leave the switch off at the cooker control unit.

4. Try resetting the MCB

With everything unplugged, push the MCB to the fully OFF position (some need a firm push downwards or to the left) and then back to ON. It should click into the ON position and stay there.

If the MCB will not stay in the ON position with absolutely nothing connected to the circuit, there is a fault in the fixed wiring. Stop here and call an electrician. Trying to reset it repeatedly will not help and may make a diagnosis harder.

If it holds with nothing connected, the fault is load-related. Move on to step 5.

5. Reintroduce appliances one at a time

Plug appliances back in one at a time, switch each on, and wait a few seconds before plugging the next one in. The moment the MCB trips, you have found the culprit.

Put the faulty appliance to one side. Do not use it again until it has been properly tested or repaired. A PAT test can confirm whether it is safe to continue using.

If you manage to reconnect all appliances without triggering a trip, the fault was a momentary overload at the time the MCB first tripped. A motor starting under load (a fridge compressor, a tumble dryer beginning a cycle) can pull several times its running current for a fraction of a second. This should only trip a B-curve MCB under unusual circumstances, but it does happen with older or borderline MCBs.

6. Check whether you are overloading the circuit

If the MCB trips again when all appliances are connected and running together, the circuit is overloaded. The MCB is doing its job by protecting the cable.

Add up the wattage of everything running at the same time on that circuit. A typical kitchen ring final circuit has a 32 A MCB, which protects cables rated for roughly 7,200 W (32 A x 230 V). That sounds generous, but a kitchen with a kettle (2.4 kW), toaster (1 kW), microwave (1 kW) and plug-in oven (2.5 kW) is already at 6.9 kW, and a fridge plus dishwasher heating cycle could push it over.

The solution is not to fit a higher-rated MCB. That would put the cable at risk of overheating. The solution is either to run fewer appliances simultaneously or to have a second circuit added by an electrician.

7. Inspect the suspect appliance

If a single appliance caused the trip, examine it carefully before deciding what to do. Check:

• The plug fuse (BS 1362 cartridge fuse). A blown fuse inside the plug is often the first indication of an internal fault.
• The flex, particularly near the plug and where it enters the appliance. Damaged insulation can cause a short circuit.
• The body of the appliance for scorch marks, burning smells, or obvious damage.

Replacing a blown plug fuse is straightforward, but only do so if the appliance appears otherwise undamaged. Replacing the fuse in a faulty appliance may trip the MCB again immediately.

When to call us

The unplug-and-reset test is something any careful homeowner can do in a few minutes. Where it becomes an electrician's job is when no single appliance causes the trip, when the MCB trips with nothing connected, or when the circuit simply cannot handle the load you need from it. Those situations call for proper testing with instruments rather than a trial-and-error approach.