What does MCB stand for?

Miniature Circuit Breaker. It is the modern replacement for rewirable fuses. Each MCB in your consumer unit protects one circuit against overload and short circuit.

Can I replace an MCB myself?

A like-for-like MCB replacement is not automatically notifiable in England, but working inside a consumer unit can expose live incoming parts and requires correct device selection and circuit testing. Use a competent electrician.

Why does only one MCB trip and not others?

Each MCB protects a specific circuit. A fault or overload on one circuit does not affect other circuits. This is the advantage of MCBs over a single main fuse — they provide targeted protection.

What MCB ratings are normal in a house?

Typical ratings are 6A for lighting circuits, 32A for socket ring circuits, 32A or 40A for cooker circuits, and 40A or 50A for electric shower circuits. The rating must match the cable size on the circuit.

How do I know if my MCB is faulty?

An electrician can test the MCB with a calibrated test instrument. Signs of a faulty MCB include tripping below its rated current, feeling loose or rattling, not latching properly, or showing signs of overheating or discolouration.

Caution

Updated 30 August 2026

MCB Keeps Tripping

A specific MCB (Miniature Circuit Breaker) in your consumer unit keeps tripping off.

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What's happening?

An MCB protects a single circuit against overload and short circuit faults. Unlike an RCD which detects earth leakage, an MCB responds to excess current. When an MCB keeps tripping, the circuit it protects is either drawing too much current (overload) or has a short circuit. Since each MCB protects a specific circuit, only part of your home will be affected.

What usually causes it?

Circuit overload from too many appliances running at once
Short circuit from damaged cable insulation, a faulty socket, or a faulty appliance
Loose connection at the MCB terminal, a socket, or a junction box
Incorrect MCB rating for the circuit (too low)
Faulty MCB that trips below its rated current
Ageing wiring with deteriorating insulation that creates intermittent short circuits

Is it dangerous?

An MCB tripping is a protective action. The danger lies in the fault it is protecting against. Overloads cause cables to overheat, which is a fire risk. Short circuits generate high currents and can cause arcing. Do not bypass or uprate the MCB — fix the underlying fault.

Can I fix it myself?

Check which MCB has tripped and note its circuit label (e.g., 'kitchen sockets', 'upstairs ring'). Reset it. If it holds, it may have been a temporary overload — try to reduce the load on that circuit. If it trips immediately, there is a short circuit. Unplug everything on that circuit and try again. Plug items back one at a time to find the faulty one.

When should you call an electrician?

Call an electrician if the MCB trips with nothing on the circuit, trips immediately on reset, or the circuit cannot be identified. Do not work inside a consumer unit unless you are competent to isolate, select the correct device, install it safely, and test the circuit.