What is this?
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.
Common causes
- 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 to call an electrician
Call an electrician if the MCB trips with nothing on the circuit, if it trips immediately on reset, if you cannot identify the cause, or if the MCB label is missing and you do not know which circuit is affected. Do not replace an MCB yourself unless you are competent and qualified — work inside a consumer unit requires a Part P-qualified electrician.
What will an electrician do?
Identify the affected circuit and its full extent
Test the MCB to verify it is operating within specification
Carry out loop impedance testing to check for short circuits
Perform insulation resistance testing to check cable condition
Inspect all connections on the circuit for looseness and overheating
Repair the fault and test the circuit to confirm it is safe
Typical cost
£80 – £200
Straightforward MCB-related faults. If the MCB itself needs replacing, add £10–£30 for the device. Complex wiring faults requiring access behind walls will cost more.
