Circuit Breaker Keeps Tripping?
Causes, Diagnosis & Fixes

Before diagnosing why your circuit breaker keeps tripping, you need to identify which device is actually tripping. In a modern UK consumer unit (fuse board), there are two types of protective device that can trip: MCBs and RCDs. They protect against completely different hazards, and the diagnostic approach is different for each.

Some modern consumer units use RCBOs (Residual Current Breaker with Overload protection), which combine both MCB and RCD functions into a single device. An RCBO can trip for either overcurrent or earth leakage. Most RCBOs have a trip indicator that shows which protection operated — check this before starting your diagnosis, as it determines whether you are dealing with an overcurrent problem or an earth leakage problem.

RCDs are also classified by type. A Type AC RCD responds only to sinusoidal AC residual currents. A Type A RCD additionally responds to pulsed DC components — the type of leakage produced by modern appliances with electronic controls, such as washing machines, dishwashers, and EV chargers. Per BS 7671 Reg 531.3.4.201, Type A (or Type F/B for more complex waveforms) is the appropriate selection for circuits supplying such equipment, as a Type AC device may not reliably detect pulsed DC leakage. This is relevant to diagnosing an RCD that trips specifically when a modern appliance is switched on.

When an MCB trips, the first step is to identify exactly which circuit is affected. In a labelled consumer unit, the tripped MCB switch will be in the middle or OFF position and the circuit label tells you which circuit it protects (e.g., "Kitchen Sockets", "Upstairs Lights", "Shower"). If your consumer unit is not labelled, identifying the circuit requires a process of elimination.

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Certain circuits in UK homes are far more prone to overloading than others. Understanding which circuits are at risk helps you diagnose the cause quickly and advise on the correct solution.

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Determining whether the fault is in an appliance or in the fixed wiring is the critical fork in the diagnostic process. The answer determines whether the homeowner can resolve the issue themselves (disconnect the faulty appliance) or whether a qualified electrician is needed.

For electricians, a definitive diagnosis requires insulation resistance testing at 500 V DC on the affected circuit with all equipment disconnected. The acceptance criterion per BS 7671 Reg 643.3.3 is a minimum of 1 MΩ — a reading below this between live and neutral (L-N) confirms a short circuit in the fixed wiring, and below 1 MΩ between live and earth (L-E) confirms an earth fault. GN3 Reg 2.21 confirms that 1 MΩ is the regulatory minimum: any reading at or above this value conforms to the Regulations. Where connected equipment is present and likely to influence the test result, BS 7671 Reg 643.3 requires a 250 V DC test to be used instead of the full 500 V test.

When verifying earth fault loop impedance (Zs) on the same circuit during live testing, note that measured values are taken at ambient temperature. Per BS 7671 Reg 253 (Appendix 3), a measured Zs is acceptable where it satisfies Zs(measured) <  0.8 × (U₀/I × C_min). In practice, electricians commonly express this as ensuring the measured Zs does not exceed 80% of the tabulated maximum — the 0.8 factor accounting for the increase in conductor resistance at operating temperature compared with the ambient-temperature measurement.

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While identifying a faulty appliance can be done by a homeowner using the process of elimination described above, several situations require a qualified electrician with calibrated test instruments.

• The MCB trips immediately on resetting with all appliances disconnected — this is a short circuit or earth fault in the fixed wiring
• You can see or smell evidence of burning, scorching, or overheating at any socket, switch, junction box, or the consumer unit itself
• The MCB trips intermittently with no obvious pattern and the process of elimination has not identified a single appliance as the cause
• The tripping started after building work, renovation, or DIY — a cable may have been damaged during the work
• The property has old wiring (rewirable fuses, no earth wire, fabric-insulated cables) — a full Electrical Installation Condition Report (EICR) is recommended

A competent electrician will carry out dead tests (continuity, insulation resistance) and live tests (earth fault loop impedance, prospective fault current) to identify the exact cause and location of the fault. If the circuit is found to be overloaded, the electrician will recommend either redistributing loads or installing additional circuits.

When a circuit breaker keeps tripping, the temptation is to find a quick fix. Some temporary measures are safe and reasonable while waiting for a proper repair. Others are extremely dangerous and must never be attempted.