An RCBO trips when the circuit it protects has an overload, short circuit, or earth leakage fault. Because each RCBO covers one circuit, you already know which circuit to investigate — a practical guide to narrowing the fault and reaching a defensible fix.
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Written and reviewed by Andrew Moore, founder of Elec-Mate, against BS 7671:2018+A4:2026, IET Guidance Note 3 and the IET On-Site Guide.
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An RCBO protects one circuit rather than a whole bank, so when it trips you already know the problem is tied to that one circuit. That makes the diagnosis cleaner than a split-load RCD trip, because you are not trying to decide which of several circuits caused the outage.
The fault could still be overload, short circuit, or earth leakage, but the search area is smaller. That usually means quicker isolation, quicker repair, and better continuity for the rest of the installation.
If the RCBO trips immediately on reset, think hard fault or solid leakage. If it trips only when a shower, immersion, or outside circuit is used, that points to demand or load-linked leakage. If it trips after rain or at certain times of day, moisture and environment move up the list quickly.
Recent kitchen work, a new outside light, a replaced socket, or a recently fitted appliance often matters more than broad theory. The last thing changed is worth checking early.
If thorough testing finds no overload, short circuit, or measurable earth leakage, arcing is a recognised cause of nuisance trips. BS 7671:2018+A4:2026 Reg 421.1.7 recommends arc fault detection devices (AFDDs) for AC final circuits to mitigate fire risk from arc fault currents. Where repeat unexplained trips occur on an otherwise sound circuit, an AFDD upgrade is worth documenting as a recommendation.
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Because the issue sits on one circuit, the close-out should be very clear: circuit description, defect found, remedial action, and test result after repair. If the client declines deeper investigation, note that limitation directly instead of leaving the job vague.
If the call-out develops into broader inspection work, move it into the EICR certificate workflow so the observation, recommendation, and sign-off stay together.
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