Where this starts
A protective device that trips has detected something. The useful first question is not why it keeps happening but WHICH device operated, because each type responds to a different condition and identifies a different family of causes.
An OVERCURRENT device — a circuit breaker or fuse — sees more current than the circuit is rated to carry, either as a sustained overload or as a sudden short-circuit. A RESIDUAL-CURRENT device sees an imbalance between the current going out and the current coming back, which means some of it is returning by another route: through earth, through a fault, or possibly through a person. An ARC-FAULT device sees the electrical signature of arcing, which can start a fire while drawing too little current to trip anything else.
The second useful question is WHEN, because the timing separates most causes on its own: immediately on closing the breaker, when a particular appliance is switched on, only under load after some minutes, or at no predictable moment.
The instruction that comes before all of it: a device that trips is working. Replacing it with a higher-rated one, or holding it closed, removes the protection without removing the fault — and the fault is a fire or a shock risk rather than a nuisance.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
Too much load on the circuit
Most commonAn overcurrent breaker, tripping after minutes rather than instantly, and reproducibly when the same combination of appliances is running.
How to check: Note what was running each time it tripped. A pattern that repeats with the same appliances together is an overload rather than a fault.
A faulty appliance leaking to earth
Most commonA residual-current device rather than the breaker, and it trips when one particular appliance is switched on or reaches a stage of its cycle.
How to check: Unplug everything on the affected circuit, reset, then reconnect one appliance at a time. The one that trips it is identified rather than suspected.
Accumulated leakage from several appliances
CommonA residual-current device protecting a group of circuits, tripping with no single appliance responsible — each is within limits and the sum is not.
How to check: Whether the device protects one circuit or several. A whole-board device covering many circuits is the arrangement in which this happens.
Water where it should not be
CommonA residual-current device, and a pattern tied to weather, to an outdoor socket or light, or to a bathroom or kitchen in use.
How to check: Isolate outdoor and wet-area circuits and see whether it holds. Tripping that follows rain is close to conclusive.
A damaged cable or a loose connection
Less commonUnpredictable timing with no appliance pattern, sometimes with a smell of hot plastic, discoloured accessories, or flickering on the same circuit.
How to check: This one is not a homeowner check. Visible scorching or a burning smell at an accessory means stop and get it looked at.
A failed or aged protective device
UncommonTripping with nothing connected and no load, after everything else has been eliminated.
How to check: Only after the causes above have been excluded — devices do fail, but assuming it first is how a real fault gets ignored.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It is the single most informative observation, and it costs nothing. A breaker, an RCD or RCBO, and an arc-fault device detect three different things and point at three different families of cause.
- 2
A reproducible pattern separates an overload from a fault without any test equipment, and a pattern tied to one appliance identifies it directly.
- 3
It converts a guess into an identification. If it holds with nothing connected, the fault is in an appliance; if it trips with nothing connected, it is in the installation.
- 4
Water ingress is a common and specific cause with an obvious signature, and it is worth eliminating before anything more involved.
- 5
Those are signs of a connection failing rather than a circuit being overloaded, and continuing to reset the device while that is happening is the scenario the device exists to prevent.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Any smell of burning, discoloured or warm sockets, switches or accessories, or scorch marks — stop resetting and get it inspected.
- A device that trips immediately with the circuit disconnected, which places the fault in the fixed wiring rather than in anything plugged into it.
- Anything requiring the consumer unit or panel to be opened. In several jurisdictions that work is restricted to competent persons, and the terminals inside remain live with the main switch off.
- An arc-fault device tripping at all, since the condition it detects is not visible and is not diagnosable by switching things off and on.
- Any repeated tripping you are tempted to solve with a larger device. That is the point at which the protection stops matching the cable, and the cable is what burns.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.