Intermittent faults are the most frustrating category for everyone involved. The lights flicker for a week and then stop. The board trips on Sunday mornings. A socket works except when it rains. Nothing is wrong when the electrician arrives. The key insight is that intermittent faults almost always have a trigger, and identifying the trigger is more valuable than any amount of testing on a good day.
Why faults come and go
Three mechanisms explain most intermittent behaviour. Thermal: a loose connection heats up under load, expands, makes or breaks contact, then cools. Moisture: water ingress into an outdoor fitting creates a leakage path that dries out afterwards. Load dependent: the fault only appears when a particular appliance runs or when total demand rises above a certain point.
Each of these has a signature. Thermal faults appear after a period of heavy use. Moisture faults track the weather. Load dependent faults track appliance use and time of day.
Keeping a fault diary
The single most useful thing a householder can do is write down each occurrence. Date, time, weather, what was switched on, what tripped, how long it lasted and what made it stop. Two weeks of that turns an unsolvable problem into a targeted investigation.
- Date, time and duration of each occurrence
- Weather conditions, particularly rain
- Appliances running at the time
- Which device tripped, and whether it reset immediately
- Anything that reliably reproduces it
Moisture related faults
Outdoor sockets, garden lights, external security lighting and cables running through damp walls or outbuildings are the usual candidates. The signature is unmistakable once you look for it: trips that cluster around wet weather and stop during dry spells.
Diagnosis is straightforward once the pattern is known: isolate the outdoor circuit and see whether the problem stops. Repair is usually replacement of the affected fitting and improvement of the gland or cable entry that let water in.
Thermal and loose connection faults
Loose terminations produce heat, which is why discoloured plates and a faint smell of hot plastic are important warnings. These faults are dangerous and should not be left. A visual inspection of accessories on the affected circuit, with terminations checked and retightened, often resolves them.
They tend to appear on circuits with sustained loads such as immersion heaters, showers, kettles and heaters, and are more likely at older accessories that have been in service a long time.
Cumulative leakage
Where several appliances each leak a small current to earth, the total can sit just below the trip threshold and cross it when one more device is switched on. Nothing individually is faulty, so appliance by appliance testing finds nothing.
This is measured with a clamp meter on the affected circuit, recording standing leakage. The solution is usually to redistribute circuits, add per circuit protection or replace the worst offending appliance.
Monitoring when the fault will not show
Where an intermittent fault refuses to present, monitoring equipment can be left in place to record leakage or voltage disturbances over days. That converts the diary into measured data and usually identifies the circuit conclusively.
An alternative is a structural change: splitting circuits across individual protective devices so that the next occurrence isolates itself to one circuit. This is useful work in its own right and it makes the next event diagnostic.
When to stop using a circuit
Some intermittent symptoms are warnings rather than nuisances. A burning smell, scorch marks, a socket that is warm to the touch, buzzing or crackling from an accessory, or a device that trips and will not reset should mean the circuit stays off until it has been inspected.
Repeatedly resetting a device that keeps tripping is not a fix. The device is doing its job and is telling you something.
Setting expectations
Intermittent fault finding is honest detective work and occasionally takes more than one visit. A reasonable arrangement is an initial diagnostic visit with an agreed time limit, a written summary of what was measured and eliminated, and a plan for the next step if the fault has not presented. That way you are paying for progress rather than for repeated guesswork, and each visit starts from where the last one finished.
Common questions
- Why does it only trip when it rains?
- Almost certainly water ingress into an outdoor fitting or a damp cable route.
- Should I keep resetting the breaker?
- No. Repeated tripping is a warning. If it will not stay in, leave it off and get it inspected.
- Can you find a fault that is not happening?
- Often yes, using leakage monitoring over time or by splitting circuits so the next event identifies itself.
- What causes flickering that comes and goes?
- Loose connections, incompatible dimmers and LEDs, or supply side issues affecting the whole property.
- Is an intermittent fault dangerous?
- It can be, especially if heat, smell or scorching is involved. Those should be treated as urgent.
Keep a diary for a fortnight and send it over. It genuinely shortens the visit and reduces the cost.
Related guides
- How Electricians Actually Find Electrical FaultsFault finding is a process of elimination, and the more information you can give at the start, the fewer steps it takes.
- Flickering Lights: Causes and FixesFlicker is a symptom with a short list of causes — and one of them is genuinely worth acting on quickly.
- Why Does My RCD Keep TrippingAn RCD trips because it detected earth leakage. It is doing its job.