Helpful video reference. UK Electrics walk through a real domestic fault-finding call in "Electrical Fault Finding - Surprising Find - UK electrics". Worth watching for the methodical approach and the reminder that what looks like the obvious cause often is not the real one.
1. Log the fault pattern before touching anything
An intermittent fault almost always has a pattern. Start a log: note the date, time, what failed or tripped, what was switched on at the time, and the weather (damp conditions can affect joints in outdoor or loft circuits). Three or four entries are usually enough to start seeing a picture.
Common patterns to look for: fault only happens under heavy load (overload or a loose terminal heating up), only in damp weather (water ingress), only at a particular time of day (a specific appliance on a timer), only after vibration (a loose conductor in a back box near a door).
2. Check the consumer unit first
Open the consumer unit cover and look at each MCB, RCBO and RCD. A tripped MCB sits at a visually different position to a healthy one -- usually slightly down or to a mid-point between on and off, depending on the board type. An RCD that has been sat very close to its trip threshold can be almost impossible to distinguish from a healthy one by eye.
Note which circuits share an RCD. If a fault is causing an RCD to trip rather than the MCB for that circuit, the RCD covers multiple circuits and the fault could be on any of them.
3. Isolate the circuit under suspicion
If the log points to one circuit, switch that MCB or RCBO off and unplug every appliance connected to it. Restore power. If the fault clears, add appliances back one at a time with the circuit energised. When the fault returns, the last appliance you added is the most likely culprit. Test that appliance on a different circuit to confirm.
If the fault does not clear with all appliances unplugged but the circuit live, the fault is in the fixed wiring, not an appliance. Move on to step 4.
4. Inspect accessible accessories on the circuit
Turn the circuit off at the consumer unit and prove dead with your voltage tester. Work along the circuit opening every socket and switch. Look for:
- Conductors that are loose in their terminals or pulling out under gentle tension
- Discolouration or black marks around terminals (arcing)
- Melted or brittle insulation on the conductors inside the back box
- Any connector block or Wago connector where the cable appears to be barely gripping
A loose terminal that only makes partial contact will run warm, eventually carbonise the surrounding insulation, and cause intermittent loss of power on that circuit. This is the most common cause of intermittent faults in older UK homes.
5. Use a multimeter to find a fractured conductor
If you found a loose terminal at step 4, retighten it and test the circuit. If you did not find anything visible but the fault remains after isolating appliances, the problem may be a fractured conductor inside the cable sheath, typically near a socket where the cable is regularly disturbed.
Set your multimeter to continuity mode. With the circuit isolated, connect one probe to the conductor at one end and the other probe to the same conductor at the far end. Flex and bend the cable at likely stress points. An intermittent beep or a continuity reading that drops to open circuit when the cable is in a certain position confirms a broken conductor inside the sheath. That section of cable needs replacing.
6. Fix what you found, or hand over
Retighten any loose terminals firmly. If a socket or switch has obvious arcing damage, replace the accessory -- the terminal contact surface will be pitted and will not hold reliably even after tightening. If a junction box or connector has failed, replace it with an accessible enclosure under BS 7671.
If you found a fractured conductor, burnt insulation, or the fault is in a location you cannot access safely (inside a ceiling void, under a floor, or at a ceiling rose), stop and call an electrician. Do not leave a circuit with known arcing damage in service.
When to call us
Richard specialises in fault-finding across east Kent. If a systematic approach leads you to suspect wiring in the fabric of the building rather than a loose accessory, that is the point to call.
Got a fault you cannot pin down in Sandwich?
Intermittent faults are what Richard spends a good part of his week tracking. If the pattern leads you to suspect fixed wiring rather than a loose accessory, call.
Contact Richard