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Fault Finding — Common Electrical Faults

Purpose

This document covers how to diagnose and resolve the most common electrical faults — tripping MCBs/RCDs, dead circuits, overheating accessories, and wiring faults. Electrical fault finding must always start with safe isolation.

Safety First

Always prove dead before working on any conductors.

  1. Switch off the circuit at the consumer unit
  2. Use a voltage indicator to confirm the circuit is dead at the point of work
  3. Where possible, lock off or remove the MCB/RCBO

Do not probe live conductors with generic multimeter probes. Use a calibrated test instrument for any live testing.

MCB / RCBO Tripping

Tripping immediately on reset — will not stay on

Cause: Short circuit or overcurrent on the circuit.

  1. Switch off the breaker and disconnect all loads from the circuit
  2. Reset — does it stay on now?
    • Yes: Fault is in a connected appliance — reconnect one at a time to find it
    • No: Fault is in the fixed wiring

Testing the wiring:

  • Disconnect the circuit at the consumer unit
  • Insulation resistance test: L-E and N-E — below 1MΩ confirms a wiring fault
  • Near-zero L-N resistance confirms a short between live and neutral
  • Locate by sectioning the circuit at junction boxes or FCUs

RCBO tripping — trips but can be reset after a moment

Cause: Earth leakage — current flowing to earth, triggering the 30mA RCD.

  1. Disconnect all appliances
  2. Reset — does it hold?
    • Yes: Fault in an appliance — reconnect one at a time
    • No: Wiring fault — insulation breakdown; use IR test to locate

Common RCBO trip causes:

  • Washing machine, dishwasher, or fridge with failing heating element
  • Outdoor socket with moisture ingress
  • Garden lighting with damaged cable
  • Extension reel run coiled (overheating, insulation degradation)

RCBO tripping only in damp weather or at night

Moisture is entering the circuit. Common locations:

  • Outdoor socket with damaged cover or unsealed cable entry
  • External lighting with cracked lens or seal
  • Cable through an external wall inadequately sealed

Dead Socket / Lighting Circuit

A socket or light is not working but the breaker has not tripped.

Check 1 — Is the breaker actually on?

RCBOs have an intermediate position — they appear on but are actually tripped at the RCD part. Push firmly to off, then firmly to on.

Check 2 — One device or whole circuit?

  • One socket/light: Loose connection in the back box is the most common cause. Check the socket with a socket tester.
  • Whole circuit dead: Use a voltage indicator at the consumer unit to confirm the RCBO output is live. If it is, the fault is in the first connection on the circuit.

Tracing a dead circuit

With the circuit on, test voltage at the first accessible junction. If voltage is present, move further along until the fault is found.

Common causes of dead sockets:

  • Loose terminal at the socket — works loose with use
  • Broken cable at a back box — cable pulled during decoration or furniture moving
  • Blown FCU fuse — replace (typically 3A or 13A)

Common causes of dead lights:

  • Blown lamp — test this first
  • Blown fuse in batten holder or FCU
  • Loose connection at the ceiling rose
  • Failed dimmer switch

Overheating Socket or Switch

A socket or switch is hot to the touch or shows burn marks or discolouration.

This is a C1 or C2 situation — do not leave the circuit energised if there are signs of burning or arcing.

Common causes:

  • Loose terminal — most common cause of overheating at accessories. Resistance from a loose connection generates heat. Retighten to torque.
  • Overloaded circuit — too many high-current loads continuously. Check what is connected.
  • Faulty appliance — an appliance with an internal fault drawing excess current.

Action: Replace any accessory showing burn marks or physical damage. Resolve the root cause — do not simply replace the faceplate.

Nuisance Tripping — Intermittent RCD/RCBO

The RCBO trips occasionally but not consistently — no obvious fault when inspected.

  1. Identify the pattern — when does it trip? At certain times? When a specific appliance starts? In wet weather?
  2. Check connected appliances — washing machines, dishwashers, tumble dryers develop progressive earth leakage as heating elements age. Note if tripping stops when the appliance is disconnected.
  3. Check outdoors — external circuits trip more in damp conditions. Inspect all outdoor accessories.
  4. Cumulative leakage — on circuits with many LED drivers or motors, the combined leakage of multiple devices can exceed 30mA even though each is individually within limits. Redistribute loads across multiple circuits.

No Power After Consumer Unit Replacement

All circuits are dead after a new board was fitted.

Check in this order:

  1. Is the main switch on?
  2. Are all RCBOs switched on?
  3. Test voltage at the main switch input — is the supply present from the meter? If no voltage here, the DNO supply or meter has a fault (outside Wilsons' scope)
  4. Test voltage at each RCBO output — voltage present but not at the socket/light means a circuit wiring fault

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