RCD Fault Finding Checklist

Purpose

To provide a structured, practical method for identifying why an RCD is tripping and locating the fault safely and efficiently.

Common Reasons An RCD Trips

Earth leakage on a circuit (most common cause)
Faulty appliance plugged into a socket circuit
Damaged cable (nails, screws, heat, crushing)
Moisture ingress into accessories or outdoor equipment
Neutral-to-earth fault anywhere on the protected side
Incorrect neutral connection (borrowed neutral / mixed neutrals)
Faulty RCD device
Overcrowded or poorly maintained wiring causing leakage
External equipment faults (garden lighting, outbuildings, showers, pumps)

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Step-by-Step Fault Finding Procedure

1. Confirm The Fault

Check what exactly is tripping:

 * RCD main switch
 * Individual RCBO

Note if it trips instantly or after a delay.
Try to identify when it started (recent work, weather change, new appliance).

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2. Initial Isolation

Switch OFF all MCBs/RCBOs on the affected RCD.
Reset the RCD.
If it holds, the fault is downstream on a circuit.
If it still trips with everything off, suspect:

 * Faulty RCD
 * Neutral issue in the board

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3. Re-Energise One Circuit At A Time

Turn circuits back on one by one.
Wait after each circuit is energised.
Identify which circuit causes the RCD to trip.

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4. Isolate The Problem Circuit

Leave the faulty circuit OFF.
Confirm other circuits operate normally.
Focus investigation only on the suspect circuit.

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5. Break Down The Circuit (Section Testing)

Disconnect all loads (sockets, appliances, accessories).
Test RCD again with circuit energised but unloaded.
If it now holds:

 * Fault is in a device or appliance

If it still trips:

 * Fault is in fixed wiring

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6. Appliance Check (If Socket Circuit)

Unplug everything on the circuit.
Pay attention to:

 * Washing machines
 * Dishwashers
 * Kettles
 * Outdoor tools and garden equipment

Plug appliances back in one at a time to identify the faulty item.

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7. Fixed Wiring Checks

Visually inspect:

 * Sockets (especially outdoor or damp areas)
 * Junction boxes
 * External lighting

Look for signs of:

 * Water ingress
 * Heat damage
 * Crushed or pinched cables

Check for neutral-to-earth contact faults.

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8. Test For Cable Faults

Carry out insulation resistance testing on suspect circuit.
Test between:

 * Live and earth
 * Neutral and earth
 * Live and neutral

Look for low insulation resistance readings indicating leakage.

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9. Check For Neutral Issues In Board

Inspect neutral bar carefully.
Look for:

 * Loose neutrals
 * Borrowed neutrals between circuits
 * Incorrect neutral pairing on RCBOs

Especially common after consumer unit changes.

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10. RCD Condition Check

If no fault is found in wiring or appliances:

 * Test RCD using appropriate test equipment
 * Check trip times and sensitivity

Replace RCD if it fails tests or behaves inconsistently.

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Common Real-World Causes

Outdoor socket full of water
Fridge/freezer developing earth leakage when compressor starts
Boiler or immersion heater insulation breakdown
Damaged garden lighting cable
Washing machine heating element leaking to earth
Shared neutral mistakes after alterations

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JME Standard

All RCD faults must be traced systematically by elimination, ensuring circuits, appliances, and protective devices are checked in a controlled and safe manner before replacement or repair decisions are made.