How to fault-find a ring circuit

Video reference. Matt from the Electrician Life channel shows a real-world ring circuit fault-find in his video "Ring Circuit Fault Finding Plus Green Goo", where a socket circuit was tripping the RCCB on a split-load consumer unit. The "green goo" turns out to be corrosion inside a socket back box -- a useful reminder that fault-finding in older UK homes often ends somewhere unexpected. The video is UK-specific, using correct terminology for ring final circuits, split-load boards and RCCBs.

1. Confirm which device has tripped

Go to your consumer unit and look for the tripped device. It will either be flipped to the off position (for an MCB or RCBO) or sitting in a mid-position (for an RCD protecting a group of circuits).

Note the type of device:

• MCB tripped: the circuit has drawn too much current, or there is a short circuit (live touching neutral or earth). Overloaded appliances and failed heating elements are the most common cause.
• RCD or RCBO tripped: the circuit has detected a leakage to earth -- current flowing somewhere it should not, often through a faulty appliance, a damp connection or damaged cable insulation.
• RCCB (the large shared RCD) tripped: any circuit under that RCCB could be the cause, not necessarily the one you suspect.

2. Switch off and unplug everything on the circuit

Before resetting any device, go round every socket on the affected circuit and switch off and unplug every appliance, extension lead and power strip. Switch off at the wall, then unplug. A switched-off but still-plugged-in appliance with a faulty element can still cause an RCD to trip.

If you are not sure which sockets are on the circuit, unplug everything in the affected area to be safe.

3. Reset and test with nothing connected

With everything unplugged, reset the tripped device at the consumer unit. There are now two possible outcomes:

• The device holds in the on position: the fault is in an appliance or in a socket outlet, not in the cable wiring itself. Continue to step 4.
• The device trips immediately again: the fault is in the fixed wiring -- a cable, connection or socket back box. This is not a DIY fault-find and you need an electrician.

If it holds, you are now powered with no load. Move to the next step.

4. Plug in one appliance at a time

Starting with the appliances most likely to cause a fault, plug them in and restore power one at a time, leaving a minute between each to see if the circuit holds. The order to try first:

1. Washing machine
2. Dishwasher
3. Tumble dryer
4. Electric oven or hob (if on the same circuit)
5. Fridge and freezer
6. Other fixed appliances

When the circuit trips, the last appliance you plugged in is the prime suspect. Try it on a different circuit (via an extension lead) to confirm it is at fault. If the appliance causes a trip on another circuit too, it needs to be repaired or replaced. Do not keep using it.

5. Check each socket outlet with a socket tester

If the circuit holds with all appliances unplugged but you want to confirm the socket outlets themselves are correctly wired, plug a basic socket tester into each one in turn. A standard plug-in socket tester shows three coloured lights that indicate correct wiring, reversed live and neutral, missing earth, and similar faults. Any indicator other than the "correct" pattern is worth investigating.

Pay particular attention to sockets in kitchens, utility rooms, garages and conservatories -- areas where moisture can get into back boxes over time.

6. Inspect for corrosion inside the back box

With the circuit isolated at the consumer unit and confirmed dead, unscrew the faceplate from any socket that showed an abnormal reading, or any socket you suspect based on location. Ease the faceplate away from the back box and look at the conductors and terminals.

Healthy wiring looks clean: copper conductors are bright or pale orange, and terminal screws are silver-coloured. What to look for instead:

• Green or turquoise deposits on copper conductors -- verdigris from long-term moisture. This is the "green goo" that trips circuits intermittently, especially when condensation is present. The conductor surface area carrying current is reduced, resistance rises, and a fault to earth develops.
• White powder on or near terminals -- a sign of arcing or oxidation at a loose connection.
• Discoloured or brittle cable insulation -- heat damage, possibly from a loose terminal that has been arcing for some time.

Minor surface corrosion on the terminal screws can be cleaned. Corroded conductors, brittle insulation or melted plastic need an electrician before the circuit is used again.

When to call us

Ring circuit faults in Sandwich, Deal, Dover and the surrounding area. If the unplug test does not clear the fault, or if you find corrosion in any back boxes, Richard can carry out a proper insulation resistance test, use a loop tester to verify the earthing, and identify exactly where the fault lies. Small jobs at £10 per 10 minutes.