Faulty wire on ring main tripping fuse identified, but what could cause it? (UK)

Question

Background

For the last 2 weeks the downstairs socket ring's breaker has been tripping (sometimes just the RCD would be down but most of the time it would be the ring's breaker and the RCD). First it was every couple of days then once a day. I had ruled out all the electrical items plugged in one by one and took the sockets of a wall that was a bit damp just in case it was that. This morning it wouldnt turn back on so I took out the multimeter split the ring and tested around the ring, disconnecting the lives until I found the 2 sockets where there is a live leg where the multimeter says its in contact with earth. I have now reconnected everything else and the ring is working again (although its not a ring anymore as I have broken the circle) On the bad wire Neutral to live is fine and so is neutral to earth.

The wire in question is newish when we renovated our house 5 years ago, the wires goes via the ceiling.

What can cause this to happen?

Water/leak from the bathroom?

If that was the case I would expect our spotlights to have issues (as we have lots in that area) and why would there be a break in the wire? Surely burying a junction box in the ceiling is a bad idea...?

Rodent / animal damage? The renovation was extensive and we put new wooden floors and skirting everywhere so I can see how rodents would get in (we have never seen any signs of them)

Any other ideas?

EDIT Update: I have reconfigured the ring so its a ring again but 1 socket now has 2 sockets spurred off of it (where the wire between the spurs is the disconnected faulty wire). So although I know its not ideal to have 2 spurs on 1 socket the sockets in question will never take much load (and have been labelled inside for suture owners )

Answer 1

You have taken the right steps to trace and identify the area of the problem, and the nature of the fault, but you still cannot know the source of the fault without further inspection.

Many things, including those you have identified, can cause faults like this. Yes, burying a junction box in the ceiling is inconvenient and inappropriate if it is not accessible for servicing, but it is still very common. If there is a junction box between the two sockets you have disconnected, the fault could be with or within it. If not, the fault is certainly in the cable.

Examples of what could cause continuity between live and earth wires in a multi-core cable are mechanical damage, insulation breakdown, insertion of a conductive item into the cable such as a nail or screw, loose, poor or incorrect connections at a junction or other accessory, presence of conductive liquid, etc.

The simplest off the cuff remark would be replace the run between the two sockets, but if this means removing tiles, chasing brick or plaster work, redecorating, lifting floorboards - it can become anything but simple.

If you can observe the cable within the ceiling, such as by removing a spotlight and looking in the hole (with a mobile phone camera, for example), or lifting floor boards above, you will be able to see whether there is any other damage or signs of damage (such as scorching, water marks) to the cable in question or the area nearby. If the fault is in that area, you have a chance of fixing it.

If however it's a nail through the cable on a wall, and it's installed in capping and clipped beneath the plaster, the only solution will be to remove it destructively, for which I recommend you contract a suitable electrician.