Short in socket - why/how?

[TheJack]

Hi. I'm in the process of getting my qualifications and am taking on some small jobs for family to try and get some experience in.

Today I was looking at a socket on a final ring circuit. The house was built in the 80s and the socket is almost certainly original to then.

There were 3 cables running through the wall connected to the socket - with all 3 conductors twisted together (ie all 3 lines twisted together and connected to the socket as one).

I presumed 2 would be part of the ring, and wanting to know where the other was spurring to I untwisted everything and with a bit of tugging worked out that one was connected to a socket in an adjacent room.

I then attempted to rewire the socket, but rather than retwisting everything I put the conductors back in individually (it was a bit of a faff as it was tight, and the old cables quite inflexible) but I got there in the end.

However when I re-energised the circuit and plugged the appliances back in there was a crack, a small bang, and a weird (perhaps acidy) smell, which I understand to be a short circuit.

Confusingly nothing tripped.

I'm baffled though as my understanding is that a short would be due to the live conductors touching, however I definitely did not incorrectly rewire it - ie lines and neutrals separate.

I'd be so grateful if someone could shed some light please?

 

[TheJack]

0.43 could be caused by an RCD socket or spur. It could also be a filtered extension lead (have you tested at 250v?).
Testing an existing circuit you should have your neutral conductors disconnected and linked to the line conductors, keep the cpcs connected. Do not test line to neutral unless everything is disconnected.

Hi Westard. I've been back to this installation again.

Re "Testing an existing circuit you should have your neutral conductors disconnected and linked to the line conductors, keep the cpcs connected" - do you mean that at the board I leave the CPC connected and remove the line and neutral conductors for the relevant circuit and connect them with a jump lead? So as per the attached illustration?

Once I'm clear on that and I'm back at the installation I'll perform the test, but in the meantime I corrected a couple of mistakes I realised I had made with the wiring and retested as per post 14 and have different (better?) results:

Neutral to earth: >999
Line to earth: 0.31
Line to neutral: >999

As I understand it my Line to Earth is my issue as less than 2 mega ohms is a fail.

I've been around the ring circuit unplugging everything and identified a few accessories that I hadn't previously been aware of: a spurred electric radiator, its switch (which has a couple of neons), a socket with a USB port. I disconnected all these and - to maintain the ring - joined the relevant conductors (earth to earth, line to line, neutral to neutral) together with jump leads / wagos (which I presume was the correct thing to do???).

This made no difference however, the reading remained at 0.31.

One theory: 4 of the double sockets have tiny neon on/off lights on the switches - could that be responsible for the reading? Or am I simply overlooking something that's still in the circuit?

 

[brianmoooore]

I disconnected all these and - to maintain the ring - joined the relevant conductors (earth to earth, line to line, neutral to neutral) together with jump leads / wagos (which I presume was the correct thing to do???).

If you've broken the ring up, leave it broken and test the individual sections.

 

[TheJack]

Hi Westard. I've been back to this installation again.

Re "Testing an existing circuit you should have your neutral conductors disconnected and linked to the line conductors, keep the cpcs connected" - do you mean that at the board I leave the CPC connected and remove the line and neutral conductors for the relevant circuit and connect them with a jump lead? So as per the attached illustration?

Once I'm clear on that and I'm back at the installation I'll perform the test, but in the meantime I corrected a couple of mistakes I realised I had made with the wiring and retested as per post 14 and have different (better?) results:

Neutral to earth: >999
Line to earth: 0.31
Line to neutral: >999

As I understand it my Line to Earth is my issue as less than 2 mega ohms is a fail.

I've been around the ring circuit unplugging everything and identified a few accessories that I hadn't previously been aware of: a spurred electric radiator, its switch (which has a couple of neons), a socket with a USB port. I disconnected all these and - to maintain the ring - joined the relevant conductors (earth to earth, line to line, neutral to neutral) together with jump leads / wagos (which I presume was the correct thing to do???).

This made no difference however, the reading remained at 0.31.

One theory: 4 of the double sockets have tiny neon on/off lights on the switches - could that be responsible for the reading? Or am I simply overlooking something that's still in the circuit?

Bumping this up

 

[Mike Johnson]

Recommend this: Electricians Guide to Inspection and Testing Book - https://www.techtrainingshop.co.uk/guide-books/electricians-guide-to-inspection-and-testing-book easiest On site guide to understand, not the easiest to use as it will not lay flat, but is still better than the Ring bound version.

This is more comprehensive and much better: https://www.amazon.co.uk/Practical-Inspection-Certification-Electrical-Installations/dp/185617607X don't pay full price can be obtained at a very large discount.

And the comment below in italics was not part of my original post, but added later by the Forum.