Dodgy Kitchen 'ring'

[pcboffinuk]

After finding and fixing a burnt through cable due to incompetent fitting of wiring in my Mother's kitchen (previous post), I decided it should be checked out and though I'd use the chance to try out my shiny new Megger MFT1552.

However, as I half expected, the testing suggests there are problems so I would appreciate running the results past you and seeking advice on my current conclusions.

The situation is that two older 2.5mm2 cables leave the CU - at some point under the floor I assumed they were joined to the kitchen 'ring' which is wired in grey (old colour cores) 2.5mm2.

I started at the CU and got the following:

Phase Continuity r1 : 0.22
CPC Continuity r1 : 0.31

so far so simple

Then the Neutral Continuity rn gave me no continuity (>999MOhm)

Given that the sockets all work correctly and all seem to be wired to correct polarity, I formed the conclusion that either one of the cable joins had a faulty neutral link, hence making the circuit effectively a long radial (running on a 30A MCB), or there was a Neutral break somewhere between sockets making it 2 radials (alebit with a complete Phase conductor)

I proceeded anyway and tested at the sockets, initially P-E (R1 + R2) with opposite Phase and Earth linked which gave readings of between 0.18 and 0.23 at the sockets, and up to 0.38 on what I think are spurred sockets - suggesting that there was a ring of sorts.

The Phase-Neutral readings with Phase-Neutrals linked ranged from 0.23 to 0.36 at the sockets.

I then tried to check my theory, so disconnected and electricall separated each individual core at the CU for this particular ring (I double checked and confirmed they were the correct cores).

I then opened a socket and did the following continuity tests:

Neutral to Neutral : 0.14 Ohm
Phase to Phase : 0.14 Ohm

and I then tested

Neutral to Phase and Phase to Neutral of the two incoming 'ring' cables, which both gave a reading of 8.43 Ohm....


At this point my headache started so I put everything back as it was and sat down with paper to draw it out.

Using my vast inexperience and some logic, my most likely situation is that the two ring ends from the CU and the two ring ends from the kitchen are joined together in one junction box, hence forming a small electrical ring in the kitchen, but connected together with with the two incoming legs from the CU, one of which may well not make proper connection between Neutrals, hence the lack of continuity on Neutral but not Phase at the CU.

Unless I'm missing something this is a 'bad thing' and I need to locate and change this end up with just the two legs feeding the ring and returning separately to the CU..

Sorry for rambling a little, but I hope I have given enough information to make sense.

If I'm missing an alternative possibility, I'd appreciate suggestions and the best way of testing to confirm before I have to dig up my Mother's nice kitchen floor!

I can draw a diagram if that would help, but not sure how much it would add to the above.

I should just add that the competence of the installer is perhaps shown by the running of 4mm2 cable from a 40A MCB to the double Gas oven, which the manual states requires a 3A FCU. I can only give him the benefit of the doubt that he ran it in case a later user wants an electric oven..

Many Thanks for any assistance - I am sure that the vast experience of the forum will find this an easy challenge (yes that was me creeping ).



Gavin

 

[plugwash]

Using my vast inexperience and some logic, my most likely situation is that the two ring ends from the CU and the two ring ends from the kitchen are joined together in one junction box, hence forming a small electrical ring in the kitchen, but connected together with with the two incoming legs from the CU, one of which may well not make proper connection between Neutrals, hence the lack of continuity on Neutral but not Phase at the CU.

yes that sounds likely, Is the floor wooden and if so is there no way you can get under the floor in an inconspicuous location and then crawl under there to follow the cables?

I should just add that the competence of the installer is perhaps shown by the running of 4mm2 cable from a 40A MCB to the double Gas oven, which the manual states requires a 3A FCU. I can only give him the benefit of the doubt that he ran it in case a later user wants an electric oven..

i hope you have applied some more suitable protection to that circuit and the appliance connected to it.

 

[jondiy]

Even still, a 40A breaker for a 4mm cable is still not on!! The cable is only rated at, what 32 Amps (without any correction factors!!) , so if you get 40 Amps of current going down it (which would be possible if somebody replaced the gas cooker with an electric one) - theres a high chance of fire before the breaker will activate! I'd advise you to advise the customer to downrate that MCB asap.

Good luck with the testing , sounds like you have your work cut out!

 

[pcboffinuk]

...Is the floor wooden and if so is there no way you can get under the floor in an inconspicuous location and then crawl under there to follow the cables?

Thanks Plugwash - glad to know that my logic is somewhere in the right area.

The floor only has a tiny gap under it - the wooden boards were added to level the existing uneven floor. The only way I am going to get to it is to raise one of the boards (screwed down very tightly and unlikely) or cut a hole in it - ho hum, it has to be done....

i hope you have applied some more suitable protection to that circuit and the appliance connected to it.

As yet no but I plan to - Once I have fixed this problem I will be changing the CU anyway as it is an old Dorman Smith metalclad unit with MCB but no RCD at all - given that an outdoor lighting feature, outdoor patio lights and security lights, not to mention a cat scarer, all run off this I think some RCD protection may be a good idea!!!

At the moment, the cable is only running the clock and ignition on the cooker, which seems to have an internal fuse as well, so its not top of my priority list but I will certainly be fixing it before long.

I'd advise you to advise the customer to downrate that MCB asap.

Wise words jon but the 'customer' is my mother, so I wouldn't dare advise her to do anything I will certainly be resolving it though, as I plan to use this job as an assessment one for ELECSA registration.

Many Thanks for the advice.

Gavin