UK Fault with ring main

[Rubberhead]

I was testing a consumer unit before upgrading to current standards. (Old re wireable fuses) when testing one of the ring mains continuity i found 38v on the cpc with that circuit disconnected. After carrying out end to end tests on Line and neutral which came back fine. I found the voltage dissapared when I removed the fuse for the lighting circuit?
Any advice on what this would be?

I also have continuity between neutral and line on a leg but haven't had time to investigate further, i e splitting the ring.
Thanks in advance

 

[pc1966]

I was testing a consumer unit before upgrading to current standards. (Old re wireable fuses) when testing one of the ring mains continuity i found 38v on the cpc with that circuit disconnected. After carrying out end to end tests on Line and neutral which came back fine. I found the voltage dissapared when I removed the fuse for the lighting circuit?
Any advice on what this would be?

If measured with a typical digital multimeter probably just capacitive coupling (the so called "phantom voltages"). DMM often have around 10M impedance so it only take 3.8uA to read 38V, getting that by two cable running side-by-side for a reasonable distance is easy.

I also have continuity between neutral and line on a leg but haven't had time to investigate further, i e splitting the ring.

L-N short? Surly it would have been blowing fuses before you got there?

If measured with a IR tester though it probably means < 0.01M ohm and that could be anything still connected with even modest load, such as filament lamp, motor in fridge, etc.

 

[Rubberhead]

If measured with a typical digital multimeter probably just capacitive coupling (the so called "phantom voltages"). DMM often have around 10M impedance so it only take 3.8uA to read 38V, getting that by two cable running side-by-side for a reasonable distance is easy.


L-N short? Surly it would have been blowing fuses before you got there?

If measured with a IR tester though it probably means < 0.01M ohm and that could be anything still connected with even modest load, such as filament lamp, motor in fridge, etc.

Sorry not line and neutral - continuity between neutral and earth with the ring pain disconnected at the board

 

[pc1966]

Sorry not line and neutral - continuity between neutral and earth with the ring pain disconnected at the board

OK, more likely and going to trip RCDs!

Probably a lack of grommet in some back box, or cable crushed up, so hopefully you can find it with a bit of systematic searching.

If it is a very low R fault you might be able to get some idea of roughly how far by measuring N1-E1 and N2-E2 and the ratio of resistance ought to put you roughly at the ratio of distances to where it is shorted.

 

[Rubberhead]

OK, more likely and going to trip RCDs!

Probably a lack of grommet in some back box, or cable crushed up, so hopefully you can find it with a bit of systematic searching.

If it is a very low R fault you might be able to get some idea of roughly how far by measuring N1-E1 and N2-E2 and the ratio of resistance ought to put you roughly at the ratio of distances to where it is shorted.

Yeah looks like I'll have to do that, what was the most puzzling is that this voltage disappeared when I removed the fuse for one of the lighting circuit.

 

[Dave OCD]

As a precaution just do an insulation resistance test between the ring and lighting neutrals, just in case someone has 'borrowed' a neutral from the ring for a light somewhere...

 

[pc1966]

As a precaution just do an insulation resistance test between the ring and lighting neutrals, just in case someone has 'borrowed' a neutral from the ring for a light somewhere...

Ah, good point!

Usually the borrowed neutral is between up/down lights, but anything is possible...

 

[Dave OCD]

I've had this very problem just recently, only found it when fitting an RCBO board and both the lighting and [sole] ring RCBOs tripped everytime the downstairs hall light was switched on..... Can't access the borrowed neutral so had to come up with a temporary solution until boards can be lifted etc - can you guess what I've done for the time being ?

 

[pc1966]

can you guess what I've done for the time being ?

I can guess, but would rather not!

 

[Rubberhead]

As a precaution just do an insulation resistance test between the ring and lighting neutrals, just in case someone has 'borrowed' a neutral from the ring for a light somewhere...

Thanks Dave

 

[DPG]

Connected the lighting circuit to the RFC using an FCU to fuse down?

 

[pc1966]

Actually I'm wondering how on earth they managed to use the ring's neutral instead of 3&E cable being used?

 

[pc1966]

Connected the lighting circuit to the RFC using an FCU to fuse down?

More respectable than (ab)using the CPC...

 

[Dave OCD]

Well there's only 1 lighting circuit as well , and the ground floor hall light comes on with the landing light, that only has the usual loops and 1 switch wire. So I reckon the linking was a more recent thing and the bottom light has the switched live tapped off the L2 of the three core from the landing 2 way with neutral from a ring link -all above the ceiling with the room above totally loaded with clutter. So for now I've spurred the lighting from the ring RCBO through a fused spur with 5a fuse right next to the CU.

 

[Aaron b]

Used a mcb and powered it from the rfc rcbo?

 

[Dave OCD]

Used a mcb and powered it from the rfc rcbo?

Very close mate, but DPG hit the nail on the head.

 

[MJPD29]

I was testing a consumer unit before upgrading to current standards. (Old re wireable fuses) when testing one of the ring mains continuity i found 38v on the cpc with that circuit disconnected. After carrying out end to end tests on Line and neutral which came back fine. I found the voltage dissapared when I removed the fuse for the lighting circuit?
Any advice on what this would be?

I also have continuity between neutral and line on a leg but haven't had time to investigate further, i e splitting the ring.
Thanks in advance

L-n load could be anything, even a neon on a spur. Are you new to testing?