Why is there a 1 volt potential difference?

[DonutHole]

Hi,
I decided to bury my 2 gang hall light switch and cables into the internal rendered brick wall. I connected the light switch back up and it worked fine, but when I came to disconnect it to cement the PVC conduit and metal back box in I noticed a 1 volt potential difference between the earthed back box, and the live on both the upstairs 2 way light, and the downstairs 1 way. Any idea what would cause this

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• DonutHole
• 28 Mar 2014
• 1

Consumer Unit

• DonutHole
• 2 Mar 2014
• 2

MK, Split load, Single RCD.

No I don't have a PME earth to the grid, the main earth is a copper earth pin below the consumer unit.

 

[DaftPunk]

Was this whilst the electricity was turned off?

 

[DonutHole]

Was this whilst the electricity was turned off?

The MCB's for the up and downstairs lighting circuits were off, while the other MCB's for Sockets, Shower, Immersion heater, Garage were all on.

I tried turning the others off 1 by 1, I had a PD of 1.2v, with all lighting MCB's off, then I turned off the Garage MCB and got 0.9v, then turned off the House sockets and got 0.2v, when I turned off all MCB's I got 0v.

 

[Lectrician]

Your worrying about 1v? I would be more worried at your "earth pin earth" and clear lack of main RCD. Unless its standalone, and not incorporated into your CU.

 

[DonutHole]

Your worrying about 1v? I would be more worried at your "earth pin earth" and clear lack of main RCD. Unless its standalone, and not incorporated into your CU.

Thanks Lectrician, I guess I didn't know if the potential difference was important, that was why I asked. No there is only the 1 single RCD, the lighting circuits and the immersion heater are not RCD protected, there are no other Consumer Units or RCD's in the house. Is it possible that a poor earth is the cause of the potential difference between live and earth at the light switch?

So you think I should worry about the lack of RCD and inadequate earth, rather than the 1v potential difference at the light switch. Presumably the fix is to put some circuits onto combined RCD, and some onto individual RCBO's, but which circuits require their own RCBO. Also I assume you are suggesting I get a PME National Grid earth sorted.

 

[Lectrician]

Either get PME, or if your rod is sound, install a main time delayed 100mA RCD, or install a new dual RCD split 17th edition board.

1v? I wouldn't be batting an eyelid! Capacitive coupling. Circuits running next to each other.

 

[DonutHole]

Thought I'd add another image, the wide angle view under my stairs.

Wide angle view under my stairs.

• DonutHole
• 28 Mar 2014
• 1

 

[DonutHole]

I think I may have miss informed you 'Lectrician', the trip to the right of the Consumer Unit says:
'240v, 50hz, 80A load, 100mA Trip, CAT.Wem 80/2 Current Operated Earth Leakage Circuit Breaker. Device must Switch off on testing if not seek advice'.

Is this what you were recommending, and if yes that must mean I had one all along?

 

[AndyPRK]

Yeah panic over!

Not sure on the cables between the meter and rcd though.

 

[JohnW2]

Thought I'd add another image, the wide angle view under my stairs.

It looks as if you may well have the standalone 'up-front' RCD to which Lectrician was referring. What does the writing on that thing to the right of the CU say?

Kind Regards, John

 

[DonutHole]

Thought I'd add another image, the wide angle view under my stairs.

It looks as if you may well have the standalone 'up-front' RCD to which Lectrician was referring. What does the writing on that thing to the right of the CU say?

Kind Regards, John

Wylex 240v, 50hz, 80A load, 100mA Trip, CAT.Wem 80/2 Current Operated Earth Leakage Circuit Breaker. Device must Switch off on testing if not seek advice

Thanks John, because it doesn't say RCD I hadn't realized that that was what it was, but I see '100mA Trip, and 'Earth Leakage' so I think I had one without knowing?

 

[JohnW2]

Wylex 240v, 50hz, 8A load, 100mA Trip, CAT.Wem 80/2 Current Operated Earth Leakage Circuit Breaker. Device must Switch off on testing if not seek advice ... Thanks John, because it doesn't say RCD I hadn't realized that that was what it was, but I see '100mA Trip, and 'Earth Leakage' so I think I had one without knowing?

It seems that you probably have - or almost (see below)

Should that perhaps be "80A load" (rather than "8A load")?

Is that all it says? No mention of "time-delayed", "TD" or "Type S"? If it is not a time-delayed one, that is not ideal. It means that a fault causing the RCD in the CU to trip might well cause that external RCD to trip as well, thereby taking out all your circuits (including the lighting ones), not just those circuits protected by the CU's RCD - which rather cancels the benefit of having a 'split' CU (with not all circuits protected by the same RCD).

Kind Regards, John

 

[DonutHole]

Yeah panic over!

Not sure on the cables between the meter and rcd though.

I took a close up of those meter tails. There appear to be 2 red and 2 black with very old looking woven insulation, and a pair with more modern grey PVC insulation. So it looks like it's been upgraded over the years, but probably due another upgrade.

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• DonutHole
• 28 Mar 2014
• 1

It looks like a job for a qualified domestic installer.

The cable between the meter and the earth rod looks like a 6mm rather than a 16mm, so that needs upgrading as well. Although if I went over to PME would I need to do both?

 

[JohnW2]

It looks like a job for a qualified domestic installer.

I don't think so - I think you'd need your electrical supplier (or their meter operator) to deal with that - a 'mere electrician' can (should) not pull the main fuse, and certainly shouldn't break the meter seals.

Kind Regards, John

 

[Taylortwocities]

Two tails into one meter termination? That's bad practice.
There really should be a 100A "Henley" block. And (per above) the fabric-covered tails could do with changing while he/she's at it.