240v Between Live And Plasterboard Wall

[john1doherty]

came across a strange one today. Had a 3 core cable to a gas boiler and there was a fault on the boiler so when tested got 145v across the live and switch wire. Disconnected the switch wire at one end and still got 145v. Thought bad insulation so replaced cable. Still there ! Touched the end it the meter prong while the other prong was on the live for the boiler and got 232v! Then measured between the live of a socket and the plaster board wall and got 235v any ideas ? The board has just been upgraded to rcbo's and earths toheating and water and gas pipe are all good

 

[amlu]

earthed metal boxes sunk into the wall, if there is little damp then the wall should be more or less earth potential

 

[ruston]

Especially as the screws are in and it may have a fly lead on too.

 

[HandySparks]

So what voltage would you expect between line and a wall (which is likely to be near earth potential)?

 

[Midwest]

Came across similar fault a while back, with 240v from wall to earth (copper pipes) in a bathroom. Eventually located to nail used to fix dado rail on landing, which had gone into strapper wire on two way lighting for landing light. Note, the nail had only shorted one strapper wire only, no earth. So fault only replicated when 2 way lighting was switched accordingly. Whole landing & bathroom wall was 240v to earth live, and only tripped RCD, when connected via tester or poor unfortunate plumber!

 

[john1doherty]

So what voltage would you expect between line and a wall (which is likely to be near earth potential)?

180v between live and a sheathed single cable lying on the tiled floor as well. And I don't expect any voltage between live and any wall. Do you ?

 

[john1doherty]

Especially as the screws are in and it may have a fly lead on too.

No fly leads on boxes but the screws are in so earthing the back boxes but if there is a short to earth then the rcbo should pop ? As the circuits are all correctly earthed. It isn't only the socket circuit but fridge washing machine etc. No potential at copper pipes in kitchen

 

[David M]

Why would you not expect any voltage between live and a wall which probably has multiple earth paths? As already been mentioned, if the wall is quite porous and contains moisture...
More worrying is if you have 230v from wall to earth like in the example Midwest posted.

 

[HandySparks]

And I don't expect any voltage between live and any wall. Do you ?

As amlu and ruston say above, if the wall is damp then it will have considerably lower resistance than normal and will be earthed via the backboxes. So, yes, under some circumstances, you'll measure a voltage between line and the wall. When the wall is dry, the voltage difference will still be present, but you won't be able to measure it because the input impedance of your meter will be low compared to the resistance of the wall.

 

[john1doherty]

As amlu and ruston say above, if the wall is damp then it will have considerably lower resistance than normal and will be earthed via the backboxes. So, yes, under some circumstances, you'll measure a voltage between line and the wall. When the wall is dry, the voltage difference will still be present, but you won't be able to measure it because the input impedance of your meter will be low compared to the resistance of the wall.

I get that but measuring around 180v on a cable just lying on a dry tiled floor not connected? Was a strange one. Walls are not noticeably damp but that explains the reading of 238v

 

[john1doherty]

Why would you not expect any voltage between live and a wall which probably has multiple earth paths? As already been mentioned, if the wall is quite porous and contains moisture...
More worrying is if you have 230v from wall to earth like in the example Midwest posted.

As the wall is not noticeably damp it never crossed my mind that is would be acting as an earth however it is clear that that's what's happening and is now clearly obvious

 

[amlu]

fire up insulation resistance check between earth and the wall...

 

[ruston]

If the walls are brick or block the fixing screws will be well in . That wall looks recently done so the structure could still be damp to wet as they will have absorbed a lot of the plaster water.
Have you checked between the wall and earth to see which one is live?

 

[Midwest]

In the instance I described, the walls were very dry old plaster. Took a couple of us a few hours to locate, sort of unimaginable set of circumstances, getting 230v from a dry plaster wall to earth. Just sat there scratching heads on the landing o/s bathroom, when someone wondered how the dado rail was fixed to the wall!

 

[Midwest]

OP back to your fault. You need to back to basics, and start your testing from the beginning in methodical order.