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Discuss Voltage up to 100v on CPC even when disconnected in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

C

chrisses

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Hi
I've never posted in one of these forums before, but have read lots of threads and found them extremely helpful.
So it makes sense seeing as I know the knowledge on this sight is very impressive to ask you guys for help.
I have a problem with a job and can't work out what the problem is I've been back there two days in a team of two and haven't fixed the problem and I'm starting to feel a bit stupid :)

It all started when I got a call from a customer I completed a full rewire and extension for last year, She claimed to be getting small shocks from the metal light switches in one room of the house. I turned up expecting it to be something simple, Maybe even static.
Turns out at that switch there was 44v on the earth wire.
Ok so at this point I'm thinking that some where on the circuit there is a damaged cable, not enough to trip but slight damage
I strongly suspect that the most likely cause is a plate screw slightly intruding on the insulation of a live so I unscrew all the plates.
I should add at this point that I went to the fuse board in the garage and discovered that they had a new automatic garage door fitted and the rails ran right in front of the fuse board, Giving me just enough space to get the fuse board cover off but not to safely work in it or test properly.
However back to the point I discovered that every earth on a feed in or out cable has voltage on it some as high as 100v!. When I turn the breaker off to this circuit this voltage disappears completely.
I disconnect the earth of this circuit out of the board and this earth has voltage, So its not coming from the mains, This is where it gets confusing to me, I disconnect all the earths from each other and from the board expecting to find which cable has the fault. All the earths are still live!
This doesn't seem possible to me and I'm hoping someone will make me feel stupid and point out something I've missed
I'm assuming the neutrals must be involved because otherwise it wouldn't be possible for all earths to be live even when disconnected from each other.
As a temporary solution I have removed all the metal plates and replaced them with plastic. Obviously the customer is not happy with this and I need to go back and remove the garage door bracket to get to the board and test properly.
Any replies of help would be most welcome (I'm a desperate man) please feel free to make me feel stupid I have no doubt in my ability as an electrician but I am also under no illusions that there are electricians which have forgotten more than I know! Seriously though I really appreciate you taking the time to read this and would appreciate ALL suggestions, I really want to sort this because how else do we learn
 

Sintra

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I would start by confirming voltages between L&N L&E and N&E both at the incomer and then on the final circuits.
 
C

chrisses

Ok last time I was there

L-E 230v
L-N 230v
N-E 63v (at board) upto 100v in other parts of the circuit
 
C

chrisses

I measured earth to neutral first then to an earth of another circuit

- - - Updated - - -

Its a TN system
 

Marvo

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There's a few things I don't fully understand from your post, especially the part about separating the earths and the neutrals being involved.

A few questions first;
Did you test the incoming supply?
What is the earth impedance of the supply?


I can give you some general advice, if you get ashock from something then there at least two faults present. Firstly there's an insulation problem causing current to leak down the CPC to earth. Secondly there's an earthing problem which allows a high touch voltage to develop because the earth impedance is too high.

Tracing this problem by measuring the touch voltage isn't the best way, rather use your mega tester to find the low insulation fault and then use your earth impedance tester to find the earthing issue.
 

spark 68

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I was thinking on the same lines as Marvo, ie. two faults, maybe a supply Earthing problem, and a fault on the house wiring.
 

NMc

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When you completed the rewire was everything tested out ok, if so, see what has been added since as a starting point.
 

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