Service Pipes showing 21volts AC to earth.

[Cnobby]

Hi,
At my mums house, she has been getting a slight shock from the metal shower valve. After she told me, i measured the shower valve to earth and had 5volts AC. I then went and measured the service pipes at the combi boiler to earth and there was 21volts AC present. There is no x bonds on these pipes. The earthing system is a TT. Could anyone give me some advice on what could be the likely cause here please? Many thanks chris

 

[welsh spark]

Hi,
what you need to do is mesure the voltage bettween your pipe work and earth and isolate one circuit at a time till the voltage drops then once the circuit that is faulty is identified you can start looking for the fault how ever i have seen a tt system where the pipe work was still live with the main swich off and we traced it to an empty terraced house next door that was leaking current down the earth rod wich was next to our instalatons eath rod very strange hope this helps welsh spark

 

[Cnobby]

Hi,
Many thanks for that advice. Hopefully when i switch off each circuit individually it will dissapear and then i can investigate further (on that circuit). Could it be a faulty appliance? i.e a washing machine, dishwasher?

 

[Bulmer]

Hi,
what you need to do is mesure the voltage bettween your pipe work and earth and isolate one circuit at a time till the voltage drops then once the circuit that is faulty is identified you can start looking for the fault how ever i have seen a tt system where the pipe work was still live with the main swich off and we traced it to an empty terraced house next door that was leaking current down the earth rod wich was next to our instalatons eath rod very strange hope this helps welsh spark

Then wouldn't you get a reading of zero as there would be note PD?

 

[Cnobby]

Then wouldn't you get a reading of zero as there would be note PD?

If there was not a fault present, then yes you would read zero, but in this case i have live voltage present on the service pipes to earth. i was just wondering if anyone had ideas on what could be causing it?

 

[Mac]

If there was not a fault present, then yes you would read zero, but in this case i have live voltage present on the service pipes to earth. i was just wondering if anyone had ideas on what could be causing it?

How long has it been happening,has she had anything changed recently.

 

[The Truth]

i have had this several times before...the cause in two cases was one of those terrible bathroom globe light fixtures...where the wires had melted above the lamp and created a high reistance path to earth via the earth wires, into the fusebox and then out on the bonding cables to the heating pipework. Nasty. check those first ( or melted mains down lighter cables - again heat problems )

check the main bonding conductors, and make sure you have 30mA RCD protection on the bathroom circuits especially and all other circuits in the property. it also common for a central heating zone valve to cause this ( also happened to me ) i would suggest upgrading the consumer unit and fault finding from there.

i bet its a melted cable somewhere though

 

[Cnobby]

No work has been done recently, and the fault has been there for approximately a month (now she tells me). The consumer unit is badly in need of replacing, it is an old wylex rewirable fuse board with no RCD protection.

 

[The Truth]

deathtrap!

change the board and check the lights for the melted cables

 

[peteludman]

its a TT system with no RCD?
should be 100ma RCD protected and would have expected the RCD to trip with earth leakage.

am i write (The Truth)

 

[The Truth]

its a TT system with no RCD?
should be 100ma RCD protected and would have expected the RCD to trip with earth leakage.

am i write (The Truth)

definately.... it must be really really old / bad

reccommend...if money permits replace the board with a 6 or 8 way consumer unit and put in an RCBO for each circuit ( probably only 6 total ) obviously the problem circuit RCBO will trip out immediately. You could go to the lengthy process of fault finding the circuits 'dead' but as the board needs to be upgraded urgently then it makes sense to do it this way. Your main bonding is almost certainly not up to scratch to that should be upgraded at the same time.....as well as a new earth rod if PME *(TN-C-S ) is not available in your area?

 

[sparky paul]

Do you know for deffo its a TT or are you assuming this because there is an earth electrode present

 

[Cnobby]

Its a TT system with an ELCB!

I am going to change the board and cross bond the pipes at the boiler aswell as run new earth bonds to the service water and gas pipes, but in the mean time i would like to identify the fault for peace of mind.

I have identified that it is the downstairs lighting circuit that is causing the problem. I have disconnected 6 out of 10 of the lights and the problem is sill there. Could it be a reversed earth neutral? I just dont understand why the ELCB is not tripping out if it is a fault to earth.

 

[Mac]

Its a TT system with an ELCB!

I am going to change the board and cross bond the pipes at the boiler aswell as run new earth bonds to the service water and gas pipes, but in the mean time i would like to identify the fault for peace of mind.

I have identified that it is the downstairs lighting circuit that is causing the problem. I have disconnected 6 out of 10 of the lights and the problem is sill there. Could it be a reversed earth neutral? I just dont understand why the ELCB is not tripping out if it is a fault to earth.

As The Truth said,possibly a melted cable.You said it's only been happening the last few months.Can't see why it would be a reversed earth/neutral.

 

[Cnobby]

Ye thats a fair point you have made (regarding reversed earth neutral) as no work has been done on the lighting circuit for years. However, I have been told that the last work my mum had done was by the gas man. When he took the oven out he had to take the built in microwave out, he did this by simply cutting the cable and connecting it back into a connector block. I havent checked this yet as i have only just been told of it. There is a possibility he has connected that wrong. But it goes back to the point, i am only getiing a voltage reading when the downstairs fuse is in.

 

[The Truth]

those old ELCB's work on fault voltage rather than current..they are virtually useless...in fact if you test them with a normal RCD tester..you wont get them to trip thats how sensitive they are! so dont assume it works at all. and any way have to tested your earth rod? its impedance is prbly so massive that the fault current has no where to go except your pipework!

lighting circuit see?? what did i say? outside light full of water and shorting to earth?? globe lights melted cables? bad ceiling rose connections above light? damaged cable in a switch backbox? melted downlighter cable not easilly spotted?

just mega through it section by section and power up the good part of the circuit as you go