75 volts in cpc

[GT1]

Guys
not sure if immissing something here, and would appreciate some help.
On a lighting circuit I've found that when energised, 70 odd volts are present in the cpc ?
Only presnt between cpc and neutral.
And it disappears once the cpc connected back to the MET in the cu.
The circuit passes all 3 IR tests at 500volts.
There lots of small little 12v luminaires as well as normal light fittings plus an outside light on the circuit.
I've done a full pir and everything checks out.
I just can't explain the prescence of potential in this cpc when it's isolated from earth. ?

 

[Sintra]

Guys
not sure if immissing something here, and would appreciate some help.
On a lighting circuit I've found that when energised, 70 odd volts are present in the cpc ?
Only presnt between cpc and neutral.
And it disappears once the cpc connected back to the MET in the cu.
The circuit passes all 3 IR tests at 500volts.
There lots of small little 12v luminaires as well as normal light fittings plus an outside light on the circuit.
I've done a full pir and everything checks out.
I just can't explain the prescence of potential in this cpc when it's isolated from earth. ?

Why would you have the cpc disconnected from the earth bar in the consumer on an energised circuit?

 

[pevvers]

Why would you have the cpc disconnected from the earth bar in the consumer on an energised circuit?

I second that thought!! Scary stuff that goes on what you hardly ever hear about....

The CPC will be showing a voltage as it has no reference to earth, maybe through inductance or other reasons. This is why all CPC's are connected to earth!

To the OP, you do realise that energising a circuit with the earth disconnected on purpose is rather dangerous? Would you be happy for someone to come to your house, disconnect your main earth and leave your system energised? In essence, that is what you have done on that circuit for the length of time that you had it live without an earth.

 

[GT1]

Yes yes I know.
And the cpc was only disconnected for fault diagnosis. After making sure it was safe to do so.
All my tests were done dead and cpc's reconnected BEFORE the circuits were energised.
The house is empty and safety precautions were taken of.
Now all that aside and ignoring the why's and wherefores and I assure the cpc IS connected !!!!
I still have potential in the cpc that shouldn't be.!

Yes yes I know.
And the cpc was only disconnected for fault diagnosis. After making sure it was safe to do so.
All my tests were done dead and cpc's reconnected BEFORE the circuits were energised.
The house is empty and safety precautions were taken of.
Now all that aside and ignoring the why's and wherefores and I assure the cpc IS connected !!!!
I still have potential in the cpc that shouldn't be.!

 

[pevvers]

Yes yes I know.
And the cpc was only disconnected for fault diagnosis. After making sure it was safe to do so.
All my tests were done dead and cpc's reconnected BEFORE the circuits were energised.
The house is empty and safety precautions were taken of.
Now all that aside and ignoring the why's and wherefores and I assure the cpc IS connected !!!!
I still have potential in the cpc that shouldn't be.!

Phew, well that's not the way that it sounded from your original post.

The only time I have got voltage between neutral and a CPC is where there is no earth to the point you're measuring at. A good example was on a periodic I had done where I found the same, and after a bit of investigation I found that someone (probably home owner) decided that he needed a loft light, but didn't think that the lilttle copper wire in the twin and earth was important, so decided to just cut them off to connect his loft light in the middle of the cable... Have you conducted an R1+R2 of the circuit, or even just an R2?

 

[chris gray]

i guess there are no rcd/rcbos as part of the installation?

 

[pevvers]

There lots of small little 12v luminaires

Are you sure that whoever installed the little SELV fittings, as they are separated from earth, didn't decide that continuity of the CPC's wasn't important?

 

[GT1]

Yeah the only thing I can put it down to is faulty equipment !!
the fault didn't show in any of the standard suite of tests.
Continuity, IR, all came back fine.
Even when energised there were no problems showing.
The earthing system is obviously dealing with it as it should. There is zero potential when everything connected properly.
Just going to have to advise that there IS a fault, that it needs further digging to find it !!

Yeah the only thing I can put it down to is faulty equipment !!
the fault didn't show in any of the standard suite of tests.
Continuity, IR, all came back fine.
Even when energised there were no problems showing.
The earthing system is obviously dealing with it as it should. There is zero potential when everything connected properly.
Just going to have to advise that there IS a fault, that it needs further digging to find it !!

 

[pevvers]

Yeah the only thing I can put it down to is faulty equipment !!
the fault didn't show in any of the standard suite of tests.
Continuity, IR, all came back fine.
Even when energised there were no problems showing.
The earthing system is obviously dealing with it as it should. There is zero potential when everything connected properly.
Just going to have to advise that there IS a fault, that it needs further digging to find it !!

If there is zero potential when all the earthing is connected then there is not fault, with earths disconnected then there will always be floating voltages, as all it now is is a piece of copper with 230V running alongside it and it will pick up inductance from this.

Now if you had earth continuity throughout, everything connected as it should be and also energised, and still had this fault then I would be stumped.

 

[Tester Steve]

I think you are looking for something that isn't there!

Steve.

 

[GT1]

Yes your more than likely right.
Probably not a fault at all and I've dug too deep until I found something I couldn't answer !
I wouldve been happy with a few a volts (all other circuits show zero)
it was mainly the magnitude that spurred me to ask the question.
I shall put it down to inductance for now and write it up.
The instalation tested very well with good readings all round other than this "issue"
thanks for the help so far.

Yes your more than likely right.
Probably not a fault at all and I've dug too deep until I found something I couldn't answer !
I wouldve been happy with a few a volts (all other circuits show zero)
it was mainly the magnitude that spurred me to ask the question.
I shall put it down to inductance for now and write it up.
The instalation tested very well with good readings all round other than this "issue"
thanks for the help so far.