100+ volts down earth ????

[JD6400]

Just had a call from two of our men who are testing an agricultural dealership , they have found 100 + volts on one of the lighting cpc's .
To cut a long story short , they have cut around 2 and a half meters out of the 10 meter the circuit ( 1.5 T+E ) and have it laid on the floor . They say that they are getting +500 between all conductors ( Two different Fluke MFT's ) , but when they put power on it ( without connecting the cpc either end , they are still getting 100 + volts on it !

Any thoughts !????

 

[Corpy76]

Just had a call from two of our men who are testing an agricultural dealership , they have found 100 + volts on one of the lighting cpc's .
To cut a long story short , they have cut around 2 and a half meters out of the 10 meter the circuit ( 1.5 T+E ) and have it laid on the floor . They say that they are getting +500 between all conductors ( Two different Fluke MFT's ) , but when they put power on it ( without connecting the cpc either end , they are still getting 100 + volts on it !

Any thoughts !????

Is the cpc connected to the MET? Have they done a R1 + R2 test? It sounds like a induction/eddy current if there is a break in the cpc. Find the break in the cpc re connect it and the 100+ volts should go away!

 

[Corpy76]

Just read your thread again no connected cpc at either end! Still sounds live a eddy current!

 

[Corpy76]

[=Corpy76;899326]Just read your thread again no connected cpc at either end! Still sounds like a eddy current![/QUOTE]

 

[Marvo]

Yeah sounds like a coupled or induced voltage. Use an analogue voltmeter for exactly the same test and it will probably show 0v.

 

[Corpy76]

Induction voltages are more common in systems wired in singles ie cpc with phase and no neutrals to cancel out phase induction. However they can occur in systems wired I. Twin & earth my guess is that the induction voltage is carried across on the switch wires (again no neutrals). If an induction voltage is carried across to the cpc it goes away when the cpc is connected to the MET. I don't know why though I'm not that clever!

 

[JD6400]

At the moment it is a 2.5m length of T+E laid on the floor ( cut out of the circuit ) , with live and neutral connected but not the earth . All conductors have megged out at +500 on two separate Flukes , but they are still getting over 100 v on the earth conductor ?
I have since spoken to them again , and they say that it appears to be the same on other lighting circuits as well .
They are bringing it back to the stores , and I will check it out next Tuesday when I am back down the yard again .

Thanks for the reply anyway .

 

[Specialist]

Yeah sounds like a coupled or induced voltage. Use an analogue voltmeter for exactly the same test and it will probably show 0v.

Nice to see i'm not the only 1 that still uses an Analogue meter

 

[Marvo]

If they've meggered the three conductors and all was okay then it has to be an induced voltage onto the CPC which is not connected therefor it's floating. If they're worried about it then connect the CPC so it's at earth potential and check the leakage current with an earth leakage clamp meter. Otherwise save a lot of messing about and try the original test with an analogue tester as I previously suggested.

 

[Marvo]

Nice to see i'm not the only 1 that still uses an Analogue meter

I'm beginning to think they should only allow sparkies to use analogue testers for the first few years when hopefully they're experienced enough to know the importance of an instruments input impedance and how it affects the results it's going to give on certain tests .

 

[Specialist]

Not trying to teach granny to suck eggs, but yet certainly sounds like induced voltages. Have they checked Earth continuity on the lighting circuits ?

 

[Specialist]

Hi Marvo: Knowing how to use an Analogue meter has certainly stood me in good stead over the years when fault finding some weird faults.

 

[Corpy76]

Hi Marvo: Knowing how to use an Analogue meter has certainly stood me in good stead over the years when fault finding some weird faults.

Whats a analogue meter? Lol

 

[Marvo]

Whats a analogue meter? Lol

Serious??? Analogue is analogue as it's name suggests, it has a moving coil inside and a needle. Digital is...well it's just digital with a digital display.


Analogue




Digital.

 

[Corpy76]

No not seriously!!! I know what one is just to young to have had the pleasure of using one! 38

 

[JD6400]

I hear what you all are saying and appreciate the input , but it is on a 2.5m length of cable .
Would that be long enough for this to happen ?

 

[Specialist]

In a word yes, tell them to connect 1 end of the cpc to a good earth and see what happens to the voltage.

 

[Specialist]

No not seriously!!! I know what one is just to young to have had the pleasure of using one! 38

Brilliant bit of kit, never leave home without mine lol.

 

[Corpy76]

Brilliant bit of kit, never leave home without mine lol.

Whats the advantages over a digital one?

 

[JD6400]

I have to admit that they did , and as expected it did disappear .
But it apparently created quite a spark on the whole circuit .
I originally suggested that they have a fair few fluorescents in the place , but none were on at the time .
It is the first time I have come across this on such a scale and especially on such a short length of cable .