Why do faulty Transformers give off a charge on the negative cable?

[mcummins7]

Hello folks,

Could anyone please explain the following circumstances to me??

A customer had their lighting breaker trip when they turned on a light. They put the breaker back on and the lights didn't work at all.

Yours truly went round to investigate and found that there was a charge coming back on the negative cable on the light fittings. I tested each leg from the board until I found an old door bell transformer on the circuit was causing this and hence isolated it to solve the problem.

Can anyone inform me please:

1) why the lights weren't working when the breaker was put back on and didn't obviously trip again?
2) why there was a charge on the negative from the faulty tranformer?
3) how there was a charge on the negative side of the lighting circuit, when at the board, the negative block hadn't?

 

[Knobhead]

Are you an electrician? Because what you have said so far wouldn’t indicate it.

Why has the neutral failed should be you first question. The bell transformer is just doing it’s job, but with no neutral it’s creating a small voltage on the disconnected neutral rail.

 

[Kiers1970]

Good Grief.

 

[mcummins7]

The neutral was not failing as soon as the bell transformer was removed. I checked all neutral connections at each break prior to that ...................

 

[Knobhead]

Good Grief.

Charlie Brown, my hero!

 

[mcummins7]

Thanks Tony, really helpful comments.........

 

[telectrix]

one suspects the bell transformer to be a red herring, and an open circuit on the neutral.

 

[mcummins7]

Thanks Telectrix - but the circuit testing fine now!! Maybe I just put back a dodgey neutral connection securely when testing each leg and that was the fault. However, I've had the same problem before with transformers giving off a charge on the neutral.

 

[telectrix]

think it must have been a dodgy neutral fixed without knowing. if the neutral is tied down to 0v in the CU, there's no way it can have a voltage on it.

 

[Knobhead]

Thanks Tony, really helpful comments.........

Had you shown a semblance of competence my response would be different.
As it was I was right in my diagnosis, added to which you found the fault not by skill but by pure chance