Strange N-E fault on armoured cable supply to sub-main

[Somebrightspark]

I've installed a garage consumer unit sub-main to an outbuilding in 16mm 3 core armoured, the insulation tests all come back fine - +800Mohms on a 500 volt test but when I power up the installation I get 90 volts leaking to Earth from Neutral and i'm not sure why. The Garage RCD also doesn't trip on the test button.

The circuit goes from a 32A mcb protected by RCD in the House Consumer Unit, in 10mm t&e to a 63A Rotary Isolator inside the house, then from the Rotary Isolator to the Garage Consumer Unit in 16mm Armoured run underground in Copex ducting.

The voltages at the Rotary Isolator are fine - 230V between L-N and L-E and 0V Between N-E.

Once the circuit is live, there's a 90 volt Leak to Earth from the Neutral, somewhere starting at the Rotary Isolator up to the Garage Consumer Unit.

The Copex has water in from the heavy rain recently so no doubt the armoured is partially submerged but surely any problem would show up on the insulation test?

The only thing I can think of is that the Isolator is faulty so when I next go back i'm going to bypass it and see if he fault clears.

Apart from that i'm out of ideas, anyone got any thoughts?

Cheers

 

[jackhammerJIM]

I agree with you that the likely outcome is bad contacts on the rotary isolator .

 

[westward10]

Have proved continuity of all conductors from source to the garage.

 

[Marvo]

the insulation tests all come back fine - +800Mohms on a 500 volt test but when I power up the installation I get 90 volts leaking to Earth from Neutral and i'm not sure why.

What tester are you using when you see the 90 volts? Make the same test again using either an analogue tester or a digital tester with a 'Lo-Z' facility.

Here's one of the many past discussions about ghost voltage;

Voltage on CPC

Went to a job the other day, where the client wanted a light fitting put up instead of the ceiling rose. In conversation they mentioned having problems with their electrics, and had had a new CU fitted. I decide to measure ZS & RCD at the said ceiling rose. However, my tester (Megger1730)...

www.electriciansforums.net

 

[Lucien Nunes]

90 volts leaking to Earth from Neutral

Sorry, this doesn't quite make sense. Neutral is already near earth potential. Do you mean: "There is a leak from line that raises the neutral conductor to 90V from earth?"
If so, then yes, the leak is probably normal capacitive leakage but the neutral conductor is not solidly connected to neutral because of a bad contact in the isolator. Any significant voltage on a neutral conductor especially when unloaded, indicates a faulty connection upstream.

 

[Somebrightspark]

I returned to the job this morning and the fault had cleared on it's own and everything tested out fine and works now so i'm not sure what was causing the voltage to appear.

Thanks for the replies though, I learned some things.

 

[ipf]

Did you operate the switch a few times?
I'd still be doing a terminal inspection.

edit. That's not electrocution whilst inspecting, by the way.