neutral live and cpc used as neutral

[captaincaveman]

Damn this heat. I just can't think straight!
Been called to an installation where the ceiling was sparking near a light. All the lights were working ok. The spark was coming from a loose earth wire from the light fitting which was touching a metal ceiling frame. I checked the light switches and found a facia screw piercing the neutrals at the back of the knockout box. I figured that could be it so I fixed the damage to the neutrals and reenergised the circuit only to find none of the lights worked! Checked all the connections and seemed to get 209v from one of the neutrals. At this point everyone was going home (office) and I said I'd return when I can isolate the ccu to do proper testing on the circuit.
Now, my hot sweaty thoughts are the polarity has been crossed over somewhere and for the last god knows how long the cpc has been used as the neutral through this screw through it. Does that sound plausible?? And if so what is the quickest route to narrowing down where the reverse polarity is? There are some junction boxes high above the false ceiling which require scaffolding to reach so if I can avoid that... Or, more likely would you not agree, that the neutral is just disconnected at the ccu (or elsewhere)? But then would i have voltage at one point of it??? Arhhh. Wish it would rain soon so i can think again!! Beer time

 

[123]

Broken neutral somewhere possibly. Is this circuit still energised?

 

[captaincaveman]

No. I isolated the neutral section at where i was getting the reading and left the circuit off. I was effectively kicked out so I could only inform the manager not to reenergise it (rather than lock it off). I should have mentioned i inspected all but two fittings/ fitting sockets to no joy but did find cpcs covered with black sleeve!
I couldn't isolate the ccu at the time because no one knew how to turn the server off but when i return tomorrow im going to have to insist on it.

 

[Gavin John Hyde]

Its a case of back tracking from the fitting as far as you can to junction boxes etc without getting a tower out. Then do from the db as close to the light.
You might then have to tell customer that there is no option but to pay for the access equipment.

 

[The Ghost]

I came across the same figures almost on the earth volt wise and forty volts on the neutral split the circuit in half and half again and found the earth/neutral had been crossed over on commercial outside lights eighty meter run. Been operating like that for twenty years until one day the lights were flashing on and off so investigated and as you say crossed wires. Just a matter of taking it methodically and splitting down and test up and downstream.

 

[Octopus]

When the neutral appears to be live it's normally a sign of a disconnected neutral ...

Dead tests required and I would start with Rn + R2 to confirm neutral continuity .... Then go from there

 

[HandySparks]

Damn this heat. I just can't think straight!

From 7:20

 

[captaincaveman]

I "think" I resolved the issue today. Got them to have an hours lunch in which time I could shut the installation down to do dead testing. I found 3 issues - 1) the facia screw through the neutral. 2) the cpc stopped at the office then started again at the toilets but weren't connected, with the toilet exposed end just lying there touching the metal conduit. 3) the neutral was not connected and lay taped over in the ccu.
I can only assume there was never an earth (or not completed on last rewire). There was a fault tripping the mcb (pierced neutral) and someone found it worked ok with the neutral removed so left it out. Unfortunately for god knows how long the conduit / building structure seems to have been used as the neutral!
Anyway, I completed the cpc run, repaired the pierced neutral and reinstalled the neutral. everything tested ok. The earth was no longer live and the lights worked. Oh my god, Japan is winning!!!
Cheers for your input guys

 

[telectrix]

nice to see a result. (on the electrics, not the bloody kick ball. had enough of it every bloody day . ).