400v at Lights

[chrisclose]

I was sent to a building as there were no lights in the lift motor room at the top... Got there, opened 2 x fluorescents and tested for voltage at them and it showed 400v, opened the light switch... got the same... So my initial thought was i had lost a neutral somewhere...

The lift motor room is supplied from a Distribution Board, the three phases come from the DB to a switch isolator, from there it goes to another switch isolator before finally going in to the Board at the top.

So I went to the local fuse board in the motor room ( 3 banks of 4 fuses, each supplied by a separate phase) isolated the board, pulled the appropriate fuse, re-energised board and tested at incoming end, again, 400v between phases, 400v between each phase to neutral, same to earth and 240v between neutral and earth.

So then i'm thinking ok, I've lost a neutral and it's shorting to earth somewhere, traced every single cable associated with that board and the supply for it, no obvious fault anywhere... Went back and re-isolated the board thinking I must have missed something, pulled every single fuse in the board and went to test again, this time i got 0v across the entire board between phases and to neutral and earth.

Eventually I narrowed it down to a particular circuit on the blue phase, when it doesn't have a fuse in it, the board is dead, however when it's got a fuse in it, the entire board reads 400v and 240v between neutral to earth... I've traced that circuit to a dead end looking for some sort of fault on it, Disconnected everything associated with it looking for any sort of fault to cause that amount of overload and... Nothing?

Basically hoping someone may have came across something similar and can point me in the correct direction?

 

[uksparks]

Were the fluorescents working at 400v? Surely not? They would have gone bang?

 

[chrisclose]

Nope, absolutely fried the lamps, starters and ballasts

 

[uksparks]

I was hoping you were going to say that.

 

[trev]

I found 400 at a light fitting in a pub once. DIY Dave had been dabbling managed to get 2 phases to it.

 

[uksparks]

Just thinking out loud here, so you obviously have 230v on the neutral somehow...

 

[chrisclose]

I must admit, This is a new fault as I was sent to relamp the fluorescents 2 weeks ago! Boss thought I was at it and hadn't relamped them when the email came through saying there were no lights up there!

 

[chrisclose]

Just thinking out loud here, so you obviously have 230v on the neutral somehow...

Yes, but only when that circuit is energised and I've traced that circuit from start to finish and found zilch!

 

[TJ Anderson]

It is lost neutral. That is why 400V across terminal at light fittings. There is 230V between neutral and earth because it has lost its connection to neutral and is actually phase to earth via connected load. Shut it down and methodically dead test to find lost neutral.

 

[micknew]

It is lost neutral. That is why 400V across terminal at light fittings. There is 230V between neutral and earth because it has lost its connection to neutral and is actually phase to earth via connected load. Shut it down and methodically dead test to find lost neutral.

Spot on, it has to be.

 

[uksparks]

I had a similar situation the other day in a house, was only single phase but still, same kind of issue, a bulb had blown, and in the process the rest of the light circuit packed up...

It tuned out to be a load of loose joints in a ceiling rose and neutral was charred a bit and hanging loose.

The point I'm making is that I had no voltage between line and neutral as both were at 230v as was getting 230v line to earth, nothing line to neutral, and 230v neutral to earth.

 

[Marvo]

It is lost neutral. That is why 400V across terminal at light fittings. There is 230V between neutral and earth because it has lost its connection to neutral and is actually phase to earth via connected load. Shut it down and methodically dead test to find lost neutral.

What Mr Anderson said.

If the panel is live with loads connected the open circuit neutral will float at a voltage approaching the phase with the highest load. This is basic 3-phase theory. Shut the entire DB down and find the break in the neutral before other equipment more expensive than a couple of fluorescents gets damaged beyond repair.

 

[snowhead]

Eventually I narrowed it down to a particular circuit on the blue phase, when it doesn't have a fuse in it, the board is dead, however when it's got a fuse in it, the entire board reads 400v and 240v between neutral to earth..

Is that with all other fuses removed and just the one "Dead end" circuit fuse in?

And what other loads are on that D.B?

 

[chrisclose]

Is that with all other fuses removed and just the one "Dead end" circuit fuse in?

And what other loads are on that D.B?

Yes.. when everything else is removed and just that one fuse in, everything's at 400v but when that's out the board is dead. It's more or less just radial circuits, few lighting circuits and a few heater circuits. I've traced every circuit up there.

appreciate everyone's replies, I think a good few hours will need to be spent on it on Monday, I isolated the full hoard before leaving yesterday and locked off so until I get it resolved it will stay like that.

 

[Engineer54]

Dead testing will show up the location of this fault quicker than you may think!!