400v at Lights

chrisclose · Mar 6, 2015

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?

telectrix · Mar 7, 2015

could the N be lost before the DB?

Engineer54 · Mar 7, 2015

telectrix said:
could the N be lost before the DB?

It could indeed!!

chrisclose · Mar 7, 2015

Cheers for your help guys! I'll report back with my findings during the week!

mnbs 4x4 · Mar 10, 2015

I had the exact same thing many years ago, same voltages at fittings and switches as you describe ,which destroyed every ballast on the board. Mine was an Independant lighting TP and N DB serving corridor, staircase lighting etc. It turned out that the old MEM switchfuse supplying th DB had a slack incoming Neutral that overtime had heated to the point all the insulation had melted and eventualy it broke off leaving a very costly bill to replace everything.
Start from the beginning of the installation , sometimes it can be the simplest of things that you don't think to check first.

Engineer54 · Mar 10, 2015

chrisclose said:
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.

Have you not found the problem/cause yet?? Hells Bells!!

chrisclose · Mar 10, 2015

I've not been sent back to it, other guy was sent this morning to find out what the deal is so I'm just waiting on the phone call!

chrisclose · Mar 17, 2015

Turned out there was an absolutely spectacular joint buried in a ceiling that had been lowered and then had a suspended ceiling hanging from it... Neutral connection had frittered away causing the 400v at lights, rewired from the main iso up to the iso in the motor room with a direct cable to replace said joint and voilà, standard readings across all phases etc.

Bit of a head pickler when looking for the fault originally but i'm glad I had it, learned from it!