Fault has stumped me!!

Eddiesparks · Mar 11, 2014

Did a board change today and put in rcbos for mcbs in common areas of a block of flats. Meggered out ok before i started by the way!!

Rcbo for outside lights didnt hold. Mcb was fine before. All others held fine. So neutral to earth fault right? Also disconnected neutral from rcbo no problems til neutral connected, with live connected all good. Connected neutral trip. Connected just neutral - trip. Earth in all the time.

Disconnected that circuit's earth and functional earth to see if it would hold. No joy. Traced earth and it goes nowhere, cut off at contactor! However outside lights gave me a Zs so earth must be taken off another circuit.

Thought about borrowed neutrals - but all other rcbo's held - didnt trip in a pair. Also meggered between neutrals came out ok. Even disconnected all other neutrals except outside lights. No joy.

It's a main board in common areas that feeds that building's power and lights and outside lights plus feeds a few submains. Flats are on separate main incomer individual fuses. Could it be a borrowed neutral from a lighting circuit in one of the flats.

Also, old board had charring round where the main neutral from 100a switch meets neutral bar. Is this probably unrelated and due to bad connection??

What a horrible day. Going back tomorrow to fault find some more. But it's hard to find the ordering of the outside lights there are groups of them all over the place. I suppose get Zs on all of them and lowest is closest to the board.

Wish me luck, but more importantly some highly experienced insight would be great!!

netblindpaul · Mar 11, 2014

Measure the leakage currents?

guest119 · Mar 11, 2014

Is there a control circuit for the contactor?
If so where does the neutral go and where is the circuit fed from?

Boydy

Eddiesparks · Mar 12, 2014

Boydy. Contactor is controlled by external PIR. Disconnected that and still fault. I'm there now. Lots of food for thought lads. Thank you. I'll get cracking! Ps def not L-E high resistance fault because fine with live connected. Introducing the neutral brings all the problems. I am doing what baldsparkies said and disconnecting neutrals bit by bit. I have a suspicion where roughly fault may be...

dazza1981 · Mar 12, 2014

Eddiesparks said:
Boydy. Contactor is controlled by external PIR. Disconnected that and still fault. I'm there now. Lots of food for thought lads. Thank you. I'll get cracking! Ps def not L-E high resistance fault because fine with live connected. Introducing the neutral brings all the problems. I am doing what baldsparkies said and disconnecting neutrals bit by bit. I have a suspicion where roughly fault may be...

Good luck eddie let us know how you get on im intrigued!!

Pete999 · Mar 12, 2014

Badged01 said:
Whats this the technical term for?

I was meggad when I went to bed last night, sorry that was due to too much Grolsh

Rampantchilli · Mar 12, 2014

Surely when testing for borrowed neutrals you don't test between neutrals? Surely the live of one circuit against neutrals of all other circuits, neutral to neutral shows only interconnection between neutrals

Richard Burns · Mar 12, 2014

Testing between neutrals would work so long as everything is on as there would be a connection through the faulty load and one of the OK loads to give the reading, but from line to another neutral would only be testing through the faulty load so gives a chance for diagnosis.
However if the load is electronic and near infinite resistance it may not show up.

This is my thought:

[ElectriciansForums.net] Fault has stumped me!!

be interested in feedback.

dazza1981 · Mar 12, 2014

Richard Burns said:
Testing between neutrals would work so long as everything is on as there would be a connection through the faulty load and one of the OK loads to give the reading, but from line to another neutral would only be testing through the faulty load so gives a chance for diagnosis.
However if the load is electronic and near infinite resistance it may not show up.

This is my thought:
View attachment 23537
be interested in feedback.

That makes perfect sense to me and the drawing makes it even easier to understand. Thanks mate

telectrix · Mar 12, 2014

nice diagram. very useful post.

mnbs 4x4 · Mar 12, 2014

is it possible that the lighting neutrals are crossed at the rcbo's , are the neutrals easily traced back at the board to match the feeds to double check

MDJ · Mar 12, 2014

Water in a photo cell or a light more than likely lol

R G · Mar 12, 2014

I would try and trace with earth leakage clamp meter with an MCB in for now, and trace it through with just that 1 circuit live then liven up other circuits and see what happens with the readings , you could try with lamps in first and all switches off , then try with switches on one at a time , trace it that way ,