Fault has stumped me!!

[Eddiesparks]

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!!

 

[Rampantchilli]

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]

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:

be interested in feedback.

 

[dazza1981]

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]

nice diagram. very useful post.

 

[mnbs 4x4]

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]

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

 

[R G]

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 ,

 

[londonlec]

Does the circuit definitely need an RCBO...

 

[Plonker 3]

Does the circuit definitely need an RCBO...

Why have you asked this?

 

[Des 56]

Originally Posted by londonlec
Does the circuit definitely need an RCBO...

Why have you asked this?

Because he wants to know if the circuit needs an Rcbo sorry dillb, couldn't resist

 

[londonlec]

Why have you asked this?

Because the landlord/Local Authorities don't want callouts due to tripping RCDs.

Obviously the fault still needs to be found. Have you IR tested the outgoing side of the contactor?

 

[Plonker 3]

Why wouldn't they?

They have a duty of care to ensure everything is safe, if it's tripping find fault repair then all is good.

Had this yesterday, RCBO keeps tripping narrowed it down to faulty light, removed fitting all ok now. RCBO still on circuit and not tripping anymore.

 

[Plonker 3]

Because he wants to know if the circuit needs an Rcbo sorry dillb, couldn't resist

[/QUOTE

i asked for that didn't I lol

 

[Matt85]

Without reading the tread in full I had this a few weeks ago. Turned out the photo cell was on a common neutral.

 

[mobile]

rcbo

Try swapping rcbo with a known good one from another circuit. I had a similar problem some time ago spent ages doing all checks and tests turned out to be duff rcbo very annoying had to go home and kick the dog!

 

[Badged01]

Re: rcbo

..... had to go home and kick the dog!

Bet she must love waiting for you to come home! Tea ready on the table, he's walking up the garden path, i'm heading down to the outside loo for a while .....

 

[guest119]

Re: rcbo

Try swapping rcbo with a known good one from another circuit. I had a similar problem some time ago spent ages doing all checks and tests turned out to be duff rcbo very annoying had to go home and kick the dog!

No need to swap if it was ramped.

Boydy

 

[Eddiesparks]

Riiiiight. So I have made a bit of an error. When doing IR test I disconnected cpc neutral and live and did it that way. The cpc wasnt going anywhere, the lights bprrowed the cpc from various local points. Not awful but very rough I know! I had a fun day disconnecting sets of lights bit by bit. Turns out they are pretty much all letting in water. Going back to change 10 lights tomorrow and Monday. Eurgh...

Cant believe i made that IR cock up. I must admit, i have a love hate relationship with IR tests. So many variables to give you low results and the wiring is slapdash in this place to say the least with so many variables I'm not sure where to start!!

 

[NickD]

Riiiiight. So I have made a bit of an error. When doing IR test I disconnected cpc neutral and live

You're meant to IR test with CPC connected to the means of earthing, I believe?

 

[Eddiesparks]

You're right there. I did learn that somewhere. But it'd been a long day and I wasnt with it.

Now this is odd: All the lights are on when PIR is overrided. When i switch a particular submain (on its own rcbo) one light goes out. Now this HAS to be borrowed neutral surely because switched live is fed from one contactor?! Or perhaps switched live and neutral from different circuit is feb from contactor? I'm a bit baffled. Lights are on mcb til i sort probs out. But rcbo feeding submain does NOT trip which is why borrowed neutral looks v unlikely. Someone help me i feel like i'm losing the plot, not slept much since son was born, i swear having young kids turns your grey matter into cauliflower....