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!!

 

[Plonker 3]

Have you IR between the neutral and all the earths and main bonds?

 

[Marvo]

Also have you IR tested the neutral of the tripping circuit to all the other neutrals whilst they're disconnected?

 

[Eddiesparks]

Yes and yes!!

 

[Eddiesparks]

Just for reference if u had a borrowed N then they would both trip right??

 

[Marvo]

Maybe but one might win the race and prevent the other one from tripping and probably both would trip only if both circuits had load on them. If one of the circuits has zero load current then maybe it wouldn't trip. I'm not very familiar with RCBO's, we only occasionally come across them where I am so hopefully someone will confirm this or give you better info.

 

[baldsparkies]

Get the offending circuit going again on an mcb. You know its not a live to earth fault otherwise the mcb would have gone. Then try and use an educated guess as to which is the first fitting in the circuit. disconnect the rest from there and get the rcbo back in. See if it holds with just the one fitting and then re connect downstream until by process of elimination you find which run of cable or fitting is giving you problems. Not very technical granted, but you will be home in time for tea.

One other point if its a borrowed neutral once the outside lights are on an mcb the borrowed circuits rcbo will start to tripp. At least you will feel like your getting somewhere

 

[Badged01]

Meggered out ok before i started by the way!!

Whats this the technical term for?

 

[Eddiesparks]

Get the offending circuit going again on an mcb. You know its not a live to earth fault otherwise the mcb would have gone. Then try and use an educated guess as to which is the first fitting in the circuit. disconnect the rest from there and get the rcbo back in. See if it holds with just the one fitting and then re connect downstream until by process of elimination you find which run of cable or fitting is giving you problems. Not very technical granted, but you will be home in time for tea.

One other point if its a borrowed neutral once the outside lights are on an mcb the borrowed circuits rcbo will start to tripp. At least you will feel like your getting somewhere

Put outsides on mcb til tomorrow. All other rcbos held. Thanks for advice. This confirms my suspicions not borrowed neutral. Will 'insulation resistance test' bit by bit (technical term ;-) )

 

[Deleted member 72666]

Whats this the technical term for?

Hoovering!

 

[thomas-1981]

If everything is showing as clear in regards to the insulation resistance test, could the rcbo be faulty?

 

[baldsparkies]

Put outsides on mcb til tomorrow. All other rcbos held. Thanks for advice. This confirms my suspicions not borrowed neutral. Will 'insulation resistance test' bit by bit (technical term ;-) )

IR's might throw you. You have a lot of resistive loads in parallel plus feedbacks from parallel earths ect. Have you got any galv conduits in use, or gear trays going down by way of fixing screws into masonry that is damp (especially with recent weather)
Use your test gear by all means but proof of the pudding is in the eating, and trying to find an IR fault thats only a fault when RCBO is connected can be time consuming and unless your really seeing methodically what your readings mean real world scenario, things will soon get frustrating.
Process of elimination rules. Good luck, and happy hunting.

 

[Eddiesparks]

Thomas 1981. Tried 2 different rcbos!

Bladsparkies- yes galv conduit in use. Are u saying parallel earth could be a problem from these?? You're damn right it's frustrating!!

 

[Guest55]

Thomas 1981. Tried 2 different rcbos!

Are u saying parallel earth could be a problem from these??!

that makes no sense , all earthing is "parallel" as it originates at the same point.

if its outside lights tripping then look for water damage in a fitting or a faulty pir / photocell , i dont see it being a borrowed neutral at all.

 

[baldsparkies]

Thomas 1981. Tried 2 different rcbos!

Bladsparkies- yes galv conduit in use. Are u saying parallel earth could be a problem from these?? You're damn right it's frustrating!!

Yes galv is conductive you have another earth return, and plenty of places on route where the cables IR might be failing or a fitting gear tray. To get an accurate insulation resistance reading you will have to disconnect so much stuff. Your far better isolating parts of the circuit and trying the rcbo to see when the fault clears. That way you can narrow your search down to maybe a chaffed neutral touching the conduit or a faulty gear tray or loss of IP rating letting moisture in. Could even be a high resistance live to earth fault. Not degraded enough to throw the mcb but enough to take out a 30ma rcbo.
Like I said real world, Proof of the pudding, vs all kinds of readings that your trying to rationalise your choice. I have my preference. Go find your gremlin.

And Biff is right, its not the parallel earth path provided by the galv conduit that will cause you problems with the RCBO. BUT the galv will provide the cable with possible earth contact points on route ie chaffed cables. And that kind of scenario makes IR readings interesting.

 

[netblindpaul]

Measure the leakage currents?

 

[guest119]

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

Boydy

 

[Eddiesparks]

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]

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]

Whats this the technical term for?

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