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HappyHippyDad

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Hello all..

I have a problem with a tripping RCBO and it has me stumped!

Yesterday I fitted 10 RCBO's into a CU which currently had an RCD main switch. This was a sub circuit from a house CU which feeds various circuits for a cattery.

4 of the RCBO's are for the cattery pens. Pens 1,2,3 and 4 (each housing 16 seperate units). Pens 1-3 are 1st fix only (awaiting heat lamps). Pen 4 has heat lamps in.

When any heat lamp in pen 4 is energised the RCBO trips, however if the conductors in the CU for pen 4 are added to the RCBO for Pen 1 all is ok. However, when I IR test between lives of Pen1 and pen4 I get >1000MΩ?

IR tests for pen 4 heat lamps test ok, all >1MΩ.

When I disconnect the neutral fly lead in the CU for Pen 1 I get 197V L-N at Pen 4 and 240V L-E. When I reconnect neutral fly lead for Pen 1 the voltage increases to 240V for Pen 4 so Pen 4 must be utilising the neutral path of Pen 1 but it has >1000MΩ of resistance between neutrals!!

Luckily the cats are not cold at the moment!

Any suggestions would be appreciated :frown:
 
Daft question, but any chance of borrowed neutrals???
 
Daft question, but any chance of borrowed neutrals???

Thats what stares at you Pete but I have put all new wiring in (all tested) bar a short length of existing 19 core SWA which I tested every single one (IR tested between all of them and to earth, found a fault to earth on one of them but have removed this from service so it has nothing to do with present situation).

Whats with the decreasing voltage when I remove the neutral fly lead from pen 1? Why am I not getting 240V with the correct neutral from Pen 4? I shall have to check continuity but i'm sure its fine! But it cant be otherwise I'd get 240V!?

I think I am going to have to return and disconnect every single RCBO and start testing from scratch!
 
Thats what stares at you Pete but I have put all new wiring in (all tested) bar a short length of existing 19 core SWA which I tested every single one (IR tested between all of them and to earth, found a fault to earth on one of them but have removed this from service so it has nothing to do with present situation).


Are you saying you have disconnected a single core of a cable which has a fault, or disconnected a whole cable which has a fault?
 
Are you saying you have disconnected a single core of a cable which has a fault, or disconnected a whole cable which has a fault?

There are 19 cores. Core number 13 read a poor resistance to earth so I have not used it in the design, I have connected both ends to earth. I also tested between core number 13 and all other cores and got acceptable IR readings. I dont think this is related to the problem but am certainly open to suggestion.
 
There are 19 cores. Core number 13 read a poor resistance to earth so I have not used it in the design, I have connected both ends to earth. I also tested between core number 13 and all other cores and got acceptable IR readings. I dont think this is related to the problem but am certainly open to suggestion.

So there is a fault between one of the conductors which is layed up in the outermost layer and the armour? And you don't think maybe that cable is damaged?
Have you established what the fault is?
 
Sounds like the heaters are causing some earth leakage.
Could be due to damp.
One solution would be to replace the RCBOs with MCBs temporarily to allow the heaters to heat up enough to drive away any moisture.
Is RCD protection a requirement?
 
There are 19 cores. Core number 13 read a poor resistance to earth so I have not used it in the design, I have connected both ends to earth. I also tested between core number 13 and all other cores and got acceptable IR readings. I dont think this is related to the problem but am certainly open to suggestion.

This would be my starting point...regardless of the one single identified fault,it is one cable,ergo;you have a faulty cable...
You would struggle to test ALL the permutations of other possible faults,with that number of cores...

How long is this cable,and could you bridge out to test?
 
Thats what stares at you Pete but I have put all new wiring in (all tested) bar a short length of existing 19 core SWA which I tested every single one (IR tested between all of them and to earth, found a fault to earth on one of them but have removed this from service so it has nothing to do with present situation).

Whats with the decreasing voltage when I remove the neutral fly lead from pen 1? Why am I not getting 240V with the correct neutral from Pen 4? I shall have to check continuity but i'm sure its fine! But it cant be otherwise I'd get 240V!?

I think I am going to have to return and disconnect every single RCBO and start testing from scratch!

Think you've answered your own question there HHD!
 
Hi HHD! was the installation on an RCD before? Is there a big ish start up current which may require a C type RCBO? If it was on an RCD before it should hold?
 
So there is a fault between one of the conductors which is layed up in the outermost layer and the armour? And you don't think maybe that cable is damaged?
Have you established what the fault is?

Yes, that cable may be damaged and most probably touching the armour so I am not using it and have connected to earth. Can you explain what you are getting at Dave?

Sounds like the heaters are causing some earth leakage.
Could be due to damp.
One solution would be to replace the RCBOs with MCBs temporarily to allow the heaters to heat up enough to drive away any moisture.
Is RCD protection a requirement?

I have IR tested all lamps and all test ok.

Time to do a few Rn + R2 readings.

what is the rating of the rcd main switch?

I have removed the RCD main switch Murdoch as all are now on RCBO's. Previously it was an 80A rated switch, now just a standard double pole 100A main switch. Not sure what you're getting at though?

This would be my starting point...regardless of the one single identified fault,it is one cable,ergo;you have a faulty cable...
You would struggle to test ALL the permutations of other possible faults,with that number of cores...

How long is this cable,and could you bridge out to test?

I have tested all permutations Peg and it took a long time. I did this before changing to RCBO's to avoid this problem! :smiley2:
I have considered bridging and may well do that, thanks.

Think you've answered your own question there HHD!

Yes, I think I have haven't I :D

Hi HHD! was the installation on an RCD before? Is there a big ish start up current which may require a C type RCBO? If it was on an RCD before it should hold?

Yes Timo, It had an RCD as main switch.
 
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What I am getting at is that the cable is damaged, regardless of how many cores appear to be unaffected you cannot sensibly use that cable for any mains voltage knowing it is damaged.
If you nail through the centre of a 3core and earth cable would you just connect the danger core to earth and carry on using the other two cores?
 
I would be tending to the damaged cable scenario.
But have you tried pen 4 in another RCBO, just in case that one is faulty.
Do you get any voltage on Pen 1 conductors if Pen 1 is disconnected but Pen 4 is connected and does the RCBO trip?
 
What I am getting at is that the cable is damaged, regardless of how many cores appear to be unaffected you cannot sensibly use that cable for any mains voltage knowing it is damaged.
If you nail through the centre of a 3core and earth cable would you just connect the danger core to earth and carry on using the other two cores?

HHD, how many circuits utilise the old multi-core cable?
Of these, which circuits are causing problems?

I would be tending to the damaged cable scenario.
But have you tried pen 4 in another RCBO, just in case that one is faulty.
Do you get any voltage on Pen 1 conductors if Pen 1 is disconnected but Pen 4 is connected and does the RCBO trip?

With regards the 'damaged' cable, I cant remember the IR results precisely however they were a little below 1MΩ to earth. I am unsure of what I could do regards this cable as the 19 core is laid under a shed and concrete, it is about 15m's in length and all other cores are in use. What would you do Richard? This cable is required and the client is not going to agree to the cattery being dug up, there is also no viable option to lay a new cable. Thinking about it further, it will not be touching the armour as the IR results were only just below 1MΩ, still a fail though!

I have not tried Pen 4 in another RCBO by itself, I will do. I did ramp the RCBO and it was fine. Also I will try your suggestion regards seeing if there is voltage on pen1 with pen 4 connected.

I had been at the cattery for 10 hours and I had got to the point where my brain was not working properly so I left. This is one of the few times when a fault is getting the better of me and I'm finding it quite frustrating. I shall go back with a clear head and slowly and methodically test.

Edit... Sorry, quoted too many people!
Spin.. All circuits utilise existing 19 core.
Dave.. my reply to Richard half answers your question. I realise you are a stickler for doing things correctly, but would you really insist to the owner that he dismantle his shed, rip up the concrete etc for an IR result of a little below 1MΩ?
 
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I shall go back with a clear head and slowly and methodically test.

That is probably the answer!

Nothing one could do about the cable now, it should have been replaced at the time of installation if the fault was present then, or replaced once the fault was found but before finishing the cattery, both of which were presumably not viable options.
A value of 1MΩ will not affect a circuit noticeably; even though it is a fail as it is indicative of excessive deterioration either by damage or age.
Probably just damp through the sheath rotting the armour away!
 

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