Difficult fault to find?

HappyHippyDad · Jun 2, 2019

I have come across a fault that I am struggling to find. I cant remember being stumped like this before as I like fault finding and have always found the fault before.

6A MCB is tripping on lights. It sometimes trips the moment it is switched on and sometimes takes up to 30 mins. There is no RCD.

IR readings are poor (o.3Mohms L/N - E) but do not show a short circuit to earth. I have removed all lamps but IR readings for L-N still suggests something is on the circuit which I cannot find (0.05Mohms).

I have tried it on another 6A MCB, it trips, so not a faulty MCB.

It will not be an over current as there is little on the circuit (2 bathroom lights, 1 x kitchen, utility, garage, perhaps 2A absolute max).

My plan is now to break the circuit in half and leave. Customer can tell me if it trips or not. I can carry on like that but it means many trips and they are not close.

I could also try it on the RCD side to see if it trips the RCD (meaning a L-E fault, but would this knowledge help?)

Any suggestions?

FatAlan · Jun 2, 2019

happyhippydad said:
Yep, that's what I think. Now tell me how to find it without any faulty readings whilst I sup my beer

Yep, supping a beer too. When I come up with something I’ll let you know

Lucien Nunes · Jun 2, 2019

whack 1000v across LN-E.....NOT L-N (!)

This is a good trick. Even if the fault doesn't avalanche, it can at least give a significantly different reading that indicates something unstable. Testing at 250V can give a similar indication by way of a much higher reading.

Here, I have a choice of insulation testers. 250V, 500V, 1kV, 2.5kV, 15kV and Cuthbert. In Cuthbert v. domestic wiring, if there are no faults, Cuthbert will create some for you. 60kV at enough current to carbonise you. He's really for testing HV lines.

HappyHippyDad · Jun 2, 2019

Lucien Nunes said:
No. If there is rodent damage to a cable, for example, there might be all manner of simultaneous problems. In fact, the 0.05MΩ might be one of them. The tripping might be flashovers L-N hence won't take out the RCD, but the 0.3MΩ is where the bared length of L is resting against brickwork so won't go low enough to trip the RCD either.

No evidence is irrelevant until the case is closed.

I did forget to mention something!

A mouse has been recently seen in the property

Lucien Nunes · Jun 2, 2019

rolyberkin · Jun 2, 2019

happyhippydad said:
I have quizzed the customer quite a bit.
No new work.
No trades in recently.
No DIY work.
They did not initially switch something on for it to trip, it just tripped.

You have simply forgotten golden rules 1 & 2

Golden Rule No 1 = Customers Lie
Golden Rule No 2 = Refer to rule number 1

See my recent post in The Arms, everything now is rodent damage!

HappyHippyDad · Jun 2, 2019

wirepuller said:
I sometimes whack 1000v across LN-E.....NOT L-N (!). It is often enough to break down an intermittent fault into a dead short, which is easier to locate.

Lucien Nunes said:
This is a good trick. Even if the fault doesn't avalanche, it can at least give a significantly different reading that indicates something unstable. Testing at 250V can give a similar indication by way of a much higher reading.

Here, I have a choice of insulation testers. 250V, 500V, 1kV, 2.5kV, 15kV and Cuthbert. In Cuthbert v. domestic wiring, if there are no faults, Cuthbert will create some for you. 60kV at enough current to carbonise you. He's really for testing HV lines.

Thanks for this.
Would you say that testing at varying voltage gives varying results ...
always, usually, sometimes, rarely? I mean this with regards a fault, not when testing a healthy circuit (as the results will be similar).
It's just useful to know how much credence to give to the results hence the options above.

Toneyz · Jun 2, 2019

happyhippydad said:
I have come across a fault that I am struggling to find. I cant remember being stumped like this before as I like fault finding and have always found the fault before.

6A MCB is tripping on lights. It sometimes trips the moment it is switched on and sometimes takes up to 30 mins. There is no RCD.

IR readings are poor (o.3Mohms L/N - E) but do not show a short circuit to earth. I have removed all lamps but IR readings for L-N still suggests something is on the circuit which I cannot find (0.05Mohms).

I have tried it on another 6A MCB, it trips, so not a faulty MCB.

It will not be an over current as there is little on the circuit (2 bathroom lights, 1 x kitchen, utility, garage, perhaps 2A absolute max).

My plan is now to break the circuit in half and leave. Customer can tell me if it trips or not. I can carry on like that but it means many trips and they are not close.

I could also try it on the RCD side to see if it trips the RCD (meaning a L-E fault, but would this knowledge help?)

Any suggestions?

Find the fault that is giving you 0.05Mohms. halving the circuit and asking the customer to let you know if it trips does not come over very well apologise for being blunt but there it is.

KEV 1 N · Jun 2, 2019

happyhippydad said:
I did forget to mention something!
A mouse has been recently seen in the property

Is the resistance of a mouse around 0.05Mohms by any chance!

HappyHippyDad · Jun 2, 2019

happyhippydad said:
I did forget to mention something!
A mouse has been recently seen in the property

Perhaps I should take my fairly newly acquired air rifle instead of my MFT!
I bought an air rifle to shoot a squirrel in the loft that was giving me sleepless nights.
I had moved my bed downstairs it was so bad.
It would go against my beliefs to just kill it without making use of it so I had read up on how to skin it and cook it.
I practised with the rifle for a couple of weeks so that I would be a good shot and not merely injure the animal.
I heard it one night.
I went up with my head torch and rifle.... into the loft.
I saw it right in the corner!
I took aim......
I continued to take aim.....
I remained continuing to take aim.....
for around 2 minutes....
It ran away!

It left soon after and I moved back into my bedroom

.
I realise this story is a bit odd, but it is true.

Andy78 · Jun 2, 2019

happyhippydad said:
Perhaps I should take my fairly newly acquired air rifle instead of my MFT!
I bought an air rifle to shoot a squirrel in the loft that was giving me sleepless nights.
I had moved my bed downstairs it was so bad.
It would go against my beliefs to just kill it without making use of it so I had read up on how to skin it and cook it.
I practised with the rifle for a couple of weeks so that I would be a good shot and not merely injure the animal.
I heard it one night.
I went up with my head torch and rifle.... into the loft.
I saw it right in the corner!
I took aim......
I continued to take aim.....
I remained continuing to take aim.....
for around 2 minutes....
It ran away!

It left soon after and I moved back into my bedroom .
I realise this story is a bit odd, but it is true.

You bought an air rifle to shoot an animal even though this was against your beliefs ? Surely a humane trap would have been a smarter buy ?

Good to know that threatening it worked well enough anyway

James · Jun 2, 2019

How about changing to a 50A breaker,
Turn it on and wait to see where the smoke comes from?
P.s. I get smarter on a Sunday after a booze lunch!!

HappyHippyDad · Jun 2, 2019

anthonybragg said:
Find the fault that is giving you 0.05Mohms. halving the circuit and asking the customer to let you know if it trips does not come over very well apologise for being blunt but there it is.

I agree Anthony.
I'm going to go down there tomorrow and find it!
Thanks for reminding me of my moral duty and I shall update you all.

rolyberkin · Jun 2, 2019

Silly question but are you testing this at 250v or 500v?

Gary latto · Jun 2, 2019

Interesting to know the outcome

Toneyz · Jun 2, 2019

A fresh on Monday you will be in a better mindset treat it as building blocks start at the c.u. and work through one part of the circuit at a time.

Octopus · Jun 2, 2019

Is the lighting circuit wired as loop in or neutrals at switches?

With so few lights narrowing it down to a section shouldn’t be too tricky.

Had this recently and mice were to blame .....

HappyHippyDad · Jun 2, 2019

Andy78 said:
You bought an air rifle to shoot an animal even though this was against your beliefs ? Surely a humane trap would have been a smarter buy ?

Good to know that threatening it worked well enough anyway

T

Murdoch said:
Is the lighting circuit wired as loop in or neutrals at switches?

With so few lights narrowing it down to a section shouldn’t be too tricky.

Had this recently and mice were to blame .....

Neutrals are in the light fittings.

Octopus · Jun 2, 2019

happyhippydad said:
Neutrals are in the light fittings.

I’d slowly taking it apart .... another one from the mists of time was where the switch drop cable had been trapped and damaged where boards had been laid in the loft, and the board pinched the cable as it entered the metal conduit .....

You will be really chuffed when you solve this

The necessary ingredients are patience, paper, pencil or pen and your test kit .. and a customer knowing that the bill will only be known when it’s done and dusted

johnduffell · Jun 2, 2019

don't let the IR test mislead, remember it would only be accurate for a linear resistance. The MCB will trip on reactive power as well as real power but with DC IR testing you'll only notice what would transfer the real power.
Could be a faulty PSU somewhere causing the issue perhaps.
Or it could be the mouse!
Can you get a clamp meter on the circuit when the MCB is on?

UNG · Jun 3, 2019

happyhippydad said:
I may have to do that, but I was hoping to track it down with fault finding/testing rather than undoing everything on the circuit.

There are only so many tests you can do before you have to undo something and test again this may need to be repeated a number of times until you find the fault
If the fault is big enough to allow a circuit tracer to function you can sometimes use this to take you to the fault without having to break the circuit down

HappyHippyDad · Jun 3, 2019

johnduffell said:
don't let the IR test mislead, remember it would only be accurate for a linear resistance. The MCB will trip on reactive power as well as real power but with DC IR testing you'll only notice what would transfer the real power.
Could be a faulty PSU somewhere causing the issue perhaps.
Or it could be the mouse!
Can you get a clamp meter on the circuit when the MCB is on?

Yes I could, but would this not just show the 1 or 2 A running through it at that time? It may change rapidly when it's about to trip but I don't think I can get away with standing there for 30 mins

Anyway, I am now leaving to find it!

Barnaby Stedman · Jun 3, 2019

found the fault?

HappyHippyDad · Jun 3, 2019

Barnaby Stedman said:
found the fault?

Just found it!
L/N -E results were now 0.03Meg. Traced the fault to between 2 lighting points.
Unfortunately this was between the main house and flat roofed extension.
Traced the cable in the loft where is disappeared into the cavity between main house and extension.
Crawled to edge as far as I could and poked the inspection camera down into the cavity.
This is what I saw..

[ElectriciansForums.net] Difficult fault to find?

Sorry for poor quality but this is me taking a picture with my phone of what the inspection camera was seeing.
Looks like the naughty mouse!
Ps.. looks like he's started on the socket cable nearby as well which you can just make out in one if the pics.

James · Jun 3, 2019

happyhippydad said:
Just found it!
L/N -E results were now 0.03Meg. Traced the fault to between 2 lighting points.
Unfortunately this was between the main house and flat roofed extension.
Traced the cable in the loft where is disappeared into the cavity between main house and extension.
Crawled to edge as far as I could and poked the inspection camera down into the cavity.
This is what I saw..
View attachment 49768 View attachment 49769 View attachment 49770
Sorry for poor quality but this is me taking a picture with my phone of what the inspection camera was seeing.
Looks like the naughty mouse!

congratulations.
bet you feel better already.

Barnaby Stedman · Jun 3, 2019

James the Spark1976 said:
congratulations.
bet you feel better already.

i expect his customer now expects him to fix it though!!

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