Difficult fault to find?

[HappyHippyDad]

The 0.05MΩ is L > N, so a load, possibly a fan timer? Won't like insulation test volts up it.

But you say the L+N > E is 0.3MΩ which is abnormally low and quite possibly at the fault location. It might be variable, a dead short at times and a meg at others. A likely scenario is that a cable is damaged with the conductors almost touching, which actually make contact when something moves, e.g. a heating pipe expands or a floorboard rocks. The arc has left carbonised insulation which is the cause of your low IR.

This is one of the things about IR testing to find a fault. It doesn't necessarily reveal the low resistance that caused / causes the bang, but it can lead you to anything suspicious - e.g. a damp section of wiring or even the aftermath of the fault - which does aid location.

I'd be splitting the circuit up in search of any section below a few MΩ.

Excellent response Mr LN
Apologies to SuffolkSpark who kind of suggested this as well and I poo poo'd it.
Do you really think tracking down the 0.3M ohms to earth will lead to the tripping MCB fault Lucien?
I was going to try terminating the line conductor in one of the MCB's on the RCD side of the board to see if it tripped the RCD. If it does trip the RCD then I know I am looking at a L-E fault and I will track down the 0.3Meg. However if it doesn't trip the RCD then would you agree that the 0.3Meg to earth is a red herring as the fault would then likely be L-N?

 

[HappyHippyDad]

Duplicate

 

[Lucien Nunes]

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.

 

[Deleted member 9648]

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.

 

[Squid]

Quote them for a full rewire......

 

[FatAlan]

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]

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]

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]

Eek!

 

[rolyberkin]

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]

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.

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]

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]

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]

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]

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]

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]

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]

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

 

[Gary latto]

Interesting to know the outcome

 

[Toneyz]

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.