Difficult fault to find?

[HappyHippyDad]

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?

 

[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.

 

[Octopus]

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]

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

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]

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]

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]

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