Strange fault/short

[stidge]

Hi all , been a mad day and last thing I needed at 6pm was this phone call ! Anyway cut it short. Signed off new extension 3 weeks ago . New ring .60 .60 1.12 >500 so all good . Had to go back today customer woke up socket ring off , went to reset ( RCD still up ! ) mcb made a pop ! I went and changed the mcb , then re tested , exact same results , still went pop ! Is it possible I have two x Wylex 32a Nsb mcb s that are faulty ??? By the way these are old type with front cut out for bus bar ! Head is spinning , not proclaiming to be an electrical genius but never come across this before ?? Back tomorrow, just wondered if anyone had any other things I can try tomorrow as testing is not helping at all ! Cheers

 

[Timbo]

If it's not tripping the rcd, I would expect a L-N short.
Check IR and continuity.
Has customer been fixing things to walls?

 

[littlespark]

Swap your suspected faulty mcb with one from another circuit that DOES work.

rule out the mcb.
other than that.... test test test!

And remember to unplug EVERYTHING don’t want 500v across brand new telly!

 

[James]

unplug everything, test for continuity between L and N
could be a faulty appliance.
could be real unfortunate where the live of one cable is contacting the N of another if they have burnt into each other whilst pulling through a tight hole in joist.

 

[Wilko]

My bet is work after you’ve gone has damaged cables. See if you get the same dead test results now .

 

[mattg4321]

IR test between L-N and L-E both >500Mohm?

 

[stidge]

Thanks lads , forgot to say everything unplugged ( even disconnected usb socket as I thought it could be that , and as I said continuity and IR all fine , been round job and nothing been fixed/ drilled since I signed off ! Thanks again
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IR test between L-N and L-E both >500Mohm?
Yes mate fine , that’s why it doesn’t make sense !

 

[littlespark]

Everything unplugged? IR checks ok? and it’s still tripping with nothing plugged in?

you must have missed something.

Try what I said, swap out with a known good breaker.

 

[James]

breakers don't go "pop" when there is no current flowing.
they go "click"
if you have disconnected all loads and the breaker wont stay on, remove the outgoing cable and try again.
if it stays on, breaker is probably ok.

 

[happysteve]

I've had preceisely one fault where the breaker itself was faulty (or rather damaged) because it had been incorrectly terminated at the bus bar, poor connection, generated heat, thermal damage to MCB -> kept tripping, even though the circuit itself was fine.

(I've also had a faulty, intermittently-tripping RCBO, too, although passed all the RCD tests and circuit was fine).

If the RCD is not tripping, but the MCB is tripping, and the MCB is not faulty, then there must be too much current L-N as others have said. You'll need >32A to cause it to trip, which means the resistance L-N would <7Ω. You'd usually measure insulation resistance L-N (with everything unplugged) as the expected reading would be >>1MΩ, but if it's tripping a 32A MCB then the resistance will be in the ohms region.

 

[davesparks]

It's a little confusing in your post, did you test the circuit or just try resetting the MCB and then replacing it?
If you did test it then what tests did you perform and what were the results of those tests. (please don't just say 'all tested ok' or similar, it is unhelpful, give us actual test results)

 

[stidge]

I've had preceisely one fault where the breaker itself was faulty (or rather damaged) because it had been incorrectly terminated at the bus bar, poor connection, generated heat, thermal damage to MCB -> kept tripping, even though the circuit itself was fine.

(I've also had a faulty, intermittently-tripping RCBO, too, although passed all the RCD tests and circuit was fine).

If the RCD is not tripping, but the MCB is tripping, and the MCB is not faulty, then there must be too much current L-N as others have said. You'll need >32A to cause it to trip, which means the resistance L-N would <7Ω. You'd usually measure insulation resistance L-N (with everything unplugged) as the expected reading would be >>1MΩ, but if it's tripping a 32A MCB then the resistance will be in the ohms region.

It's a little confusing in your post, did you test the circuit or just try resetting the MCB and then replacing it?
If you did test it then what tests did you perform and what were the results of those tests. (please don't just say 'all tested ok' or similar, it is unhelpful, give us actual test results)

hi, sorry for rubbish explanation ! First tried another mcb , still tripped , then tested cont- .60 .61 1.12 then IR LE >200 NE>200 and LN/E >200 ( wasn’t sure what was plugged in and 1 x usb socket )
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breakers don't go "pop" when there is no current flowing.
they go "click"
if you have disconnected all loads and the breaker wont stay on, remove the outgoing cable and try again.
if it stays on, breaker is probably ok.

Tried that mate and mcb stayed in with line disconnected

 

[James]

Odds are you have a live to neutral fault as said before.
Take away the usb sockets and replace with wago to keep the ring.
Test L to N continuity, find the fault .
With everything disconnected there should be open circuit both at low voltage and at ir test voltages

 

[davesparks]

hi, sorry for rubbish explanation ! First tried another mcb , still tripped , then tested cont- .60 .61 1.12 then IR LE >200 NE>200 and LN/E >200 ( wasn’t sure what was plugged in and 1 x usb socket )

You have an MCB tripping, but the RCD doesn't trip, it should be obvious from this information alone that the cause of the tripping is going to be overcurrent flowing between L and N.
But you have carried out a bunch of tests which won't identify this type of fault?

This just seems illogical to me, what was your logic behind carrying out these tests?

 

[Pete999]

Hi all , been a mad day and last thing I needed at 6pm was this phone call ! Anyway cut it short. Signed off new extension 3 weeks ago . New ring .60 .60 1.12 >500 so all good . Had to go back today customer woke up socket ring off , went to reset ( RCD still up ! ) mcb made a pop ! I went and changed the mcb , then re tested , exact same results , still went pop ! Is it possible I have two x Wylex 32a Nsb mcb s that are faulty ??? By the way these are old type with front cut out for bus bar ! Head is spinning , not proclaiming to be an electrical genius but never come across this before ?? Back tomorrow, just wondered if anyone had any other things I can try tomorrow as testing is not helping at all ! Cheers

Any outside lights fed from this RFC via a FCU?

 

[radiohead]

As already stated the evidence points to a L-N fault.
You need to think logically.
You re-tested ring continuity, a break in r1,rn or r2 will not cause an mcb to trip.
You re-tested insulation resistance to earth, when it should be apparent that a fault to earth will trip the RCD before an mcb trips.
You need to carry out an insulation resistance test L-N, but do so at the 250v setting on your meter initially. If there is a short circuit this should still identify it with less risk to any connected equipment you may have overlooked.
Bear in mind if you do get a low reading this could mean two things, a fault, or you still have a connected load.

L-N faults can be tricky, if the 250v test is inconclusive sometimes going to 500v will pick it up....and I've even whacked 1000v across an inconclusive or flickering reading which has broken it down into a dead short which is easier to trace.
Only do that if 250v is inconclusive and you are absolutely certain nothing is connected.

 

[stidge]

As already stated the evidence points to a L-N fault.
You need to think logically.
You re-tested ring continuity, a break in r1,rn or r2 will not cause an mcb to trip.
You re-tested insulation resistance to earth, when it should be apparent that a fault to earth will trip the RCD before an mcb trips.
You need to carry out an insulation resistance test L-N, but do so at the 250v setting on your meter initially. If there is a short circuit this should still identify it with less risk to any connected equipment you may have overlooked.
Bear in mind if you do get a low reading this could mean two things, a fault, or you still have a connected load.

L-N faults can be tricky, if the 250v test is inconclusive sometimes going to 500v will pick it up....and I've even whacked 1000v across an inconclusive or flickering reading which has broken it down into a dead short which is easier to trace.
Only do that if 250v is inconclusive and you are absolutely certain nothing is connected.

Thanks I’ll let you know how I get on later today , hopefully with a clear head ! Cheers

 

[radiohead]

Good luck! Often a fresh start and clear head gets it sorted straight away.

 

[mattg4321]

If it’s going ‘pop’. You probably don’t even need to IR it to start with. Continuity test between Land N will give you a more accurate result. It’s going to be only a few ohms. Possibly less than 1 ohm. Might even help you find the fault quicker as if it’s under 1 ohm you can work out if you’re getting closer/further from the fault by the reading

 

[stidge]

Any outside lights fed from this RFC via a FCU?

No mate , new ring , approx 8 double sockets , 1 x outside, 1x usb , 2x fcu for utility fan and underfloor heating