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?

magnoliafan89 · Jun 3, 2019

happyhippydad said:
This one RIDGID micro CA-150 Inspection Camera - https://ridgidtoolshop.co.uk/hand-held-video-inspection/ridgid-1597-micro-ca-100-inspection-camera.html?gclid=Cj0KCQjwitPnBRCQARIsAA5n84ny7m9xdD6ESuZvE-RkBuYkSHX6ae3u0_qG0_uAQh_h4tJ-pTyxQZoaAuPlEALw_wcB
I'm sure it must be able to take a picture somehow at that price but I dont know how to do it!

----ing how much?!?!?!

HappyHippyDad · Jun 3, 2019

magnoliafan89 said:
****ing how much?!?!?!

Yeah but I'm sure it does all sorts of clever things.... I just don't know how to use them and probably never will!

ipf · Jun 3, 2019

happyhippydad said:
Yeah but I'm sure it does all sorts of clever things.... I just don't know how to use them and probably never will!

Don't give upon it...……...after all, you've just learned another way of finding a fault....and that was impossible 24 hrs ago.

PEG · Jun 4, 2019

FatAlan said:
Too right Tel. They’re also a family from the Manchester area I believe, one member is quite good at boxing

Not just one

johnduffell · Jun 4, 2019

happyhippydad said:
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!

Glad you found it! If you put 10a through a 6a mcb it will take a while to trip, and it wasn't clear if the trip was from overload or fault. The idea was also to work out if it's going down l-n or l-e or l-cpc.
Glad you're sorted though!

Lucien Nunes · Jun 4, 2019

it wasn't clear if the trip was from overload or fault

I think it was though. HHD reckoned on a couple of amps load on the circuit, and anything else consuming 1500VA or so would have made its presence felt. Had the MCB been tripping through being on the threshold of its current rating, a) it would have been warm which HHD would hopefully have noticed and b) the tripping would be more cyclic than random. It's useful to consider the power dissipated at a suspected fault and what damage it would cause. 1500W dissipated at a point fault would soon set it on fire or melt it so extensively that it could not have remained as it was to cause another trip. So it had to be an intermittent short-circuit, that did not blow itself clear completely on account of the low rating of the MCB and moderately high resistance of the small cable. Had the same damage occurred on a cooker circuit near the board, it would be more likely to blow itself apart at the first bang.

It had classic symptoms of a damaged cable - very low IR L-E and N-E with occasional tripping of the MCB. There was no certainty that each of the abnormal symptoms was related to a single fault but the principle of Occam's razor would suggest so. Easy to analyse with hindsight, of course.

johnduffell · Jun 4, 2019

Agree totally and thanks for the analysis, i only wanted to add it to the mix as a consideration as no point in saying "i thought that originally" after the fact!
Just a couple of points that may be interesting..
If an MCB trips in overload it can trip again quickly due to it already being very warm
Also the fault i suggested was a faulty psu for lights or similar. A transformer with shorted windings can warm up a lot without causing a fire