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?

 

[Deleted member 9648]

Break the circuit down and find the 0.05 meg. I would not want to find the cause of an OCPD tripping by trial and error, there are likely to be fault currents sufficient to cause a fire. If you can isolate the low reading energise the rest of the circuit and see what doesn't work. Sometimes if everything appears to work I leave the offending part disconnected until the owner notices what doesn't work
Not a criticism by the way, I know how frustrating some faults can be.

 

[SJD]

I once had something like you describe, that turned out to be an outside cable on the circuit with bare corroded ends that were touching. Mostly hardly any current due to the corrosion, but sometimes more current would flow and then rapidly increase and trip the MCB.

 

[KEV 1 N]

Could be water ingress some where......?

 

[buzzlightyear]

ask hammer jack the customer did they put a nail in the wall.

 

[HappyHippyDad]

Break the circuit down and find the 0.05 meg. I would not want to find the cause of an OCPD tripping by trial and error, there are likely to be fault currents sufficient to cause a fire. If you can isolate the low reading energise the rest of the circuit and see what doesn't work. Sometimes if everything appears to work I leave the offending part disconnected until the owner notices what doesn't work
Not a criticism by the way, I know how frustrating some faults can be.

How would finding the 0.05Meg help? I could find out which 2 points it's between but not the item itself. Also, the fault may be at this lost item but it's a bit of a long shot.
A bit embarrassing to say, but it may actually be quicker to remove all fittings (a total of 6 lights) and physically look at them. I cringe when I hear electricians doing this rather than testing, but what happens when the tests are not faulty and cannot lead you!

Could be water ingress some where......?

I thought that too. I thought I had found the fault with a poorly fitted outside light but it wasn't this.

 

[HappyHippyDad]

ask hammer jack the customer did they put a nail in the wall.

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.

 

[SJD]

Might be interesting to monitor the current on the circuit (with all the lights turned off) when the MCB will hold. Is it zero then gradually creeps up; or increases very rapidly as the MCB trips.

 

[HappyHippyDad]

Might be interesting to monitor the current on the circuit (with all the lights turned off) when the MCB will hold. Is it zero then gradually creeps up; or increases very rapidly as the MCB trips.

Interesting Yes but how does it help?

 

[suffolkspark]

Just split the circuit in half and see which half that low reading is on and so on? Am I missing something

 

[SJD]

Interesting Yes but how does it help?

If there is a substantial current with everything turned off, you can find where is it going (before the MCB trips). If it just rapidly increases & trips, it is probably no help.

 

[FatAlan]

Live - neutral short somewhere with a loose varying amount of contact thus affecting the time it takes the MCB to trip?

 

[HappyHippyDad]

Just split the circuit in half and see which half that low reading is on and so on? Am I missing something

The low reading is 50000 ohms. I believe this is something 'plugged in' not the reason for the tripping MCB, therefore I would be tracking down this item rather than finding the fault.

 

[HappyHippyDad]

Live - neutral short somewhere with a loose varying amount of contact thus affecting the time it takes the MCB to trip?

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

 

[SparkyAndGeorge]

Have you checked the condition of all the fittings including any transformers and fans connected to that circuit?