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mattg4321

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Hi guys (and girls)

Have a job for a long term customer, finding a pretty awkward fault. I'm almost to the point where I'm a bit stumped. Generally speaking IR testing, RCD testing and experience hasn't let me down in the past and I've found plenty of faults other 'electricians' haven't.

Symptoms are:

Loud bang, possibly from kitchen area, happens around once every 3-4 weeks. Downstairs RFC RCBO trips out. This covers all sockets downstairs, bar 3. It has apparently happened twice overnight, when no appliances are being used - customer has come downstairs to find circuit off. This has been ongoing for a few months now. The consumer unit is at the other end of the house to the kitchen.

Testing so far:

All loads disconnected Line-Earth, Line-Neutral and Neutral-Earth all reading over 70Mohm.
All loads connected L&N connected together tested to Earth - 5.5Mohm
I thought I found it when I found an old fridge with a very low resistance line to earth. Sadly not. The fridge has been replaced.
Nearly new Hager consumer unit (fitted last year) RCBO ramp tested. All ok.

Conclusion:

The customer is a liar! Highly unlikely in this case.

or

Appliance is causing the issue. What would cause this at any time of the day randomly when nothing is operating though? How to confirm this without a scattergun approach of renewing appliances/plugging them in to different circuits?

or

Something is happening to the fixed wiring once a month to case a 'bang' and a fault. Seems highly unlikely to me with the IR readings I'm getting.

Anybody got anything to add? Hopefully I'm missing something blindingly obvious here! One downside of fitting RCBO boards is I don't actually know whether or not I'm probably looking for a L-N or a L-E fault, which is slightly unhelpful.

Cheers
 
One of the big "plus" points with RCBO boards is that you know which circuit is affected. Just imagine if this was a dual RCD board or worse a singe up front RCD board!

Had this once in a commercial premises ............. the muppet builder had removed a socket from a cable (a spur) and the cable was left live in the wall ........... every now and again a loud band and the MCB popped.

With this in mind have you been round all the "points" looking for any "disappearing" cables from the accessories you know about?

EDIT: You should consider swapping the RCBO for a MCB for a trial period - then you'd know what was more than likely causing the problem.
 
I suspect a twin and earth squashed in the floor or behind a skirting or cupboard.

It's flashing over L to E and clearing the fault when it does.
You'd need to test closer to when it's likely to go not just after it's gone.
Or isolate the section in the suspected area ( and temporarilly derate the RCBO.) then test just that section after a couple of weeks.

Is it an older potentially damp house?
 
There is something in Murdoch and Snowheads posts but I don't think to change from RCBO to MCB would prove anything as an MCB should still disconnect a fault in 0.4s. Something that you have not said is doing a ring test if there is a compression fault and one of the conductors is blown apart this may be contracting at night and causing a fault at night. Another thought is a slight leak/small piercing to a pipe that is only happening when the pressure is greatest at night and getting into a junction box or socket just my thoughts.
 
There is something in Murdoch and Snowheads posts but I don't think to change from RCBO to MCB would prove anything as an MCB should still disconnect a fault in 0.4s. Something that you have not said is doing a ring test if there is a compression fault and one of the conductors is blown apart this may be contracting at night and causing a fault at night. Another thought is a slight leak/small piercing to a pipe that is only happening when the pressure is greatest at night and getting into a junction box or socket just my thoughts.

If its a N-E fault then it would go away

That said, most N-E faults don't go "bang"

Wonder if the OP has looked in all the back boxes - for "scorch" marks ............ seen this before.

EDIT: The OP doesn't say if its a ring or radial - wonder if they have done any continuity tests?
 
Ring continuity was r1 - 0.74, rn- 0.74 and r2 - 1.04 a few weeks back when testing. Can't read anything into that unfortunately.

I've only been there a couple of times since the CU replacement. 1st time I put down to a one off or customer being economical with the truth or faulty appliance and didn't do any testing. This was a quick courtesy visit on the way home.

When it happened again I went and spent a couple of hours there. The results Ive posted on this thread are from this visit. This was when I found the fault on the fridge. The fridge was replaced a day or two later.

3/4 weeks have passed and assumed that was problem solved. It's happened again a couple of days ago!

In answer to a couple of questions.

Yes it is an old house and damp is likely present. However IMO IR testing will find this. Also its been very warm and dry for a few days here now so that seems to rule that out.

No sign of rodents, although thats not to say they arn't present.

I haven't removed all accessories to visually inspect. This will be pretty time consuming as quite a few are located behind awkward appliances and it seems a bit like clutching at straws when the test results are so good.

Boiler system is an old floor standing gas boiler (in garage). Apparently the fault has occurred without this being on. The timing of the fault appears to be totally at random currently.

I suspect I've reached the limit of testing now though and will need to do some more inspection. Probably removing all sockets and seeing if I've got any spurs that are unaccounted for and checking for damage/scorching inside all accessories. Any further ideas?
 
Any further ideas?

Keep very good notes each time you visit so your invoices are full of facts ........... they they should get paid no question

I would also be looking, especially around washing machines, dish washers etc for evidence of a any very small water leaks close to sockets etc and also inside back boxes...
 
I had one like this. It eventually turned out to be a length of T&E behind a dishwasher. The D/W was pushed up against the wall, and the cable. Every so often, it went POP!
 
I had a fault before (I've told the story many times :rolleyes:), where the previous owner had put up dado rail, and nailed right through one of the strappers in a tw&e, in the right switching arrangement made the whole wall live. Wasn't discovered until the new owner, had his bathroom refurbished, and the plumber was getting a belt off the wall, when touching his pipes. Took ages to find the fault, and only with the help of another electrician, actually he found it :oops:.

PS; I would get the owner to record the time the fault manifest itself, and what the household we doing/using. Get them to do that a few time before returning.
 
I had a similar fault a couple of years ago, only started occurring after the customer had new CH installed, a cable under floor damaged by new pipework, gradually crept towards the pipe, then the fault moved it away, and so it repeated.

To help locate it, split the RFC into two radials, and left for a while to see which section had the fault. Then moved the split to a different point, to narrow down the location. Obviously needed several visits but access to some of the cabling was quite difficult.
 
As SJD posted the only thing that I would add to that is do a sketch of the rooms and outlet positions and indicate on it which is on what protective device.
I had a fault that I attended once RCD keeps tripping every morning about the same time. It ended up the un-suit bathroom was above and the seal had gone and every morning when her husband got up and had a shower before going to work it used to leek into the garage door socket on the ceiling below ( I took my time on that on after I had found what it was as she was easy on the eye).
 

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