Finding a very awkward fault

[mattg4321]

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

 

[SJD]

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.

 

[Toneyz]

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).

 

[HandySparks]

I'd be tempted to try IR testing at the maximum voltage that your meter will produce. Make very sure that you've disconnected all loads on the circuit. You might just be able to provoke the fault.

 

[littlespark]

I had this problem recently. Breaker tripped at strange times, never the same. This was a combination of rodent damage and the cable being squashed between floorboard and heating pipes.

 

[Toneyz]

It looks like the pipe and the cable on the left has been damaged as well.

 

[Lucien Nunes]

You might try a higher voltage insulation test. Faults that blow themselves clear can sometimes acheive a very high resistance on a dry day, but the creepage distance is small. Once the test voltage gets up towards flashover, the resistance avalanches. I have a selection of tools for this. The Megger (500V), the Big Megger (2.5kV), the Very Big Megger (10kV) and Cuthbert (30+30kV). Cuthbert is not allowed out to play very often. Many kinds of insulation cannot stand him at all!

 

[Marvo]

We have a 10kV megger tester and I keep it under lock and key. Letting the wrong person loose with Cuthbert could be a very expensive exercise.

 

[Rpa07]

View attachment 42110
I had this problem recently. Breaker tripped at strange times, never the same. This was a combination of rodent damage and the cable being squashed between floorboard and heating pipes.

With the damage to that pipe, imagine the damage if that was gas hence the ‘distance from services’ regulation.

We have a 10kV megger tester and I keep it under lock and key. Letting the wrong person loose with Cuthbert could be a very expensive exercise.

Yes you don’t want to be the only apprentice on the crew with this around!

 

[MFS Electrical]

I wonder what the apprentice would look like in circuit with the 10KV

With the damage to that pipe, imagine the damage if that was gas hence the ‘distance from services’ regulation.


Yes you don’t want to be the only apprentice on the crew with this around!

Man took the thoughts right out of my head

 

[ImpededLoop]

When you carried out the IR test, were the ring final cpc's still terminated to the earth bar? Sometimes people remove them to to do the continuity tests but forget to reconnect them before doing the IR tests. If the fault is between a live conductor and a pipe, it may not show up on the circuit cpc's but would to the earth bar.

Otherwise, I agree with the suggestion of splitting the circuit into two radials.

 

[marconi]

MF-110A Electric AC/DC OHM Voltmeter Ammeter Analog Multimeter - https://www.banggood.com/MF-110A-Electric-ACDC-OHM-Voltmeter-Ammeter-Analog-Multimeter-p-952406.html?gmcCountry=GB&currency=GBP&createTmp=1&utm_source=googleshopping&utm_medium=cpc_elc&utm_content=zouzou&utm_campaign=pla-uk-ele-pc&gclid=EAIaIQobChMIi7fj2tT12gIVC5ztCh1eRAXUEAQYAiABEgK9nvD_BwE&cur_warehouse=CN

Arcol RCC025 510K J 0.25W Carbon Composite Resistor - https://www.rapidonline.com/arcol-rcc025-510k-j-0-25w-carbon-composite-resistor-63-2372


This is an idea for how to investigate a tracking/sparking/arcing fault in the FRC between L-N or L-E or both. The problem at the moment is the fault is intermittent and when it manifests it activates the RCBO immediately because the source is very low impedance and the fault has low resistance so a large fault current flows.

The intermittent fault might be represented as a high resistance for the tracking and a spark gap in parallel with that resistance for the sparking and arcing event. It would be nice to be able to monitor the potential difference across this parallel arrangement of resistance and spark gap without the RCBO activating.

If one uses an analogue moving coil and needle voltmeter (see reference) it might be possible to observe the movement of the needle as current flows through the fault. The voltmeter I refer to above as a resistance of 2kOhms per volt so on the 1000V ac range it has a resistance of 2000 x 1000 = 2MOhms.

If a 500kOhm resistor is inserted in series with the L feed to the ring circuit and N connected as normal to the N bar, and then the voltmeter on 1000Vac connected across the L and N (or E) on the FRC side of the resistor one will be able to observe the potential difference over a period time. If there is no tracking or sparking/arcing the potential difference will remain constant at circa 2/(0.5+2) x 240V = 192V. If there is a current flowing through the tracking it will be lower. If there is sparking and arcing the needle might be observed to flicker downwards in voltage from 192V every time there is a discharge of the cable. (I have ignored the cable IR reported as 70MOhms).

Beforehand, one should do a trial on a new, perfect 50 or 100m reel of 2.5mm2 T&E to provide some comparison between meter observations. Remember to isolate the end of the reel of cable.

 

[Lucien Nunes]

Are you offering to sit there watching?

If you're aiming to induce partial discharges you might as well hit it with the highest voltage available, which is probably 1kV from the MFT. Although your moving-coil meter will give a clearer response in the time domain, the MFT will be more sensitive reading to hundreds of megs.

The flashes might be visible, so could I recommend an alternative kit list:
1 iPhone running Facetime
1 VDE approved insulated ferret
duct tape

The tape is just for strapping the phone on. Do not try to insulate a regular ferret with tape, use a proper live-working ferret.

 

[Dave OCD]

I once told someone that birds don't suffer from standing on HV overhead lines because their feet are made of a natural rubber like substance which insulates very well.

 

[marconi]

Re #27. Mmm.

I am suggesting a way ahead involving components costing £2.75 + £0.10 (ex VAT). The homeowner/a friend could look at the meter while the OP prods, pokes, shakes, stamps, rattles, taps, plugs in/unplugs around the house. The nice thing is if the needle does move then one can repeat the stimulation and integrate the results.

Then again one can always just rip up all the carpets and floorboards, damage all the wallpaper and remove all the tiles.......or simply quote to rewire.

What is to be lost by giving it a go?

PS: My idea does not involve potential cruelty to animals.

 

[mattg4321]

IR tests carried out at 500V with CPC's connected to earth bar.

Will try at 1000v on my return.