Finding a very awkward fault

[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

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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.