Rubber Flex Insulation Resistance Failure 

Some natural and man made rubber cables over time in damp or wet conditions go hygroscopic and actually absorb moisture. Then you get exactly the effect you got with the Megger. The insulation “breaks down” the longer a standing voltage is left across it.

John , When i worked at a local electrical wholesalers 15 years ago, we had a batch of Collingwood spike lights ,where the paralleled leakage from the porous cable caused the rcd to keep tripping ,the customer brought them all back , and i offered to replace all the cables with some quality British rubber cable, problem solved, But probably invalidated their warranty

Not far off the 100k subscribers now john!! well deserved youve given free quality informative videos to help so many of us and for that im very grateful keep up the good work sir 👏

Very interesting video as always thanks!

Great to see a real world example. Thanks John.

I had exactly the same thing on a garden lighting call out last week, I ended up stripping the flex down to visibly see what was wrong,I even think it was the same make of cable,poorly made cable= moisture ingress I presumed

PVC insulation often contains some (normally VERY small) quantity of lead. PVC insulation from China often has considerably more lead than normal. Lead is, of course, conductive, and combine that with moisture leaching through it, the minerals in the water may be reacting with the lead and forming conductive lead salts, which create numerous tiny current paths. I'd get a cheap lead test kit from a hardware store or similar, and rub the swab from it all over the insulation off those wires to see if there's lead in there.

Oh what a fun job we have! Thanks John. Never take anything for granted eh? I found some conductive silicon sleeve recently.

Interesting video thanks mr ward. I’ve come across some 6242Y cables that have a much higher r1 r2 and r n than should be.

Hi John, Can you test the inner resistance of that black outer shell? The wires were inside, not on the outside. I'm interested to see if degraded rubber caused some of the issue. Thanks Larry

@JW: Do you have a Leslie connected to that M-102?

Do you have any tips for outlet setup for using a welder at home? I have heard all kinds of things, such as that the cable inside the wall will heat up!

Great video JW.. I find this often with T&E connected to outside light fittings (which obviously is not permitted unless contained) the UV light breaks down the pvc insulation, cracks and moisture gets in. IR fails or is well below 2 megohms. I suspect that cable is not to spec or a Chinese fake cable 🤔 would be interesting to see what curren/voltage it could take before failure of the insulation? You could pinpoint the weakness in the conductors insulation then.

I'm surprised that such a class of cable is allowed in such an installation. If armoured cable is too much of a pain and mains power is needed for garden lighting then what is done down here is to use 12V lights. For larger installations I think 24V or 32V is used. This way anyone can install the lights and an oops with a mower or other implement isn't a major issue.

I used T&E 4core few months ago doing emergency lighting and exactly same things happened and according to NiC there’s over 20m meters of faulty cables come to uk from Turkey and there are still in many wholesalers to be discovered

Are you having your globe light on at the beginning and off at the end on purpose ?

Hello John, hope you are well. I've been looking into a techinque or equipment that would allow us to find an insulation damage on a buried conductor. We have voltage on the end but when drawing power it drops to 0 and the insulation resistance to earth is 0, so we believe it's partially broken, because we receive voltage but when current flows it drops so there must be a few wires still good but others cut. Any thoughts? It's a 1000 m run by the way.

So I’m in a couple UK electrical groups on Facebook and observe all kinds of things that are very different than here in the states. Here in the states there is a stamp on pretty much every cable from what certifying lab has accepted that cable, like UL, etc. also most of our cables here also show voltage rated for. The other thing that blows me away about the UK and apparently Australia also and probably many other countries that I don’t know. The fact that you do all these tests on your installations. We do nothing like that here in the states. Nobody carries a meter like everybody I watch in the UK. We don’t do an Any kind of certification when we install stuff and we have nothing on any kind of regular basis. If it screws up your call us so we come out and find it. If you do a new install that needs a permit from the city or county or whoever, all they’re really looking for Is how you installed it. Usually it’s two inspections. A rough inspection for your cabling and conduit, and then a final inspection after the walls and finish are done.

I need to see John play that keyboard!

i once worked to seal up the discharge of shower sump pump while it was running, not bad but quiet noticable tingel in the water

JW can you do a video on how to calculate how far along the cable drum the fault is?

some years ago it was found the rubber scan coil 'wedges' in some crt tvs became conductive and even 'leaked' out corrosive 'goo' onto the scan coil windings which turned them green with corrosion ., i've seen this for myself !

We’ve had a few odd faults where moulded on plugs have had insulation breakdowns, they have been all the same ‘Deka’ brand. Seeing that drum of flex rolled up, was wondering if you could do a video explaining why an extension cable shouldn’t be used rolled up?

Hi John. I'm intrigued how the flex would have behaved in a 250v test, closer to actual operating conditions. I'm not denying that it would probably still fail, but would have been interesting to see.

What brand is the tester?

It would have been interesting to apply pressure to the cable while twisted or in the sheath and see what happened to the resistance/leakage? There may be one particular point where the leakage is affected more - or indeed over its full length.

You mention that some of the wires tested ok. I wonder if they had water in the junction boxes. Maybe the inner insulation is not water resistant and relies on the water resistance of the outer sheath.

If one end of this flex terminates inside a the spot light fittings that gets hot and the other end terminated inside a flooded with water junction box then the cable is turned into a 1metre long drinking straw.

What happened to old wire colours code .which red as hot line and black as neutral

You could also try putting those wires into the oven for 30min or more at the cable´s max temperature rating. Just to see if some moisture does indeed leave it or wheather the insulation is just degraded in some other way.

Good example of Chinese gutta percha. Or crap rubber. Thanks for sharing.

How does Britain handle certification of components? I was expecting to see some sort of testing lab logo on that cable: UL, CSA, ETL, or whatever.

I wonder if some ionic salts have migrated into the insulation material from the soil? Maybe actually an electrolytic reaction occurring when a voltage is applied. Does a Megger test with DC or AC at 500 or 1000v and what current can it source? Surprised the ‘good’ cable showed such a long charge up time. Capacitance at maybe 1nF per mètre so should charge almost instantly unless the current put out by the Megger is at the micro-amp level.

Yay recycling dirty milk bottles and posting them back.. You could have done the same test you did on the exterior insulation on the inner stuff after removing the copper. It's been lovely weather for the neighbors to have a BBQ ,time to take the heather and big transformer into the garden with the other half of that cable.

Expandable rubber sleeves also, I heard in the past

Can you please explain why electricians in UK tie Line and Neutral together for insulation resistance test? Thanks.

So if the readings on the machine is low it means there is a breakdown of the insulation. The higher the reading the better the insulation?

Why does the Megger warn when using 1000 volt test? Is it only because it sounds fancy? Can it upset you it you get that voltage between your hands and you can drop something? I couldn’t find the current used in the leakage measurement but it must be quite small.

Could have taken one core and done IR test on the insulation of it.

It would be interesting to cut 15cm from both ends of the cable and test again. This way we could see if the insulation is deteriorated only on the parts that were not fully enclosed inside the black outer isolation.

I installed about 10 of GU10 spike lights with the same flex. Can't remember the make but bought them from TLC Electrical. Got called back about 3 times with the RCD tripping thought it was to do with damp getting in the fitting but eventually narrowed it down to the flex. Had them 13 months so manufacturer wouldn't take responsibility.This flex should not be allowed. Wasted my time and money.

I have some generic power cords that look like this junk. The insulation on the wires is very soft. It's so soft you can cut it with your fingernails. I don't know what type of plastic this is but it's dangerous. It also burns very well.

The key to the issue was on the outer sheath. Its Chinesium.

Would be interesting to slow heat those wires to drive out any moisture and test again.

Made by good old Ningbo Huashun, what could go wrong?!

Have we discovered a new type of semiconductor?

another example of fine chinese quality manufacturing!!

was the cable copper coated aluminum?

Same thing actually happened to my original figure of 8 Xbox power cord.

Smeg !

This cable will self destruct in thirteen months.

Thanks John Wow now im now better of test and how to do it far better Im so proud of my mentor Thanks now my mind is test eyes open Your videos is my go back when im stackt Then i go check fast tru your videos and found my answer Thanks I no people will said what a idiot better a idiot as im set up a fire Then i said if anny one how not makes misstacks wow you a creat That this people where i found plentty fualts misstacks

If you have no resistance in your flexing. . . Try yoga

Next, we shall test the lead and organic toxics content of this piece of Chinesium.

Thanks for showing us another reason not to buy Chinese rubbish. Well explained and proven with your trusty Megger. (It takes no prisoners)

According to BS7671 it was within minimum required

Could elaborate why the earth is a globe?

Faulty flex but okay

Made in China!

Testers faulty 😛