Weird extension lead test results?

[mattylad]

Howdy Y'all.
Today I had 2 long extenion leads to PAT test (dont go getting all pedanticky on the term now ya hear!).

One is a 240v rolled one with 4 sockets in the side and a 16A commando plug on it (yes I know that its not capable of 16A but it never will be used so).

The other is a 110v extension.
Both are 25m+/- in length. (they are long.)

Earth continuity tests OK
Insulation resistance tests OK.
They both fail on "short" on my UT-528 tester after the above tests both pass OK (its doing the continuty check).
However manually testing them with a digital multimeter and a continuity tester show them both as being wired correctly & 1:1 with no shorts.
Using a megger (Avo BM121) shows them as passing with no shorts between any wire.

I have taken the 240 socket cover off the reel holder, inspected and cleaned that up and its all in good order.

Any suggestions on what may be causing this or how to manually test for this?
What else can I use to test them to find what is actually shorting?

They work fine, they are not causing any problem yet fail on my meter (changed batteries too).
I also tried them on a Metrel AutoPAT MI2143 which is 12 month out of calibration (should be fine) and when doing a test on that it does the 25A earth test and flashes a pass (showing that the result is between the 2 limits of 100uA and 25A).

The only thing I can think that may be the cause is:
The 240V one has a crush in the middle of the cable - but surely I'd be able to measure the resistance between cores using my multimeter or the megger?

The 110V one had a nick in the outer insulation a while back and has a double layer of heatshrink over this. No copper was exposed so I cannot see it being this.

Do I just declare them failed/scrap them (kept for parts ) or can I do some other test to identify what and where the issue is?
I don't have a TDR to hand
????

 

[bernardgreen]

If they are coiled up on a reel then maybe the UT-52B is being "confused" by the inter conductor capacitance and inductance of the coiled up lead.

Try the same test with the leads laid out straight.

 

[mattylad]

They were no on the reel, laid out on the floor. Maybe a little coiled but not as if they were on the reel.

 

[ericmark]

A 25 meter lead will only pass if supplied with a RCD protected circuit, and many PAT testers allow the user to build a sequence of tests to suit. I did have a problem with long leads, we know on camp sites the 16 amp supply should be RCD protected but in industrial premises one has to use some common sense.

110 leads have a problem, with the brick transformer it is common to have a trip in the supply only, so with for example a 10 amp thermal overload that is 2300 VA at 55 volt that is 42 amp. A 1.5 mm² cable will not stand that current, and it is common to find damaged 110 volt cables, however the worse place to be damaged is in the plug or socket as outer insulation is stripped back there so can't get rid of heat as easy, so I always open plug to inspect never rely on the meter.

 

[JohnW2]

Circumstantially speaking, is not the finger pointing at your UT-52B as the most likely 'culprit'?

Kind Regards, John

 

[ericmark]

I have not used the UT-52B my firm had a Robin, and I know with the Robin unless you enter your own program with the lead test it fails all long leads, not got the IET book to had giving all the limits, but I know the standard resistance is too high with a long lead, I would often insist on the fitting of an RCD plug to pass them, unless no sockets in the building were without RCD protection.

Loop impedance for 13A fuse was 2.42Ω and a 16 amp outlet would pass with 2.87Ω so there is no real leeway for any extension lead, however the PAT testing book does give a limit, I don't have book to hand but I am sure if you are PAT testing you will have a copy of the book anyway. I consider with a ring final we had 1.44Ω so limit was around 0.98Ω but of course could be a radial in which case a lot lower. I seem to remember it was 0.5Ω?

 

[mattylad]

Not really as I have tested a similar long 110v extension with clear results.

I also always open both plugs too to ensure they are still fitted in the cord grip (if there) and the terminals are tight.
Both these cables were tested in exactly the same conditions 12 months ago.

Something weird is going on, Ill get to the bottom of it.

 

[JohnW2]

Not really as I have tested a similar long 110v extension with clear results.

Fair enough - but I only made that suggestion because you seemed to be saying that at least two other means of measurement indicated that there was nothing wrong with the cable.

Kind Regards, John

 

[SUNRAY]

A 25 meter lead will only pass if supplied with a RCD protected circuit, and many PAT testers allow the user to build a sequence of tests to suit. I did have a problem with long leads, we know on camp sites the 16 amp supply should be RCD protected but in industrial premises one has to use some common sense.

110 leads have a problem, with the brick transformer it is common to have a trip in the supply only, so with for example a 10 amp thermal overload that is 2300 VA at 55 volt that is 42 amp. A 1.5 mm² cable will not stand that current, and it is common to find damaged 110 volt cables, however the worse place to be damaged is in the plug or socket as outer insulation is stripped back there so can't get rid of heat as easy, so I always open plug to inspect never rely on the meter.

Come across this quite often, especially where 5KVA or bigger transformers are used with 32A yellow sockets and suddenly those figures are doubled.

I did some subbing for a controls company who specialised in water treatment, we had a green 'cabin' in the workshop, this sort of thing:
View attachment 185542
Their own electrician, as opposed to controls wireman, had wired in a 5KVA 110V isolating transformer feeding 4 16A yellow sockets each with their own 16A MCB in a seperate ('110V') CU. It was powered by a C20 MCB in the 230V CU.
I instantly spotted he'd used SP MCB's for the 110v and queeried it with him. I was told to mind my own ******* business and go back to my 'little toy wires'. Bearing in mind I was only temping there and hoping to continue I dropped it until the following day when the the Contracts Manager came to the workshop and pointed it out to him. He advised it was normal and done by the qualified electrician so it must be OK. After that I spoke to MD who asked what the problem is, but he didn't understand.

I got a long (20m sort of length) piece of 1.5mm singles from the scrap bin and poked it in the 'E' & '-' holes of a yellow socket and waited. The MD switched it off when things started smoking and called the Contracts Manager for a demo. Then they called the electrician and asked him to come in, He wasn't happy as it was Friday and already finished for the day.

 

[ericmark]

Does the tester check insulation with just 500 volt DC, or does it also check earth leakage with 230 volt AC. I have not really measured but there is a difference between resistance and impedance and although it may not leak DC the capacitive and inductance linking may mean there is some AC transfer, which could change due simply the way a cable is unwound from the reel. I know my PAT tester does measure earth leakage in mA but never really looked at the comparison between leakage and insulation resistance. Son has my tester or I would try it.

 

[JohnW2]

Does the tester check insulation with just 500 volt DC, or does it also check earth leakage with 230 volt AC. I have not really measured but there is a difference between resistance and impedance and although it may not leak DC the capacitive and inductance linking may mean there is some AC transfer ...

I'm a little confused/uncertain, since the OP indicated that the cable failed on "short", which seems to imply that it found a very low resistance/impedance between something and something else, rather than an 'unsatisfactory' (but still 'very high') IR measurement.

Maybe the OP could clarify?

Kind Regards, John

 

[Gasguru]

In the electronics world Uni-trend equipment is regarded as bottom end, poorly designed, poorly manufactured and as a company they seem happy to apply fake approvals to their multimeters (Cat ratings etc).
Much of their test gear is really only safe for the hobby electronics market ie. on low voltage DC.
Have a look at the many teardowns to see the evidence. I'd bin it and use a better regarded tester.

 

[RF Lighting]

Is there a neon or any sort of surge suppressor on either of the leads?

 

[SUNRAY]

One of my cable testers does AC tests of 'open', 'load' & 'short' and the connectors used on the other end are female to female barrel, 50Ω or 75Ω dummy load & short circuited female, BNC's respectively.

I wonder if the term 'Short' may be a short circuit test as above rather than looking at IR.

If that is the case, the 'fault' will be high resistance conductors which may be the case for leads of 25m.

 

[JohnW2]

I wonder if the term 'Short' may be a short circuit test as above rather than looking at IR.

That's what I was suggesting when (by implication) I asked what "short" meant.

If that is the case, the 'fault' will be high resistance conductors which may be the case for leads of 25m.

I don't get that. How could high resistance conductors get described as a 'short circuit'?

Kind Regards, John