Mystery mains lead

[BurningTantalum]

A while ago I was given a new-looking 25m mains extension lead that had had its trailing socket cut off. I have cut lengths off it to replace short mains leads on all manner of tools- recently a band saw. Today I acquired an unused but second-hand induction hob/cooktop. I clipped temporary wires to the terminals and my trusty Safebloc to test that it lit up and performed its POST. This was all OK so I cut a length of the aforementioned mains lead and fitted it to the terminals to check that that the hob would heat a pan. It promptly tripped the RCD which was a surprise as it didn't even perform the POST. A check showed that L, N and E were shorted so I removed the earth lead at the terminal and found no short in the appliance. I removed the N lead and the short went away. Strange I thought, as I put my meter on the open-ended lead and it was short all ways round. The 2m length appeared unblemished and new, so I pulled it through my hand and felt a very slight bulge in it. I opened the outer cover with a craft knife and the attached picture shows what was within.
[ Attachment Invalid Or Does Not Exist ]
I have worked on process machinery that splices material together when a roll runs out- sometimes on the high speed fly- but the splices was always flagged by some method that could be detected and then ejected further down the line. I guess this got missed as the cores went through to the moulding of the cover, but I have not seen a cable manufacturing process. Has anyone come across this 'fault' ?
I guess that the original owner was a bit baffled by the extension lead and threw it out. I know the store that it came from but I'm not sure if I could get a refund!
BT

[thm_w]

Can you fix the photo?

[BurningTantalum]

I don't know what happened there- apologies!

[thm_w]

wow, that is bad. I think you are right about it being a splice, its clearly not intentionally joined or there would be a proper crimp/solder/etc.

They might have expected QC testing to catch it, with a continuity or hi-pot type test.

[Shock]

I would say that is the splice for a long run of cable. The fact it got through QC is amazing, they probably had no such thing. This kind of stuff starts house fires in certain circumstances.

I avoid dirt cheap extension cables, power strips, smps. I have a supply of wall outlets from 20-30 years back and the plastics are so much better than the crap they are putting out today. All bullshit ratings on cheap consumer stuff as well.

[chris_leyson]

If you use wire by the km it's not uncommon to find a drum of wire with a splice. Most manuacturers will add an optical knot and splice detector or similar to their automated wire handling equipment to detect and reject spliced sections of cable.

[Bark]

Holy SMOKES!  Literally!

[coppercone2]

good work finding the problem, you need to feel the entire cable (making cable work a complete bastard on your hands).

scary fault because most people would say 'thats not possible and you are wasting your time looking for it'.

i recommend getting a folding sawhorse/work table with the little footrest on it so you can unspool the cable and run it through if you are working remotely (also looking for cracks in the insulation). Also, a good headlamp

[Shock]

Why would you do it manually? Isn't there like a handful of different ways to find the join using test equipment? Not that I've ever needed to do this myself, I don't mess with big runs of cable.

[coppercone2]

because you can always find a nick in the wire if you touch it, or find something suspicious like embedded material in the cable jacket.. i wonder if its possible to have a wisker of wire pulled through the insulation too

also, when you work with wire and your hands you should bend it around a bit give it a bit of a mechanical feel

[chris_leyson]

After reading coppercone2's comment I remember mention of high voltage wire testers for detecting such defects but never seen one in the wild. Knot and spiice or lump and bump detectors are used as part of automated systems in the manufacture of aircraft wire harnesses for example. Sometimes the wire manufactures strip the end of a wire and twist it onto the next wire and then cover it with insulation tape but sometimes they just knot the two ends.

[mcovington]

because you can always find a nick in the wire if you touch it, or find something suspicious like embedded material in the cable jacket.. i wonder if its possible to have a wisker of wire pulled through the insulation too

also, when you work with wire and your hands you should bend it around a bit give it a bit of a mechanical feel

Yes, one wants to find defects that have not yet caused an electrical short or open, but are likely to.

[coppercone2]

yea the problem is, it means sore hands (given the fact there will never be enough people to do the job )

flex those guns

I thought of really insidious things too, like chipped bits of factory maintenance tools being stuck to wire with grime (i.e. razor blade bit, screw driver chipping, etc) that can dig in during pulls etc

I saw some weird stuff with wire before that requires IMO microwave testing, like several strands (not all) being corroded down the wire line for a few feet. Not sure why it progressed as a narrow corrosion channel way deep........... logically you would think it would tend to screw up the wire in a 'plug' form, not shoot down the line like a dendrite.  if that happened inside the wire away from the cut end, you would need a NLJD to find a big high surface area diode in the wire.... water must have entered a 'fracture' between the strands or something, you would think the insulation would automatically close/eject that by pulling the strands close.

i mean like

WWWWWWWWWWWWWWWWWWWWWWW
WWWCCCCCCCCCCCCCCCCCCCCCCCCCC
WWWCCCCCCCCCCCCCCCCCCCCCCCCCC
WWWWWWWWWWWWWWWWWWWWWWW
WWWWWWWWWWWWWWW


it makes me wonder if it has a nonlinear time constant, i.e. a corrosion channel that is long develops deep along the wire but only effects a few strands (making the ohm reading good enough) but then corrosion causing conditions change so there is fast crosswise corrosion spread (rather then further axial spread) that drastically increases resistance in a short amount of time.

(i saw this axial growth happen in a old extension cord)

[james_s]

I avoid dirt cheap extension cables, power strips, smps. I have a supply of wall outlets from 20-30 years back and the plastics are so much better than the crap they are putting out today. All bullshit ratings on cheap consumer stuff as well.

Are there not multiple grades there? Here we have the cheap builder grade stuff, the sort of receptacles that are under $1 each and they're crap, have been for decades. Then there is spec grade, they're around $3 each and are much better quality, they're typically used in commercial buildings though they're readily available. If you really must have the best there is hospital grade, these are top notch, no expenses spared but you pay for it, typically $10-$20 each.

[Shock]

It's a race to the bottom, the only thing I've noticed an improvement on is name brand LED lighting, getting rid of compact fluorescent lighting was the best move ever. Plumbing and electrical seems to get nastier each year.

[BurningTantalum]

I was given the 25m lead by a friend who told me that "it didn't work". He has less than zero technical knowledge so I grabbed it as it was new. I tossed it into the shed to use as donor mains lead. I never tested it of course, so it was a surprise when I tested the cut length as dead short all ways round!
I recall that the leads were $18- 'Shock' will know where it came from: the ubiquitous green and red Oz DIY store (like 'Wickes', for UK readers).
I'm going to have a go at taking it back... should be fun but I know some of the staff.
BT

[coppercone2]

It's a race to the bottom, the only thing I've noticed an improvement on is name brand LED lighting, getting rid of compact fluorescent lighting was the best move ever. Plumbing and electrical seems to get nastier each year.

its not breaking everywhere, lets remove components and make things thinner!