Crumbly PVC cable (red only)

[SJD]

Today on an EICR I came across some old (1960's, I think) PVC lighting cable - twin red/black, no earth - where the red was crumbly. It behaved like old perishing rubber, any bending and it cracks - but it is defintely plastic of some sort. The black was perfectly OK, as was the sheath. Curious to know if anyone else has seen this in PVC cable.

 

[7029 dave]

What colour was the sheath, is that Aluminium or tinned copper.?

 

[SJD]

Definitely tinned copper (the tinning can be scraped off). The sheath was vaguely grey-ish, but I didn't bring a sample home to recall exactly.

 

[7029 dave]

Yeah I have only ever seen PVC twin/E crack, not as bad as your sample where it was installed outside.

 

[Lucien Nunes]

Interesting. The left-hand end of that specimen has a glossy sheen that initially made me wonder whether it was PE, rather than PVC, but is probably migrated plasticiser. PVC has multiple possible degradation modes such as plasticiser evaporation, hydrolysis and dehydrochlorination. How fast any of these processes occur depends on the material composition, temperature, environmental exposure etc. The processes interact and can be self-catalysing especially if the exudate cannot escape easily.

The plasticiser used in the batch of red insulation might have been of unusually low molecular weight, resulting in accelerated migration and evaporation, despite performing to spec during manufacture. Unlike cables with green goo, there won't have been any reaction with the conductors because they are tinned (the extreme loss of phthalate in green goo cables is usually the result of the antioxidant.)

Is the red insulation or the inside of the sheath sticky? Does it smell of anything?

 

[SJD]

No smell and nothing sticky on the red insulation inside or out, it is quite dry, has perhaps a waxy feel to it.
Unfortunately I didn't bring back a sample of the sheath but I don't recall it being sticky either.

 

[Dustydazzler]

Fake Cable ?

Fake Cable

 

[brianmoooore]

The earliest plastic cable was polythene, and the red does deteriorate in a similar way to rubber, especially if exposed to UV, when the colour will be bleached out of it
Seen (and replaced) loads of it in my area, where a lot of it was installed when mains power arrived at the beginning of the '60s.

 

[Clive P]

I have some in front of me, Red/white/blue which I replaced some time ago which was used for two way switching in the house I live in. Kept it just for interest, but it is definitely more plastic than pvc and much harder to strip.

Regards.

 

[Clive P]

Just to add from my other post, I have also got some Black/red and sheathed green from the same era.

Regards.

 

[Lucien Nunes]

I thought it might be PE hence my first comment.
@SJD, try burning a tiny bit outdoors. Does the smoke smell like candle wax?

 

[7029 dave]

I have some in front of me, Red/white/blue which I replaced some time ago which was used for two way switching in the house I live in. Kept it just for interest, but it is definitely more plastic than pvc and much harder to strip.

Regards.

Good to see you Clive long time no see

 

[SJD]

I thought it might be PE hence my first comment.
@SJD, try burning a tiny bit outdoors. Does the smoke smell like candle wax?

Yes. Looks & smells like it.

 

[Lucien Nunes]

OK so it's polyethylene, rather than PVC. I'm not 100% sure what the degradation mechanism is here, but I would think it's likely to be polymer chain cleavage. When the average molecular weight decreases to a certain point, it rapidly transitions from a plastic form to a brittle one. The trigger might be oxidation. Was that sample exposed to the air e.g. in an accessory back box or from within a sheathed section of cable? Are you able to get any more samples of complete cable or is that job finished?

I wonder why the red and black behave differently, given that @brianmoooore has experience of the red failing first too. We don't know whether the correlation is causative or coincidental.

 

[SJD]

The sample was from the end of a cable, part exposed, part sheathed - the red seemed equally crumbly in both cases - while the black was fine.

I don't currently have the chance to get any more samples. There is still some of this cable in service, recommended to be rewired, but that is probably not happening for a while.

 

[brianmoooore]

I wonder why the red and black behave differently, given that @brianmoooore has experience of the red failing first too. We don't know whether the correlation is causative or coincidental.

My assumption has always been that the red is translucent and the black is opaque, so light doesn't get past the surface of the black.

 

[Clive P]

Good to see you Clive long time no see

Always around Dave. I just don't post a lot.

 

[Lucien Nunes]

However in a sheathed cable the incident light on the insulation is much reduced and in many installed situations effectively zero. Unless the effect is shown to be absent in cable permanently in the dark, I'm inclined to think that the difference is caused by composition, rather than exposure. The question is then whether the red compound is inherently less stable, had some intentional difference in formulation that was unrelated to the colour and could equally have affected the black, or whether it was actually defective in manufacture.

 

[SJD]

The crumbly red section was under a light fitting and not exposed to light, same as the black non-crumbly. I'm inclined to believe the thought that either the red was a different formulation or defective. If I get called back to rewire, I'll certainly collect a better sample next time. Thanks for the comments.