Crumbly PVC cable (red only)

[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.