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.

 

[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.