Polystyrene

And the question is, has the cable been overheating, or is it a chemical reaction?

Mr. HandyAndy - Really

 

That's a very good point HA

 

Chemical reaction, not caused by overheating.

 

Chemical reaction. No sign of the polystyrene melting due to heat.

 

Makes you wonder what is reacting to what up and down the country.

 

Plasticizer in the sheathing reacting with the polystyrene, no overloading/cable caused by overheating

 

It's the classic reaction of the plasterciser. The plasterciser leaches out of the PVC, making it brittle.

 

The type of PVC was changed in the 80s. So for more recent cables than those in the pics above it's not a problem.

I found some of the older cable stuck all over with polystyrene insulating beads in a house refurb I'm doing at the moment... Not too common a sight these days thankfully.

 

Not true, state. The chemical composition of PVC was changed principally to avoid the reaction with copper causing green goo. Polystyrene still reacts with PVC. It is best to avoid laying cables in the stuff, tbh, because it causes the PVC to stiffen when the plasticizer is reduced by the chemical reaction.