In article ,
David H-S writes:
This discussion has been about lead wiring.

Another type of cable sometimes used c. 50 years ago for ring mains
consisted of a copper sheath, a ceramic insulator and then the live and
neutral leads inside that, cased I thinkk in rubber. The whole about
1/3 inch in diameter.

I think you are describing MICC (Mineral Insulated Copper Covered)
cable, or Pyro to use one of the trade names.

while lead cable was used for lighting in our house, this copper/ceramic
cable was used for the ringmain power crcuits in our old house. I think
it was installed in the late 50s for my parents.

What we saw by 2000 was increasing incidents of short circuits (via the
copper exterior) and blown fuses. Eventually whole circuits became
unusable. Regular slight movements, duw to floor boards & joists
flexing as people moved about, had caused the ceramic to crumble. then
maybe some damp penetrated and shorts were the result.

It lasts forever providing the ends are sealed against any
moisture ingress and the copper sheath doesn't get punctured.
I guess frequent movement could cause work-hardening and eventual
fracture of the copper sheath.

The insulation is magnesium oxide, which is hygroscopic (will
absorb water from the air given the chance), hence the need to
make sure the cable ends are well made.

MICC will survive being heated up to just short of copper's
melting point, and you can hammer a piece almost flat and it
will still be fine (providing you don't puncture the sheath),
as the conductors and compressed magnesium oxide all deform
in exactly the same proportion. Indeed, the stuff is all
initially made much thicker, and taken down to the required
final thickness by running it through pressure rollers.
It is still used today in situations where any of its outstanding
physical properties are required, but it is expensive and
installing it is a rather more specialist task. This and its
tendancy to absorb water into the insulation are its weak
points. Also, it doesn't handle voltage spikes from inductive
load switching well -- this isn't an issue today, but it was a
problem when fluorescent lamps first started appearing and
fast-break switches were still around (which were required
for earlier DC circuits).

--
Andrew Gabriel