On 28/05/2019 08:20, PeterC wrote:

Where the cables are in the wall (historic routes) they're steel capped but
only the thin steel - just enough for a bit of a warning - and open at the
back.

The metal capping is very soft.

Believe me, if you hammer the pin for a picture hook into the wall and
it goes through a bit of metal capping you will be totally unaware of it
- until the pin hits the line conductor.

As others have said the capping is only for temporary protection against
plasterers.