Ian Stirling wrote:

Yeah - I know, I was just pointing out that it needs to be earthed
metal.

It doesn't need to be earthed if it's substantial enough to "prevent
penetration of the cable by nails, screws or the like." Otherwise you
can can used earthed steel conduit or one of the prescribed cable types
with earthed armouring or screening, etc. [see Reg. 522-06-06].

--
Andy