this is getting to the stage where it needs to go in the electrical forum.
steel capping doesn't need to be earthed although you would be crazy to install it and not earth it, there is no good way to earth it either, i HATE the stuff, it is leathal imo, should it get penetrated with a drill a chard of the steel capping could quite easily come in contact with a live cable, not touch earth and remain live if the capping is not earthed.
For best protection then steel conduit is ideal, due to it being round most drillbits will slide to either side of it.
In the real world most people are unwilling to invest the extra money in installing conduit in domestic houses, it is alot more work and in all fairness, I dont know anyone who has had a house rewired twice, i usually end up just clipping the cables in the chases and filling in with one coat plaster, with two or three coats of jointing compound over top, if the wall was good in the first place then you wont even notice this, if the wall was bad then the chase is only at worse, as bad as the wall. The only places i have used buried conduit have been on new self build and it's very rare.
In the perfect world you will have everything chased in and then have the chases filled in with bonding, s&c, hardwall, whatever, and the whole wall skimmed, whilst you are at it you may as well bury any pipes and get some new skirting and architrave haha!!
