I don't have the byb on me as not at home but iirc the relevant reg (522 something I believe) specifies a bs number that the earthed metallic containment must meet the requirements of as it's expected to be penetrated hence having to meet the requirements of a protective conductor. From my point of view earthed metallic capping is not satisfactory. One other option the reg gives us is that if the cable is burried in a wall at a depth of less than 50mm then RCD protection can be ommited if it has sufficient mechanical protection to stop the penetration of nails screws and the likes of.
In the absence of a relevant product standard, e.g. British Standard, the designer should obtain suitable evidence from the manufacturer that any product selected for the purpose is suitable to prevent penetration by nails, screws and the like.
As an example, steel of 3 mm minimum thickness is generally considered to provide sufficient mechanical protection, except where shot-fired nails or similar fixing methods are likely to be used. I did post a video once from clarkson Evans which shows some thick metal capping that he drills, hitting a nail through etc to no avail but can't seem to find it. The key thing is that sufficient mechanical protection doesn't need earthing where as other methods require earthing.....Cables can be run out of zones as long as they have sufficient mechanical protection.
