I have been asked to rewire an office which was once 4 shops. Each shop was fed of an overhead trunking with a tap off. The tap off is basically a small DB with a breaker in fixed to the trunking. The trunking has 4 x 50mm GP cables running through it. The trunking itself has stagered terminated (R,Y,B and Neutral. The 50mm cables are clamped in a fixed insulated Allen key head terminals and a 16mm goes from that to the tap off box which houses a breaker which then went on to feed the shop DB. The Earth for the tap off is a fixed point on the trunking - a purpose made terminal fixed to the trunking. Now I know you can't use wireways as earth (c.p.c.'s) according to the new regs. This trunking is designed to be used as the earth. I wouldn't use metal conduit, tray or normal trunking in a new install as the earth but would it be considered acceptable seeing as this is specifically designed as such? The trunking metal is thicker than usual. Can I consider it as a bus bar trunking? The Earth fault loop test results are text book perfect at the tap off I want to use. What do you think?
Again I know wire ways can't be used as earth but if the product was specifically designed as such would it be considered ok.