OP
Engineer54
Just a note for engineer 54, it doesnt half to be an earth rod to make it TT,any permenently fixed earth source will suffice.
So long as it meets the requirements of an electrical permanent electrode you'd be right. Such a suitable electrode would be for instance a Ufer earth, or say a steel building where the steel posts have been driven or concreted into the ground.
What isn't acceptable, is the use of extraneous service pipes that can be replaced by plastic service pipe at any time. It is also not generally permissable to use metallic service pipes, as the sole means of earthing to an electrical installation. So even in this case, it cannot be deemed a TT system!!