I am intrigued to how you would discover it by measuring L-N, L-E, and N-E at the incoming side of the Main Switch, as the Earthing conductor would be connected to the Line at the cut-out and give 0V, the same as if it were correctly wired and you were measuring N-E?

Indeed it would. It becomes more obvious at the "edges" of the equipotential zone where there's some true earth reference about. Holding class 1 equipment while standing on damp solid floors indoors or outside touching a bonded tap.

I haven't come across the situation personally, but I do vaguely recall a report of a case ... detected via an unwisely bonded metal garage door if I recall correctly - with an unfortunate result.

If in doubt, test to an independent earth reference - e.g. screwdriver in a flower bed - is often the best course.

   - Andy.

I am intrigued to how you would discover it by measuring L-N, L-E, and N-E at the incoming side of the Main Switch, as the Earthing conductor would be connected to the Line at the cut-out and give 0V, the same as if it were correctly wired and you were measuring N-E?

Indeed it would. It becomes more obvious at the "edges" of the equipotential zone where there's some true earth reference about. Holding class 1 equipment while standing on damp solid floors indoors or outside touching a bonded tap.

I haven't come across the situation personally, but I do vaguely recall a report of a case ... detected via an unwisely bonded metal garage door if I recall correctly - with an unfortunate result.

If in doubt, test to an independent earth reference - e.g. screwdriver in a flower bed - is often the best course.

   - Andy.