potential on earth & N

[the pict]

I have a ( brief over view) 5 Db's served from main board by C50 OPD,s inadequate main earth 6mm self tappered to metal of CU's I can get a decent ZS/PFC/ DB if I clamp onto brass nut of SWA (and set my tester to a B50) but not on disconnected main earth, would this poor earth cause a potenial on E N,
Pict

 

[londonlec]

Sounds like poor connection of an SWA with no CPC.

This will cause metal work to float above 0v.