Hi Paul
I had thought that the isolation cad on the lenze controller gave that protection.
If you look at the wiring diagram I inserted earlier in this thread , the 0v connection from the iso board ( term 5) come back to a common 0v buss which picks up the the drivers and rectifiers on its way back to a direct connection to the Mains Earth.

I will be using the Bob which was repaired for me by Doddy which has the facility for a 0-10v signal but only a make/break relay.
Does the DIYCNC spindle board generate its own signal output, so no need to connect the BOB 0-10 signal ? Or might it be better to take the speed regulation signal straight from the BOB and cut out the middle man as it were?