The advice I got on this subject by a drive specialist a number of years ago was to forget about the VSD and design the circuit as if the drive wasn’t there. Any earth fault downstream of the VSD would be handled by the VSD itself. An internal fault in the VSD upstream of its own protection would be handled by the CPD. We were installing the units in a fertiliser factory with the VSDs quite some distance from the motors they controlled. We just did a loop to the VSDs and an R1+R2 to the motors from the VSDs and ensured that the total would be low enough to take out the CPD. Hardly a sophisticated approach but acknowledged as satisfactory by our NICEIC engineer.
With regards to checking Zs on an existing VSD arrangement, I doubt that bypassing the VSD is a practical consideration. Even checking R1+R2 from VSD to motor might be totally unnecessary and might only be worth the effort if access to the motor terminals was being considered for other reasons. Otherwise confirmed to be earthed using a wandering lead seems an optimal approach.
The advice I got on this subject by a drive specialist a number of years ago was to forget about the VSD and design the circuit as if the drive wasn’t there. Any earth fault downstream of the VSD would be handled by the VSD itself. An internal fault in the VSD upstream of its own protection would be handled by the CPD. We were installing the units in a fertiliser factory with the VSDs quite some distance from the motors they controlled. We just did a loop to the VSDs and an R1+R2 to the motors from the VSDs and ensured that the total would be low enough to take out the CPD. Hardly a sophisticated approach but acknowledged as satisfactory by our NICEIC engineer.
With regards to checking Zs on an existing VSD arrangement, I doubt that bypassing the VSD is a practical consideration. Even checking R1+R2 from VSD to motor might be totally unnecessary and might only be worth the effort if access to the motor terminals was being considered for other reasons. Otherwise confirmed to be earthed using a wandering lead seems an optimal approach.
