ZS for an motor run off an inverter

No, I don't really like it Graham, because the manufacturer does not give instructions on this, because it is not relevant. The output (motor) side of an inverter is only connected to the mains supply by electronics, there is no direct connection at all. This means that Zs at the motor has no meaning, but the CPD will provide the final circuit protection against overload or Earth fault if the electronics fail in some way.  The electronics provide all the protective functions necessary for a motor fault, usually including phase disconnection, current-limiting / disconnection on fault, overload, etc. Obviously the motor itself needs an Earth connection, but it has no direct effect on the disconnection time of the primary circuit. That phrase in BS7671 is a wonderful "catch-all" phrase, but it has no proper meaning. Inverters check the motor parameters on power-up automatically (they need more information than any electrician could ever measure) to work at all. If any parameter is out of range, not symmetric, etc. they will not power the motor.


As I said above, the only valid measurement which can be made is the Zs at the inverter input. If you think otherwise you are welcome to explain how and what instrumentation is to be used, what the limits are, and how you get a loop reading through the electronics, which essentially isolate the motor from the input voltage anyway. I would also like you to explain what the BS7671 phrase really means because it is easy to say but impossible to achieve. You and I both know there are a number of such phrases in the book, it is time they were removed by policy. Inverters are one of the most complex but common devices in wiring systems, particularly after the latest EU directives on motors. The lack of understanding shown in BS7671 is amazing and should be corrected with an explanation. You will be saying that a loop figure for class 2 power supply outputs is needed next!

No, I don't really like it Graham, because the manufacturer does not give instructions on this, because it is not relevant. The output (motor) side of an inverter is only connected to the mains supply by electronics, there is no direct connection at all. This means that Zs at the motor has no meaning, but the CPD will provide the final circuit protection against overload or Earth fault if the electronics fail in some way.  The electronics provide all the protective functions necessary for a motor fault, usually including phase disconnection, current-limiting / disconnection on fault, overload, etc. Obviously the motor itself needs an Earth connection, but it has no direct effect on the disconnection time of the primary circuit. That phrase in BS7671 is a wonderful "catch-all" phrase, but it has no proper meaning. Inverters check the motor parameters on power-up automatically (they need more information than any electrician could ever measure) to work at all. If any parameter is out of range, not symmetric, etc. they will not power the motor.


As I said above, the only valid measurement which can be made is the Zs at the inverter input. If you think otherwise you are welcome to explain how and what instrumentation is to be used, what the limits are, and how you get a loop reading through the electronics, which essentially isolate the motor from the input voltage anyway. I would also like you to explain what the BS7671 phrase really means because it is easy to say but impossible to achieve. You and I both know there are a number of such phrases in the book, it is time they were removed by policy. Inverters are one of the most complex but common devices in wiring systems, particularly after the latest EU directives on motors. The lack of understanding shown in BS7671 is amazing and should be corrected with an explanation. You will be saying that a loop figure for class 2 power supply outputs is needed next!