Zs testing

[digital_boy]

Evening all.

Is there a regulation or any guidance regarding live zs testing with loads connected....

Eg zs testing on a 3phase motor whilst it is running. Could the current being drawn damage the meter if testing at the end of line.... Or doing a live zdb test, is there any guidance to say there must be no live loads or a maximum current being drawn at time of test ?

I hope this makes sense I am new to testing and don't want to damage my mft1730 Megger.

Thanks

 

[Deleted account]

Good question. I don’t think there is an actual regulation, but it does make sense to turn off or unplug any equipment. After all you are testing the resistance of the cable itself not the equipment.

 

[pc1966]

The running motor won't damage your MFT but it can lead to a higher than expected PSCC as the motor has inertial and on slowing works as a generator. As a result it is not really giving you the circuit's Zs value for fault clearing at the supply, since you have in effect a 2nd power source nearby.

In some larger systems where you are trying to work out max fault currents (either for checking MCCB/fuse/switch limits, or arc-flash safety calculations) then the contributions of nearby motors can be significant, at least for a few cycles of the AC as its magnetic field collapses (or it stops, probably later).