Breaker tripping when machine turned off.

[The Murch]

Hi All,

Recommissioning an old Control Cabinet and retrofitted it with some new control gear. I'm looking to put it back into use, however when i turn off the isolator on the Control Panel, it trips the 16A Breaker for the Socket its plugged into.
Checked for Ground Faults and Short-circuits, everything seems to be fine. Isolator is 4 Pole (3Ph + N), Plug wiring looks good.
Cabinet has a Siemens Simanics V20 VFD and Logo8 Controls, motor isnt running when powered down so there shouldn't be any inductive kickback. However when I E-Stop the machine before turning off the isolator, which in turn cuts power to the VFD via the branch contactor, the breaker doesn't trip.

Trip Switch is a Hager ADM420c B20.

Any suggestions?

Thanks.

 

[junkhound]

Will reply since there are a lot of views and no replies and a VFD in the system. -

Have seen cases where on shutdown the VFD frequency goes lower faster than the voltage declilnes, resulting in a motor or transformer magnetic saturation and overcurrent. Have even seen PT burn out due to this, especially on MG sets where there is rotating inertia to feed the lower frequency energy to the saturated part vs. just the capacitance in the dc link of the VFD.

Your last line already gives you the solution,