1 direct short to ground... unexpected result.

[Chris egbert]

Had a 3 phase VFD feed with one leg shorting to a ground bushing. There was obvious burn marks where it shorted which was about three feet from the load side of the breaker.
Approximately 200’ away, inside the VFD, it also caused significant damage. Any thoughts?

 

[GoldDigger]

Had a 3 phase VFD feed with one leg shorting to a ground bushing. There was obvious burn marks where it shorted which was about three feet from the load side of the breaker.
Approximately 200’ away, inside the VFD, it also caused significant damage. Any thoughts?

If it did not happen instantaneously when the fault occurred, one possibility is that when one feed leg shorted out the VFD continued trying to drive the motor with input from the DC bus coming from only one line to line phase instead of three, severely overloading the input rectifier bridge.
Alternatively the arcing as the short occurred might have caused voltage spikes that destroyed some input elements of the VFD, or even output elements if the bus voltage also spiked.

 

[Chris egbert]

Thanks for the input. I couldn’t figure out how to post the photos, but it completely destroyed the lugs at the VFD input terminals and there was lots of burn residue as if there was a fire inside of the enclosure.

 

[kwired]

Thanks for the input. I couldn’t figure out how to post the photos, but it completely destroyed the lugs at the VFD input terminals and there was lots of burn residue as if there was a fire inside of the enclosure.

starting to sound like lightning or other surge damages. Short at the ground bushing may have been a weak spot in conductor insulation for some reason.

 

[milldrone]

Had a 3 phase VFD feed with one leg shorting to a ground bushing. There was obvious burn marks where it shorted which was about three feet from the load side of the breaker.
Approximately 200’ away, inside the VFD, it also caused significant damage. Any thoughts?

You didn't state what kind of power supply you have. If you have a ungrounded delta or high resistance grounded wye and you did not remove or disable the MOV's in your drive. The MOV's sacrificed themselves in an effort to keep L1, L2, L3, at the same potential from ground. Sometimes this can be very spectacular with a lot of damage.