Hi Guys,

I’ve just had a VSD for a supply fan trip on over current.
Upon meggering the motor cable it was discovered that the shielding and one of the phases was shorted.
I searched the length of the cable (around 50metres/150ft) and found no obvious signs of damage.
I checked the motor connection box, no signs of water ingress or other damage.
Next morning when I resumed my fault finding I discovered the short had disappeared. So I started the fan again and it run ok until about midnight when it tripped again.
Morning number 3 checks found the cable shorted again.
Morning number 4 clear again and fan motors been running fine ever since.
My question is, “Is there a phenomenon where a cable insulation can temporarily break down due to the high frequency nature of a VSD output power?”

 

Unlikely just a cable failure as such.
What is the wiring gage, and insulation type, voltage rating.

 

Hi Dave,

Thanks for your reply. Cable is 16mm2/6awg 3core shielded copper xlpe. Voltage is 690v

 

Is the short present with both ends of the cable are disconnected?
Advanced cable testers are available that test for cable faults electronically and report the fault location.

 

https://www.fluke.com/en-us/learn/blog/predictive-maintenance/beyond-the-multimeter-part-2

 

Hi Dave,
Sorry about the late reply, but yes when the fault was present, it was present when tested from both ends.
But then as if by magic the fault disappeared.
The VSD tripped again last night, after running ok for the last 2 weeks, the cleaner that was nearby reported seeing blue sparks when it happened, but when I checked it there was no smell or burn marks.
I rechecked the cable which was OK. Restarted the drive, no issues so far.
The mystery continues.

 

This reminds me of a problem I recently had. I had a 100 Ft length of SO cable transmitting 460 Volts between 2 cabinets. As soon as power was supplied, an immediate trip. Bottom line, we ended up stripping the insulation of the cake and fount that two wires were actually spliced together under the insulation. Thank goodness for circuit breakers. You have a defective cable, replace it.