Ka-boom

ElectroChem · Dec 7, 2014

Callout to a farm today, motor won't go. This is what greets me when I open up the control box. Lightning storm yesterday, seems the poor relay didn't appreciate the jolt. Anyone care to share their pics of utterly destroyed bits and pieces?

Besoeker · Feb 22, 2015

Tony said:
What voltage? They look like 3.3KV fuses but the wiring doesn’t. Interesting how you can see the flashover points on the fuses. Is the white mark across the back plane tracking?

I’ve not seen anything like that for many years, nothing to do with lightning but inductive discharge when a motor stopped.
The motor had been uprated three times as I can remember, pity the draw out starter hadn’t. The fuses had gone up from 150A to 300A, obviously the fuse diameter was bigger making the gap between them far to small.
Intermittently when the motor stopped the back EMF would cause flashover taking out the 600A busbar riser fuses.
I wasn’t based on this plant but got lumbered with finding a cure. I fitted insulating shields between the fuses and everything was fine.
It ran like that for a couple of years and then the problem started again? Some pillock had replaced the nylon nuts and setscrews with brass! To make it even better they’d cut the setscrews with pliers leaving a nice point aimed directly at the adjacent fuse. It was just like a spark gap used for MV over voltage protection.

Our plants took several direct strikes but they never did any damage, the building acting as a Faraday cage.

The fuses are 3.6kV and so is the wiring. It's part of the speed control system for a 3.3kV slip ring induction motor. A transformer in the adjacent panel, also part of the speed control system smoked and the hence the ensuing damage. The white mark is where somebody wiped the back panel. Parts were changed as required and it all cleaned up rather nicely.

Besoeker · Feb 22, 2015

Here's another nice mess.

We don't what happened. The kit had been stored in an unheated room in the jungle in West Africa for about six years. It is, or was a 315kW AFE VFD. It comprises two panels split to make shipping easier. We suspect that the power connections between the two were not done up correctly leading to arcing/overheating. But who knows? Given the history, it could have been full of vermin or snakes or bees nests....

We were asked to bid for a replacement unit but that was a year or so ago. Nothing since.