The project we were working on was the replacement of a damaged secondary bushing in an induction furnace transformer with a throat connected fused disconnect switch. The switch was our lock out and grounding point for the primary of the transformer. Upon opening the door on the unit, a distinct smell of ozone was present. At first, this was ignored as it was thought to have come from a load break operation. As the day progressed, working around the unit, the smell was still present. As the line side of the switch was tied to the plant main, it was not easy to isolate. In discussion with our plant contact, it was decided to investigate further and an outage was arranged. Below is what we found.

Rear of the switch—note the right phase with the burning around the insulating material. Also the surface tracking on the other phases.

A close up of the right phase. Note how the arcing had eaten away the glass polyester insulation. The unit was located outdoors on an elevated platform. The cabinet heaters were working but the unit had holes in the top of the switch cabinet allowing water to leak in.

The shield that was removed from the right phase.

Surface tracking on center phase.