Insulated bus is a desirable feature in dusty enviroments.
I once witnessed an MCC section arc over due to a combination of an exceptionally humid day and a buildup of fine sawdust on the horizontal insulators supporting the uninsulated vertical bus bars. (I actually saw the second flash-over that resulted from unseen damage done by the first flashover.) The arc rose until it was stopped by the next higher horizontal insulator. It burned completely through one bus bar and then jumped up and burned under the next higher horizontal insulator. It burned through another bus bar and jumped again. The primary fuses on the transformer bank went out before the arc could burn through a third time.
We were called out one night because of an arc and subsequent power failure in an MCC. We removed a switch can and inspected the vertical busses. We could see arc damage to the lower part of the bars, but the bottom switch was quite high and the area that it connected to the bus was undamaged. The bus meggered clear. We re-energized without incident. We gave the operators the all clear and they started their start-up sequence.
They had about 80% of their motors started and all heaven broke loose. The lights went out and we were standing in the dark in the control room facing the MCC outside through the safety glass. All of the vertical joints between sections and the horizontal joints between switch cans on the MCC were outlined in brilliant white by the internal arc.
Our best guess was that the first flashover left a splatter of aluminum, sort of like tinfoil, stuck to the back of a switch can and the shock or vibration of a large contactor closing shook it loose and fell onto the bus to start the second event.
Of course it is just my personal opinion but I have been a fan of insulated bus for MCCs and switchgear ever since.
respectfully
We always specify insulated bus in 600V MCC's for power plant applciations.
We had a couple of MCC burn downs that may have been prevented with insulated bus. We had one burn down that occurred on an insulated bus MCC, so it is not fool proof. (The entire MCC didn't actually burn down or melt, just one cubicle or vertical section had arc damage, but there was enough smoke damage to require 100% replacement of the equipment.)
Our safety group believes insulated bus minimizes the chances for arcing faults and arc flash.
