Interesting question...
I've been in the "burn rooms" at UL, Northbrook, and I recall:
Temporary test rooms had no electric whatever; construction was done with extension cords and halogen lights;
The main test room, where pallets were burned, had all electric in pipe. The receptacles were well below the level of the fire (fire was raised maybe ten feet up). The lights were well to the side of the test ares, with an exhaust directly above the fire.
Oddly enough, the major issue seemed to be corrosion, rather than heat. Some materials, when burned, create very acidic fumes and ashes. This was not a problem for the electrical, however- soot from countless fires had well sealed every inch of pipe!
