A journeyman electrician and an electrical helper were at a worksite to install a new three-phase run of wire between an existing energized 480-volt circuit breaker panel and a new piece of machinery. During the process, the journeyman electrician attempted to install a missing bolt from a breaker mount on an energized 480-volt bus bar. The electrician used protective insulating gloves, but did not use safety glasses, shields, or any other safety equipment.
While attempting to screw the bolt into place, he lost control of the bolt. Either the bolt or the mount (or both) then contacted another phase in the circuit breaker panel resulting, in an arc flash.