An employee was de-energizing the main distribution switch board. The breaker had been opened and locked. He tested it and deemed there was no voltage present at the lugs on the supply side of the breaker. A leg of a ground cluster was attached to the neutral bus. Another leg of the ground cluster came in contact with the energized "A" phase on the supply side of the breaker, resulting in an arc flash (480v/277v). The employee sustained first degree burns on the right side of his face and neck and second degree burns on his lips, under his chin, and behind his right ear.