Abstract: Employee #1 was installing outside lights from a rented upright TL-33 personnel boom lift. He was elevated approximately 14 1/2 ft above ground. When he experienced operation problems with the platform controls, he transferred control of the platform to his coworker, who was on the ground using the ground control box that was mounted on the base of the lift. As the coworker started to move the boom to position Employee #1, a loud noise was heard from the base and the lift rolled onto its right side. Employee #1 sustained nerve damage in his left ear and arm, and a slight injury to his right arm. The right rear outrigger was partially folded up. No parts were found to be broken or deformed. The rental company took the lift to its maintenance shop for a full checkup, and found all systems to be functioning, including the outrigger's safety interlocks and leveling system. According to the lift company, the outrigger was not fully locked and a safety bypass was used to operate equipment. The investigation was unable to confirm the cause of the accident. Employee #1 is familiar with aerial lifts, but was not trained to operate this one. No competent person inspected the lift. Employee #1 was wearing an attached safety belt at the time of the accident.