Abstract: An engineering firm had installed a power-line-carrier control system at a manufacturing plant. The system controlled a rooftop air conditioning and heating unit. Two electricians for the engineering firm had been at the jobsite for about 3 weeks, installing the electronic controls. Over the course of 2 days, they were troubleshooting the control system. The unit would not transmit a signal. After troubleshooting the control system, they determined that the control system ground was bad. Looking for a good ground, they entered a high-voltage electrical room (a substation). Two maintenance employees working for the manufacturer accompanied them into the substation, which was posted with signs warning of high voltage. (The two maintenance employees did not know the operating voltage of the lines in the substation.) One of the electricians used his 600-volt voltage tester to check a 13-kilovolt, three-phase power line. The voltage tester failed, creating an electrical fault. The ensuing electric arc burned the electrician who was using the tester. He sustained first-degree burns on his face and third-degree burns on the back of his hands. He was hospitalized at a burn center. The other electrician sustained eye flash. He was transported to a hospital, where he was treated and released the same day. The employer held safety meetings once a month, posted some safety lists, and provided on-the-job safety training.