Abstract: A maintenance supervisor and a senior maintenance worker were installing a 120-volt receptacle. They opened and tested one junction box. They found the voltage in that box to be 480 volts. They searched for a 120-volt circuit and located two lights used for signaling purposes. They knew these lights operated at 120 volts. They followed a conduit, which was located overhead, 5.5 meters to a junction box. They installed another conduit and pulled conductors through it. One of the two employees turned on the two lights, went to a panelboard, and started opening circuit breakers. When the lights went off, the employee at the junction box yelled, "That's it." The employee who had opened the circuit breaker went to the parts room for wire nuts. When he returned, he found his coworker slumped over the ceiling joist, electrocuted. It was later determined that the junction box the employees were working on contained 480-volt conductors supplying power to a machine below and did not contain the wiring for the 120-volt lights. From the floor, it appeared as if the conduit from the junction box led to the lights, but it did not. The employees had not tested the circuit even though they had a voltmeter with them.