Abstract: An underground crew working on a tunnel excavation project was reinstalling linear utilities after blasting earlier at the intersection of two tunnels. One employee was reconnecting the water hose to the rear of a tunnel boring machine (Dosco MK II-A Roadheader) when he heard a crackling sound and noticed arcing coming from the attachment plug on the machine's trailing cable. He yelled for someone to turn off the power. As he was walking away, the electrical fault became more severe and produced an electric arc. The employee lost consciousness at this point. It could not be ascertained whether he was knocked down by the arc or just lost his footing and was thrown toward the tunnel wall. He landed on his knee with his back toward the rib and had the wind knocked out of him. He sustained low back strain and contusions on his left pelvis and left shoulder but did not suffer any electrical burns. The tunnel crew always disconnected all linear utilities including the electric power supply cables before blasting, and this occurred two or three times a week. Examination of the ground wire at the machine and at the surface main circuit breaker by a licensed electrical contractor did not indicate any fault in the system. It is believed that the constant connecting and disconnecting of the cables caused two of the six conductors inside the sheathed trailing power cable to short, as evidenced by some beading.