Jeff Conover, asphalt performance manager for Ohio-based asphalt plant Oldcastle materials, was charged with fixing a seemingly straightforward problem with the plant’s dust filtration system. Something was causing a breaker to trip intermittently, interrupting power to one of two 100-horsepower fan motors. Safety topped Conover’s priorities as he and plant foreman and electrician Charlie West locked and tagged-out the motor control center and set up their tests, which included using a Fluke Connect® wireless test tool system.
Here's what happened..."We had all stepped out of the motor control center. We asked the plant operator to go ahead and start the exhaust fan. As soon as he hit the start button a small shotgun blast back from the control center sounded. Something had burned up, or flashed. My first thought was I was glad nobody was in the room. My second thought was the Fluke test tool, even though we didn’t get any readings, it did what it was supposed to do. We were able to leave the motor control center room and step around the corner with the remote tester because we were able to read the meter without being in that location."
Because the team had stepped out of the room and were 15 feet away, they didn’t see the arc flash, but the damage when they returned was obvious.
"The weakest link in our circuit was the thermal overloads in the motor control center," Conover said. "When that shorted, it caused that thermal overload…to explode."
"We found that the motor lead cable going out to the motor, probably 75 to 100 feet (22.8 to 30.4 meters) away from the control house, had shorted internally from one phase to ground…causing the arc flash that happened back inside the motor control center," Conover said. Tracing along the cable, the team found a hot spot under the insulation where the short occurred. Conover estimates the failed cable, located outdoors, at 20-plus years old.
The timing of the short Conover attributes to luck—it just happened to occur while testing was underway. The fact no one was injured? That came from good safety practices and using the right gear. "It was just luck as to the time that it did happen," he said. "It also gives you an understanding that even though the short was 100 feet away, it doesn’t mean that’s where the arc flash was going to happen. It happens at the weakest link, and we don’t always know where that weak link is."