Q: Recently an event occurred during a trouble job that surprised us. We had an underbuild phase down that was broken midspan. Our crew was working from an insulated bucket, and we grounded both the feeder we were working on and the one above. While our crew was beginning to crimp the splice for the repair, an energized line a few spans away came in contact with the grounded phase our lineman was in contact with. The lineman was in an insulated bucket, but he still received a shock. He was not seriously injured. Can you help us understand this?
A: The explanation is simple. Grounded circuits will still have current flowing through them if they are energized. Where there are resistances in parallel paths with the grounded circuit, there will be a voltage drop and there will be current flow through the parallel path. The current level is limited by the resistance in the path. Insulated bucket trucks are not totally isolating. To confirm that, all you have to do is look at the electrical tests performed on insulating booms. The current flow on an insulating boom is limited to a value well below the current necessary to injure a worker. In your case, there was voltage drop in the gap between the grounded, energized phase and the insulating boom that was a path to ground. Your lineman bridged that gap when he was in contact with the phase while standing in the bucket. The voltage was high enough to penetrate his skin so that current could flow. He was protected from injury by the current-limiting function of the insulating boom. We know it takes about 50 milliamps of current through a worker to rise to the level of injury. Depending on the electrical integrity of the boom and the voltage involved, there were – and this is just a guess based on boom-test protocols – perhaps 50 microamps to 1 milliamp of current that could flow on the boom. That is well below the level of injury. Electrical integrity of the boom is paramount in protecting workers. That is why it is critical to wash and maintain the boom.