It's spirits from the other side attempting to cross back into the living world.
It's a little known fact, but back when power systems were first being developed spirits (either of the deceased or those who have never lived) were able to cross back into our realm during lightning storms. Hence the popularity of seances at the time. The spirits, not being anchored in this reality, would fade back into the ethereal planes after a few weeks.
Edison, annoyed at the ghost of his great grandmother constantly nagging him to eat his vegetables, eventually engineered a solution. He found that the most probable location of a cross-dimensional vortex forming, was at locations where there was a split or change of wire gauge. By surrounding these locations with insulated boxes, you could keep the ghost at the vortex until it closed, forcing them to stay on the other side. An unfortunate side effect was that compressing the vortex and spirit together in this manner created an interruption in electrical flow.
That is why things flicker on and off during thunderstorms, and why fuse boxes, meters, and other high voltage switches are locked in metal boxes.
