I am not an electrician, I am an operator apprentice at a municipal power plant, which gives me a pretty fair understanding of how things are supposed to work, and these forums are a great source of info and amusement.
Today a mechanic came to me to have me put a breaker back in service for a cooling tower fan after it had been serviced. The mechanic also had an electrician standing by to monitor the amps to be sure all was well. Normally this is no big deal, but operations staff has to be the ones to take equipment out of service and put it back. When I arrived at the panel the lower door where the starters are was open. Before I closed the CB, the mechanic reached over and closed the door and latched it. Thank goodness he did! When I closed the CB there was a loud bang, and the deadfront puffed outward, then rattled for a bit. After waiting a moment to be sure the fireworks were over, I put on a glove, reopened the CB and locked it back out. When we opened the door for the starters, a cloud of smoke rolled out, but that was it. The door did not have any debris or scorch marks on it, so we are pretty sure the CB or one of the connections to it blew. This is a 75A 480v 3phase CB. The panel is in the neighborhood of 50 years old, and I have no idea how old the CB is (was!). The 225A CB on the upstream distribution panel did not trip, so it was probably a pretty quick fault.
Just goes to show you, even on routine operations, the potential is there for a disaster.
Robert.
