Many years ago Otis had a problem with the 6754 switches in escalator controllers. They were Silver moveable to carbon stationary. Some one decided to make the stationary silver also for longevity. Silver to silver welds on AC. They had to go back to a carbon stationary. Safety circuits could open all day long and motor kept running. When you pulled the main line the relay contacts were welded together and relay was locked in. Always make sure the contactor, if replaced, is rated for AC and not just DC. May end up with some silver soldering and relay locked in.