Just curious about something. I work on elevators and there are countless safety circuits to prevent accidents. One example is an elevator hatch door. There are contacts that have to "make" in order for the elevator to run, to prevent the elevator from taking off with the door open. These contacts usually are 120AC and are easily the cause of most shutdowns on any elevator. After so many times of opening and closing the door, carbon builds up on the contacts and the circuit eventually fails. Why is it necessary to send voltage through these contacts? Cant you have a relay make up the safety circuit based on continuity instead? I wondered if any such type of relay exists but then i thought they must. How else would your meter beep when you test continuity. Without arcing the contacts would not build up carbon and be much more reliable. On the other hand i guess it is job security for guys like me.