One of the modules that are used to work with high voltages, is the Relay. On the front it has three ports: NO, COM and NC. It works like a regular switch: when triggered, it closes the connection with the NO (Normally Opened) and opens the one with the NC (Normally Closed). For example, if we connect two wires, one on the NO and one on the COM, they’re not connected until we give the .on() command to the relay. If we put them on COM and NC, they are always connected until the .on() command is given.
Via Axel, Damien Rivoal