NO = Normally Open, NC = Normally Closed. This describes the state of a contactor when the voltage in the system is off. Normally open (NO) is mostly used in electrical systems where, for safety reasons, you want to be sure that the contactor opens when the power is cut in the system in case of a power failure. Normally Closed (NC) is often used when you want to be sure that the contactor closes a circuit in the event of a voltage drop, such as a power failure. to drain the system of energy to earth.
Read more about our DC contactors for both high and low voltage here.