Nc is a normally closed symbol. NC is the abbreviation of normally close in English, which means a contact that is in a closed conductive state when it is not energized, usually called a normally closed contact. The normally open symbol is NO, and the English name is normal open, which corresponds to the initial o of the English letter "Open".

Contacts that are normally open when not energized are called normally open contacts. Specifically, it refers to the output form that the load current is not connected when the detector is not detected.

Whether it is normally open or normally closed, it refers to its original state. Once electrified, its state is actually reversed, the normally open point will be closed and the normally closed point will be disconnected.

The relay (multi-way switch) in the circuit is generally provided with a plurality of normally open points and a plurality of normally closed points to control the automatic operation and stop of electrical equipment.