Normally open: Its symbol is NO, corresponding to the first letter O of the English letter "open". A contact that is in a normally open state when not powered on is called a normally open contact.

Normally closed: Its symbol is NC, corresponding to the first letter C of the English letter "close". A contact that is in a closed and conducting state when not powered on is called a normally closed contact.

Normally open contacts refer to contacts that are in an open state without external force. When an external signal acts on a normally open contact, it will close, energizing the circuit. Normally open contacts are commonly used in circuits to control switches, plugs, relays, etc. to achieve control of the circuit.

Normally closed contacts refer to contacts that are in a closed state without external force. When an external signal acts on a normally closed contact, it opens, causing the circuit to power down. Normally closed contacts are commonly used in circuits to protect circuits and equipment. When a circuit malfunction or equipment malfunction occurs, the normally closed contacts will automatically open the circuit to avoid damage to the circuit and equipment.

How to distinguish between normally closed points and normally open points

1、 Different references

a. Normally open contact: The contact in the disconnected state when the relay coil is not energized.

b. Normally closed contact: The contact in the closed state of a relay when the coil is not energized.

2、 Different principles

a. Normally open contact: When a certain voltage is applied to both ends of the coil, a certain current will flow through the coil, resulting in electromagnetic effects. The armature will overcome the pulling force of the return spring and attract to the iron core under the attraction of electromagnetic force, thereby driving the movable contact of the armature to engage with the static contact (normally open contact).

b. Normally closed contact: When the coil is powered off, the electromagnetic attraction also disappears, and the armature will return to its original position under the reaction force of the spring, causing the moving contact to engage with the original static contact (normally closed contact).

3、 Different uses

a. Normally open contact: When the contactor is not working, this contact is open. When working, the closed contact first opens, and then the normally open contact closes.

b. Normally closed contact: When the contactor is not working, this contact is closed. When the contactor coil is powered off, the normally open contact first opens, and the normally closed contact then closes and resets.

Source: Xianji Network, Refrigeration, Guide Car Robot Technology