by
noel famador , Site Supervisor , Woo sun Electric Co. Ltd
Normally Open (NO) and Normally Closed (NC) describe the state, or position, of contacts in a spring-loaded switch (forces other than springs can be used, but spring-loaded switches are by far the most common).
Switch contacts can be open or closed. Closed contacts can conduct current through the switch, while open contacts can not conduct current. A spring-loaded switch uses spring force to hold the contacts in one position or the other. The switch can have multiple sets of contacts, so some contacts can be NC, while others are NO on the same switch.
NC contacts will be closed when the spring is in its more relaxed (resting) position. The contacts will open only when something (a solenoid or some other mechanical force) causes the contacts to move from their resting position.
NO contacts work the other way. They are normally open when the spring is in its more relaxed position and close only when the switch is forced to its other position
The switch will always return to its resting position when the mechanical force is removed. Note that it is not true that the non-energized state is NO. The non-energized (resting) state can be either NC or NO, depending on the design of the switch. And as mentioned earlier, many switches come with one set of contacts NO and another set of contacts NC.
