Normally open and normally closed contacts (or circuits) are descriptions of electrical contacts in their resting state.
For example a normally open contact while at rest will be an open circuit, when it is triggered by something (to take it out of its resting state) it will become a closed circuit. The same is true for normally closed except in reverse.
Use The Cheat Sheet!
If we look at the above cheat sheet, we can see that if a “NORMALLY OPEN” contact meets “NO FORCE APPLIED” the result is an “OPEN” circuit and electrical current can’t go through. When a “NORMALLY OPEN” contact meets with “FORCE APPLIED” the result is “CLOSED” and electrical current can go through.
Force Applied
The force that could be applied to the contacts could be anything from someones finger at a light switch to a solenoid in a relay or a mechanical arm on a limit switch. Anything that a contact is attached to from a push button to a relay or a switch can be acivated by some sort of mechanical or electrical force.
To Recap
NO & NC contacts are electrical components that change their state once a force is applied to them. They can be used in many different applications from plain old light switches which are operated by hand to a contact on a relay that is electrically activated.
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