Normally Open Contact Numbers

Normally open no contacts allow current when the relay or contactor is energized.

Normally open contact numbers.

Normally open is a contact that does not flow current in its normal state. In other words when voltage is applied to the relay contactor terminals this contact closes. If it is a relay then it is not energized. Out coils a plc output is really just an internal relay with a physical connection that can supply electrical power to control an external load.

This is simply the state that the contact is in when something else is not affecting it. A relay contact in the ladder diagram can be normally open or normally closed and there is no limit to the number of contacts a relay can have. Form a contacts make contacts are normally open contacts. If it is a high limit such as a temperature alarm then the current temperature is below the limit.

When the energizing force is present the contact will close. If it is a switch then it is off. An alternate notation for form a is spst no. 87a normally closed contact.

Energizing it and switching it on will close the contact causing it to allow current flow. 85 relay coil 86 relay coil 87 common contact. Din 72552 defines contact numbers in relays for automotive use. The contacts are open when the energizing force magnet or relay solenoid is not present.

87b normally open contact.