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.
Normally open contact numbers.
If it is a switch then it is off.
Normally open no contacts allow current when the relay or contactor is energized.
87b normally open contact.
This is simply the state that the contact is in when something else is not affecting it.
The contacts are open when the energizing force magnet or relay solenoid is not present.
Din 72552 defines contact numbers in relays for automotive use.
Normally closed nc contacts allow current through when the relay or contactor is not energized.
85 relay coil 86 relay coil 87 common contact.
Of these contact forms the following are particularly common.
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.
Normally open is a contact that does not flow current in its normal state.
If it is a relay then it is not energized.
Energizing it and switching it on will close the contact causing it to allow current flow.
If it is a high limit such as a temperature alarm then the current temperature is below the limit.
An alternate notation for form a is spst no.
Form a contacts make contacts are normally open contacts.
When the energizing force is present the contact will close.