The contacts are open when the energizing force magnet or relay solenoid is not present.
Normally open contact numbers.
Din 72552 defines contact numbers in relays for automotive use.
If it is a relay then it is not energized.
If it is a switch then it is off.
When voltage is applied this contact opens and interrupts current.
An alternate notation for form a is spst no.
When the energizing force is present the contact will close.
87a normally closed contact.
Energizing it and switching it on will close the contact causing it to allow current flow.
Normally closed nc contacts allow current through when the relay or contactor 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.
87b normally open contact.
This is simply the state that the contact is in when something else is not affecting it.
In other words when voltage is applied to the relay contactor terminals this contact closes.
Normally open no contacts allow current when the relay or contactor is 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.