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