87a normally closed contact.
Normally open contact numbers.
85 relay coil 86 relay coil 87 common contact.
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.
In other words when voltage is applied to the relay contactor terminals this contact closes.
When the energizing force is present the contact will close.
Din 72552 defines contact numbers in relays for automotive use.
Form a contacts make contacts are normally open contacts.
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.
If it is a relay then it is not energized.
If it is a switch then it is off.
Of these contact forms the following are particularly common.
Normally open is a contact that does not flow current in its normal state.
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.
When voltage is applied this contact opens and interrupts current.
Normally open no contacts allow current when the relay or contactor is energized.