Normally Open Contact Numbers

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.

87a normally closed contact. In other words when voltage is applied to the relay contactor terminals this contact closes. 85 relay coil 86 relay coil 87 common contact. Din 72552 defines contact numbers in relays for automotive use.

87b normally open contact. When the energizing force is present the contact will close. When voltage is applied this contact opens and interrupts current. Energizing it and switching it on will close the contact causing it to allow current flow.

An alternate notation for form a is spst no. If it is a high limit such as a temperature alarm then the current temperature is below the limit. Normally open no contacts allow current when the relay or contactor is energized. If it is a relay then it is not energized.

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. Normally open is a contact that does not flow current in its normal state. Of these contact forms the following are particularly common.

If it is a switch then it is off.