Glossary
What is Interlock?
A condition that prevents a device from starting, or stops it, to protect the process or people, whatever the operator wants.
For example: the pump does not run with a full tank, the valve does not open above a pressure limit. An interlock is not an operator choice; it is enforced by the program.
It differs from a permissive, which only allows a start when conditions are met but does not stop a running device. Safety-critical interlocks belong in an independent safety system, not in the process PLC.
Related terms
- Seal-in (latch)
- A pattern that keeps an output running after a momentary button press, by feeding the output back into its own start condition.
- Permissive
- A condition that must be true before a device is allowed to start, but which does not stop it once it is running.