Glossary · PLC and control programming
Permissive
Also known as: Start permissive, Release condition, Enable condition
German: Freigabebedingung
In control engineering, a permissive is a condition, or set of conditions, that must be fulfilled before an operation may start; unlike an interlock, it usually does not stop an operation that is already running.
- PLC programming
- Process industry
In one sentence
A permissive is a condition that must be met before an operation may start, such as a valve open before a pump starts.
Example
The pump start permissive requires the suction valve to be open and the tank level above minimum; without it, the start command is ignored.
How it applies
- Engineering: Permissives guard the start of an operation: a pump, a sequence step, a batch phase. They are listed in cause-and-effect matrices and implemented in the control logic, often together with interlocks.
- Operation: Operators need to see which permissive is missing. HMI faceplates commonly show a permissive list with the status of each condition.
- Safety: A permissive in the basic control system is an operational condition. If a condition protects people or equipment from a hazard, it belongs in a properly designed Safety function or Interlock, not only in a start permissive.
- Documentation: The operating manual should list the permissives for each start with the operator action that fulfills them. Using the same condition names in the manual and on the HMI reduces calls to service.
Permissive vs. interlock
A permissive is checked before a start. An Interlock is effective continuously and stops or prevents operation whenever its condition is not met, also while running. Some plants use the word interlock for both; the design documents should define the terms.