Imagine pressing the START button for a large compressor.
Should it start immediately? 
Not necessarily.
Before allowing the equipment to start, the control system checks whether all required safe conditions are satisfied.
These conditions are called Startup Permissives.
What is a Startup Permissive?
A startup permissive is a predefined condition that must be satisfied before a piece of equipment or process sequence is allowed to start.
In simple terms:
All required conditions TRUE → Start permitted
Any required condition FALSE → Start blocked
Example: Pump Startup
Consider a process pump.
The DCS may require:
Suction valve OPEN
Minimum tank level available
Lubrication system healthy
Motor breaker healthy
No active trip
Pump in AUTO/REMOTE mode
Only when all required conditions are satisfied:
Permissives = TRUE
The pump start command is allowed.
If one condition is missing:
Permissives = FALSE
Pump startup is blocked.
Typical Startup Permissives
Depending on the equipment and process, permissives may include:
Adequate lubrication pressure
Cooling water available
Minimum suction pressure
Valve position confirmed
Motor ready
No active shutdown
Instrument healthy
Adequate tank level
Seal system available
Correct operating mode
Required auxiliary equipment running
Startup Permissive vs Interlock
These terms are related but should not automatically be treated as identical.
Startup Permissive:
A condition that must be satisfied before starting.
Interlock:
A logic function that prevents or changes an equipment/process action when specified conditions are not satisfied.
For example:
Low lubrication pressure → Pump cannot start
This can act as a startup permissive.
If lubrication pressure subsequently becomes dangerously low while the pump is running, a separate running interlock/trip may be required depending on the design.
Simple Logic
Suppose a compressor requires three startup conditions:
Lube Oil Pressure OK
AND
Suction Valve Open
AND
No Trip
Then:
TRUE + TRUE + TRUE = START PERMITTED
But:
TRUE + FALSE + TRUE = START BLOCKED
Important Point
A permissive does not necessarily mean the equipment is safe under all operating conditions.
It only confirms that the specified condition(s) required for that particular action are satisfied.
Other protection functions—such as alarms, trips, interlocks, and SIS functions—may still be required during operation.
Interview Question
What is a startup permissive?
A startup permissive is a predefined condition or group of conditions that must be satisfied before the control system allows equipment or a process sequence to start.
Remember:
No permissive → No startup
All required permissives satisfied → Startup allowed
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