Interlocks vs Trips vs Permissives — Explained Simply
These three terms are everywhere in PLC, DCS, and SIS systems…
But many engineers still confuse them 
Permissive
Condition required to START equipment
“Can I start the machine?”
Example:
Lube oil pressure healthy
Cooling water available
Valve in correct position
If all conditions are OK → Equipment can start
Interlock
Prevents wrong or unsafe operation during running condition
“Should this action be blocked?”
Example:
Prevent motor start if valve closed
Prevent pump operation at low tank level
Mainly used to avoid equipment/process damage
Trip
Immediate shutdown during dangerous condition
“Stop NOW for safety!”
Example:
High-high pressure
High temperature
Fire & gas detection
Equipment/system shuts down automatically
Simple comparison
Permissive → Allows START
Interlock → Prevents wrong action
Trip → Forces SHUTDOWN
Real example — Pump System
Permissive → Tank level healthy → Pump allowed to start
Interlock → Discharge valve closed → Pump start blocked
Trip → Motor overload → Pump trips immediately
Video Lessons:
Key takeaway
Permissive = Start permission
Interlock = Operational protection
Trip = Emergency shutdown
Simple understanding
Before Start → Permissive
During Operation → Interlock
During Danger → Trip
#Instrumentation #PLC #SIS #ProcessSafety Automation #ControlSystems #IndustrialAutomation #Engineering dcs #FunctionalSafety 