What is an Interlock?
An interlock is an automatic protection or blocking function that prevents a hazardous or undesirable action from being performed in an installation or machine. It is often implemented in PLC or DCS systems as part of safety and operational logic.
Interlocks prevent human error, unsafe situations and damage to process installations.
🔐 What does an interlock do?
An interlock checks whether certain conditions are met before an action may take place. If they are not, the action is blocked or aborted immediately.
For example:
- A pump may only start if all valves are in the correct position
- An oven may only open once the temperature is below 50 °C
- A motor must not run while the machine cabinet is open
🎯 Types of interlocks
| Type | Description |
|---|---|
| Safety interlock | Protects people or installations from unsafe actions |
| Process interlock | Ensures the correct process sequence (e.g. fill the tank first, then heat) |
| Mechanical interlock | Physical blocking of a component (e.g. key switch, lock) |
| Software interlock | Logical blocking in PLC or SCADA software |
| Electrical interlock | Relay or contact arrangement that electrically prevents an action |
🏭 Real-world examples
| Situation | Interlock description |
|---|---|
| Running a centrifuge | Only allowed if the lid is closed and locked |
| Pumps in a CIP system | Only start when valves and levels are correct |
| Starting a burner | Only allowed when the gas valve is open and airflow is present |
| Rotating machine | Drive switches off when the guard is opened |
🔗 Interlocks and systems
| System | Role in interlocks |
|---|---|
| PLC | Holds the interlock logic in the program |
| SCADA | Visualises interlock status and provides warnings |
| SIS (Safety Instrumented System) | Executes critical interlocks with high reliability |
| DCS | Coordinates process interlocks in complex systems |
⚠️ Difference between interlock and alarm
| Interlock | Alarm |
|---|---|
| Actively prevents a hazardous action | Reports that something has gone or may go wrong |
| Action-blocking or shutting down | Passive: alerts the operator |
| Automatic and immediate | May require manual follow-up |
📌 In summary
An interlock is a critical safety function that automatically prevents an installation from entering an unsafe state. It is a fundamental element of process safety and reliable automation.