HARDWIRED INTERLOCK: When Safety Depends on Physical Wiring! 
Imagine a motor must never start unless the lubrication system is healthy.
Instead of relying only on PLC/DCS software logic, the permissive can be implemented using physical electrical contacts and wiring.
This is a Hardwired Interlock.
What is a Hardwired Interlock?
A hardwired interlock is an interlocking function implemented using physical electrical connections between field devices, relays, contactors, switches, or other control/protection devices, rather than relying solely on software logic in a PLC/DCS.
In simple terms:
Example: Motor Interlock
Suppose a pump motor should start only when:
Emergency stop is healthy
Lubrication pressure is OK
Suction valve is open
No motor fault exists
A simplified hardwired arrangement could be:
E-Stop NC Contact
Lube Pressure Switch NC/NO Contact
Valve Position Contact
Motor Starter Control Circuit
If any required condition is not satisfied:
The control circuit is interrupted.
Motor cannot start.
Why Use Hardwired Interlocks?
Hardwired interlocks can provide:
Fast response
Simple and deterministic operation
Independence from software execution
Protection against certain PLC/DCS software failures
Clear electrical troubleshooting path
They are particularly useful where a specific interlock needs to remain effective independently of the normal control-system software, depending on the design philosophy.
Hardwired vs Software Interlock
| Hardwired Interlock |
Software Interlock  |
|---|---|
| Physical electrical wiring | Programmed in PLC/DCS |
| Uses contacts/relays/devices | Uses software logic |
| Independent of normal program execution | Depends on controller/software |
| Easy to understand electrically | Easier to modify/configure |
| Wiring changes required for modification | Logic modification may be sufficient |
| Troubleshooting requires electrical checks | Troubleshooting includes logic/configuration |
Hardwired Interlock vs SIS
Don’t automatically assume that every hardwired interlock is a Safety Instrumented Function (SIF).
A hardwired interlock may be:
Process protection
Equipment protection
Motor protection
Operational interlock
Emergency shutdown function
If it is intended to perform a SIF, it must be designed and managed according to the applicable functional-safety requirements.
Real-World Example
Consider a compressor:
Low Lube Oil Pressure Switch
Hardwired Relay
Compressor Trip Circuit
If lubrication pressure falls below the defined limit:
Low pressure
Contact changes state
Relay de-energizes
Compressor trips
The trip does not need the DCS operator to manually intervene.
Interview Question
What is a hardwired interlock?
A hardwired interlock is a protection or control function implemented through physical electrical wiring and devices such as switches, relays, and contacts to prevent or initiate an equipment action when defined conditions occur.
Remember:
Software Interlock = Logic in the program
Hardwired Interlock = Logic in the wiring
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