Imagine a field input is OFF, but an engineer needs to test the logic.
Instead of changing the actual field condition, the engineer may temporarily force the signal to ON.
This is called a Force.
What is a Force?
A force is a temporary command applied within a control or safety system to override the actual state of an input or output with a defined value, such as ON/OFF or TRUE/FALSE.
In simple terms:
Actual signal = OFF
Forced signal = ON
System logic sees ON
The exact behavior and terminology depend on the PLC/DCS/ESD platform.
Force in PLC
Suppose a PLC receives:
Level Switch → DI → PLC
Actual field condition:
Level Switch = OFF
For testing, the engineer may force the PLC input:
DI = ON
The PLC program then processes the forced state as though the input were ON.
This can be useful for:
Logic testing
Commissioning
Troubleshooting
Simulation
I/O testing
Force in DCS
In a DCS, forcing may be used during:
Commissioning
Functional testing
Troubleshooting
Control logic verification
For example, an engineer may temporarily force a digital status to verify:
Input → Control Logic → Output → Graphic/Alarm
However, forcing a process input in a live plant can make the DCS display a condition that does not represent the actual field condition.
Force in ESD / SIS
This is where extreme caution is required.
Consider:
Pressure Switch → ESD → Shutdown Logic → SDV
Actual condition:
Pressure = Normal
If the ESD input is forced to indicate:
High Pressure = TRUE
the ESD logic may interpret this as a genuine trip condition.
Depending on the configuration, this could:
Initiate an ESD
Close shutdown valves
Stop equipment
Create process consequences
Conversely, forcing a signal to a safe-looking state could prevent a safety function from responding to a genuine hazardous condition.
Therefore:
Forcing an ESD/SIS signal is not a routine troubleshooting shortcut.
It should be performed only under the approved testing/maintenance procedure, with appropriate authorization, risk assessment, control of bypass/override status, and clear communication.
Force vs Bypass
These terms are often confused.
| Force |
Bypass |
|---|---|
| Overrides a signal/state with a specified value | Prevents a specified function from acting normally |
| Common in PLC testing | Common in SIS/ESD maintenance/testing |
| Can affect input or output behavior | Usually associated with a defined function/path |
| May make logic see a simulated condition | May temporarily remove a protection function |
| Exact behavior depends on platform | Exact behavior depends on SIS philosophy |
Do not assume Force and Bypass are equivalent.
The Biggest Danger: Forgotten Forces
A force is applied for testing:
Test completed
Engineer leaves the system
Force remains active
Plant returns to normal operation
Actual field condition occurs
System may not respond as expected
That’s why forced points should be:
Identified
Clearly indicated
Authorized
Controlled
Recorded
Reviewed
Removed after testing
Function verified after restoration
Interview Question
What is Force in a PLC/DCS/ESD system?
Force is a temporary mechanism that overrides the actual state of a selected input or output with a defined value for purposes such as testing, commissioning, or troubleshooting. In an ESD/SIS, forcing can affect safety functions and therefore requires strict procedural control.
Remember:
PLC Force → Test the logic
DCS Force → Simulate/override a signal for testing
ESD Force → Potentially affect a safety function
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