Sunday: Control Systems: The Danger of "Forces" in PLC Logic
Treat software forces like physical safety bypasses. They can be just as deadly.
The Core Issue: During troubleshooting or commissioning, standard practice is to place a software “force” on a PLC output or input to bypass a sensor.
The Lesson: If a technician forgets to remove the “force” after testing, the automated safety interlocks rely on false data. A tank could overfill, or a motor could start while someone is working on it, overriding the physical emergency stop relays in badly designed systems.
Actionable Takeaway: Treat software forces like physical LOTO locks. They must be aggressively tracked, documented, and never left active at the end of a shift without full sign-off.
Post Conclusion
Failure Mode — Do Not Ignore This post describes a failure mode or active hazard. Do not ignore the warning signs described.