The Danger of Forced Contacts and Bridged Interlocks
How the normalization of deviance leads technicians to bypass critical safety interlocks, leaving permanent blind spots in PLC logic.
1. The 3 AM Troubleshooting Shortcut
It’s 3 AM. A critical conveyor system is down because a proximity switch verifying a guard door is closed has failed. The replacement part is in a locked crib, and the production supervisor is demanding the line restart immediately.
To get the process running, the maintenance technician logs into the PLC and applies a software “Force” to the proximity switch input, tricking the logic into believing the door is closed. Or, lacking PLC access, they install a small physical jumper wire across the relay terminals, bridging the interlock entirely. The line starts, production resumes, and the technician moves on to the next fire.
2. The Normalization of Deviance
This scenario is common, but it represents one of the most dangerous human factors in industrial controls: the normalization of deviance.
What started as a “temporary fix just for tonight” is forgotten by shift change. The software force remains active in the PLC. The tiny alligator clip remains hidden in the wire duct. The safety system has now been permanently blinded. Weeks later, an operator opens that very guard door to clear a jam, trusting that the machine will halt. It doesn’t.
3. The Illusion of Control
Forcing a contact in a PLC or bridging a hardware interlock completely bypasses the Safety Instrumented System (SIS).
- Loss of Visibility: When an input is forced in software, the physical state of the machine is disconnected from the logic solver. The PLC operates on a lie.
- Cascading Failures: Many interlocks are tied to sequential processes. Forcing a pressure switch to “OK” might allow a pump to dead-head against a closed valve, causing an overpressure explosion.
- The Invisible Threat: Unlike a physical lock and tag (LOTO), a software force or a tiny jumper wire is completely invisible to operators and mechanics on the floor. They interact with the machine assuming the safety boundaries are intact.
4. Actionable Takeaways
- Strict Management of Change (MOC): Bypassing any safety interlock—whether hardware or software—must require formal MOC authorization, documentation, and a hard deadline for removal. It cannot be an ad-hoc decision by a single technician.
- Audit the Logic: Program PLCs to continuously alarm on the HMI if any forces are active. Controls engineers should routinely audit the PLC logic online specifically to hunt for forgotten forced bits.
- Physical Bypasses Require Tags: If a hardware jumper must be used for testing, it must be highly visible, brightly colored, and physically tagged with the name of the installer and the removal date. Never use a standard piece of wire that blends into the cabinet.