PLC Sequence & Safety Modernization

This safety-critical project modernized fire detection and emergency response systems for railway tunnel infrastructure. The existing legacy system suffered from unclear state machine logic, difficult maintenance, and limited diagnostic capabilities. The modernization delivered a fully SIL2-compliant architecture meeting EN 50129 (Railway Applications – Safety) and IEC 61508 (Functional Safety) requirements.

The new Safety PLC architecture implements a hierarchical state machine with clearly defined states, transitions, and interlocks. Fire detection inputs from multiple sensor types—smoke detectors, thermal sensors, and flame detectors—are processed through a voting logic system that prevents false alarms while ensuring genuine fire events trigger immediate response.

Emergency response sequences coordinate tunnel ventilation systems, evacuation signage, traffic signals, and emergency service notifications. Each sequence follows strictly defined timing requirements with fallback modes ensuring safe states even under component failure scenarios.

The implementation uses fail-safe design principles throughout—all outputs default to safe states on communication loss, power failure, or software watchdog timeout. Extensive diagnostic coverage monitors sensor health, communication link integrity, and actuator feedback, with all events logged to a historian database for incident analysis and regulatory compliance.

Development followed V-model methodology with formal requirements traceability, FMEA analysis, and comprehensive test coverage including fault injection testing. The system includes a maintenance interface for authorized personnel to perform diagnostics without affecting safety functions.