faultsafe

Faultsafe is a term used in safety engineering to describe systems and software designed to ensure safe behavior in the presence of faults. It combines aspects of fail-safety, which places the system into a safe state after a fault, with fault tolerance, which allows continued operation despite faults. In practice, faultsafe design aims to prevent unsafe states, detect faults promptly, isolate them, and transition systems to deterministic, safe modes.

Design principles include defining safe states, redundancy, and diversity to avoid common-mode failures, continuous health monitoring, failover mechanisms, watchdog timers, and formal verification. Systems are often designed to degrade gracefully rather than fail catastrophically, performing essential functions while preserving safety.

Common implementation patterns include redundant hardware with voting logic (such as triple modular redundancy), software health checks, exception handling with safe recovery paths, safe-state controllers, and certified safety cases. In embedded and cyber-physical systems, faultsafe approaches align with functional safety standards and risk assessment processes.

Applications span aviation, automotive and railway systems, medical devices, industrial control, and critical software services. Certification bodies evaluate faultsafe designs through hazard analysis, safety requirements, testing, and validation to demonstrate adherence to relevant standards such as IEC 61508 and ISO 26262.

Critics note that faultsafe increases complexity and cost, and that safety gains depend on rigorous implementation, comprehensive testing, and avoidance of common-mode failures. Ongoing research seeks improved modeling of fault behavior and automated verification techniques.