failclosed

Fail-closed is a term used to describe a failure mode in which a system or component defaults to a closed, blocked, or locked state when normal power, signals, or functionality are lost. This state is intended to prevent unsafe or unauthorized operation. The exact meaning of "closed" depends on the context: closed to fluid flow, closed an electrical circuit, closed to physical access, or closed to traffic.

In industrial engineering and process safety, fail-closed valves and actuators are designed so that loss of power or actuator fault causes the valve to close, limiting unintended release or contamination. This contrasts with fail-open designs, where the same fault would permit flow. In safety interlocks and emergency shutdown systems, fail-closed behavior reduces risk by not allowing a process to continue in an unsafe state.

In building and information security, fail-closed devices or policies default to denying access when control hardware or software fails. A fail-closed door lock, for example, remains locked if power is lost; a network firewall may default to blocking traffic in the event of controller failure. Some implementations use redundant controls or health monitoring to detect and mitigate unintended lockouts or maintained secure states.

Designers must weigh safety, availability, and usability when selecting a fail-closed approach. Potential drawbacks include risk of unintended deadlock, nuisance outages, or hazardous closures. Standards and risk assessments often guide whether fail-closed is appropriate for a given system.