Interlocks

An interlock is a device, system, or arrangement that prevents one operation from taking place or ensures a process cannot proceed unless a prerequisite condition is satisfied. Interlocks are used to enforce correct sequencing, protect personnel, and prevent equipment damage by making unsafe states impossible or unlikely.

Common forms include mechanical interlocks, electrical interlocks, pneumatic or hydraulic interlocks, and software or logic interlocks. Mechanical interlocks rely on physical barriers, latches, or keyed handles to block access or motion until conditions are met. Electrical interlocks use circuits, relays, sensors, and safety switches wired to disable power or motion if a guard is opened or a fault is detected. Pneumatic and hydraulic interlocks use pressure-based actuators to lock components in place or release them only after a sequence. Software or logic interlocks enforce sequencing and mutual exclusion within automated control systems, preventing incompatible actions from occurring concurrently.

Interlocks are used across industries: guarding on machinery and doors to prevent access during operation; elevator and escalator control to prevent movement with doors open; railway signaling and interlocking to prevent conflicting routes; and process industries, such as chemical, oil, and nuclear facilities, where shutdown or containment interlocks help maintain safety.

Key design considerations include fail-safe behavior, diagnostic monitoring, redundancy, and ease of testing. Interlocks are part of broader safety systems and are often defined by standards such as ISO 13849-1 and IEC 62061, which guide risk assessment and the assignment of safety integrity levels. Despite their value, interlocks do not replace good engineering practice; they can fail, be bypassed, or be defeated by improper maintenance. They should be complemented by other controls and regular verification.