Safety Interlock
How does a safety interlock differ from a fail-safe?
An interlock prevents initiation unless conditions are met; a fail-safe ensures a safe state after failure occurs.
What standards govern industrial interlock design?
ISO 14119 (electrical/mechanical requirements) and ANSI B11.0 (North American defeat-resistant mandates).
What is the role of the logic solver in a SIF-based interlock?
It receives sensor signals, evaluates safety action necessity, and triggers outputs via SIS/ESD systems.
Why must interlocks default to blocking?
If a precondition (e.g., sensor offline) is unverifiable, the system must halt operation to prevent hazardous conditions.
A safety interlock is a mechanical or electronic device that prevents equipment operation unless predefined safe conditions are met, defaulting to a blocking state if any precondition cannot be verified. In reliability engineering, it functions as a Safety Instrumented Function (SIF) comprising sensors, a logic solver, and final control elements to enforce safe states.
On the shop floor, safety interlocks isolate power when a guard door opens, ensuring machinery stops before operator access. In CMMS/ServiceGrid, interlocks are logged as critical safety assets; maintenance tasks include verifying SIL/PLr ratings, testing switch integrity, and ensuring defeat-resistance per ISO 14119/ANSI B11.0. They are used in process flowsheets/P&IDs to visualize monitoring of pressure, temperature, or flow before permitting operation.
Defeat/Tampering: Operators bypassing interlocks with cable ties, tape, or scrap metal to avoid downtime, violating defeat-resistant standards.
Sensor Drift/Offline: Undetected sensor failure causing the system to default to blocking, halting production unnecessarily, or failing to block if logic solver misreads.
Inadequate SIL/PLr Validation: Using non-safety-rated components for interlock circuits, leading to unreliable shutdown during abnormal conditions.