Start with the hazard and task
A requirement should identify what exposure is being controlled and during which task. “Fit an interlock” is a component request, not a complete safety requirement.
- Identify the hazardous movement or energy
- State who may be exposed and during which task
- Define the access route or initiating event
Describe the intended safe response
The response may involve stopping drives, removing torque, closing valves, preventing restart or retaining a guard locked until a safe condition exists.
- Name the final elements that create the safe state
- Define which machine zone is affected
- Include retained energy and run-down behaviour
Define reset, restart and operating modes
The requirement should explain how the safety function is acknowledged and how normal operation can resume.
- Separate reset from start where required
- Address whole-body access and unseen occupancy
- State behaviour in manual, teach and maintenance modes
Make validation possible
Write requirements in a form that can be tested in normal conditions and with relevant faults.
- Provide acceptance criteria
- Identify required measurements or timing checks
- Record expected diagnostic and fault behaviour
Apply the guidance to the specific machine.
This guide supports early decision-making but does not replace a project-specific risk assessment, safety-system design or validation. Machine condition, tasks, controls and modification scope can change the correct solution.
Questions
Direct answers.
What is the difference between a requirement and a design?+
The requirement states what the safety function must achieve. The design describes the devices, architecture and logic used to achieve it.
Do all functions need a response-time requirement?+
A response time is particularly important where protective-device position depends on total stopping performance. Other functions may be defined by state and sequence.
Who should approve the requirements?+
The responsible parties should review and agree the assumptions, machine interfaces and acceptance criteria before implementation.
