How to draft a Safety Requirement Specification according to EN ISO 13849

Prior to the validation of an SRP/CS, which performs one or more safety functions, the specifications of the requirements for each safety function must be drawn up and verified to ensure their consistency and completeness for the intended use.

Content of the Safety Requirement Specification of an SRP/CS

Safety requirement specifications must be drawn up and analyzed before starting design, since any other subsequent activity must be based on the requirements, starting with preparation of the SRP/CS.

The safety requirement specifications of the SRP/CS should contain the following information at a minimum:

• Expected risk scenarios and consequent risks to be reduced
• PLr and category to be attained
• Requirements arising from other applicable standards (e.g. Type C standards)
• Safety status of the machine to be reached after the intervention of each safety function
• The behavior of the machine in the event of a power failure
• Software requirements
• Demand rate of safety functions
• Response rate of safety functions
• Intended use and measures against reasonably foreseeable misuse
• The priority of functions that can be active at the same time and that can cause conflicting actions
• Environmental conditions
• Maintenance constraints

All these requirements must be specified in a formal and documented manner and should be available before the design process and therefore also before the validation process.

Some details of the requirements may be modified, adapted or extended during the design process.

This means that the applicability of the entire specification needs to be reviewed and corrective actions can be applied for a project that has already been developed.