Machine Safety: Verification is not validation
To perform functional safety and comply with safety standards like ISO 13849-1 and 2, design engineers need to know how to perform verification and validation measures. But aren’t they the same step?
The most immediate answer is NO!
Best-in-class companies are generally adopting the “lifecycle” design model when approaching functional safety design requirements. An example is shown in this graphic of the functional safety lifecycle:
1) Perform risk assessment
2) Examine functional requirements
3) Design and verify mitigation
4) Install and validate
5) Manage, change, and improve.
ISO 13849-1 requires that during the design stage a verification step is performed (lifecycle step #3). For example, see clause 4.7 on page 26 of the standard [talking about Safety Related Parts of The Control System (SRP/CS)]:
4.7 Verification that achieved PL meets PLr
“For each individual safety function the PL of the related SRP/CS shall match the required performance level (PLr)…. The PL of the different SRP/CS which are part of a safety function shall be greater than or equal to the required performance level (PLr) of this safety function.”
Validation is a different step. While verification is often performed by someone other than the original designer during the design phase, validation is performed during the build and/or installation stage (lifecycle step #4). The goal of validation is to acknowledge that the required safety function is actually achieved for any one hazard in all modes of operation. For example, see clause 8 on page 41 of the standard:
“The design of the SRP/CS shall be validated (see Figure 3). The validation shall demonstrate that the combination of SRP/CS providing each safety function meets all relevant requirements of this part of ISO 13849. For details of validation, see ISO 13849-2.”
The above requirements are for the safety related parts of the control system (SRP/CS) for a machine’s overall control system. Additionally, ISO 13849-1 and 2 also cover the software of a control system, which also includes requirements for verification and validation of application software for all safety functions.
It is my opinion that this information could help clear up possible confusion over the terms "verification" versus "validation" for functional safety applications. If you have additional understandings or clarifications please submit your ideas, experiences, and challenges in the comments section below.
Related articles linked in the text and the bottom of this post, follow:
The Safety Standards Maze, 3 Comments: More Standards, Validation
Machine Safety: safety system validation and daisy divination
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