Machine safety: Confusion amuck, quantitative circuit design versus qualitative risk assessment

When talking about qualitative versus quantitative as they relate to machine safety standards and compliance requirements, are we talking about the risk assessment process? Or do we consider the process to engineer and mitigate hazards related to the Safety Related Parts of Control System (SRP/CS)?


Is everyone confused about the terms qualitative versus quantitative as they relate to machine safety standards and compliance requirements? Weekly we hear the entire industry talking about the transition from qualitative to quantitative hazard analysis and mitigation. Is anyone confused as to whether we're talking about the risk assessment process or the process to engineer and mitigate hazards related to the Safety Related Parts of Control System (SRP/CS)? Wow – that’s a mouth full!

The relatively new machine safety standard that brings this question to the front of discussion is – ISO 13849-1:2006. A big hint to resolve this question is on the cover page. The title of this standard is – Safety of machinery – Safety-related parts of control systems – Part 1: General principles for design. Now, having said this, the entire international standard is focused on the design and integration effort. Paragraph 1 of clause 1 reads:

1    Scope

This part of ISO 13849 provides safety requirements and guidance on the principles for the design and integration of safety-related parts of control systems (SRP/CS), including the design of software. For these parts of SRP/CS, it specifies characteristics that include the performance level required for carrying out safety functions. It applies to SRP/CS, regardless of the type of technology and energy used (electrical, hydraulic, pneumatic, mechanical, etc.), for all kinds of machinery.

Based on the above paragraph isn’t it positively clear that this standard is directed at the design and integration of the SRP/CS including the design of software. Yes, this standard moves these efforts and compliance requirements into the realm of quantitatively designing safety-related circuits introducing such terms as Performance Level (PL) and much more. In the past, engineers and maintenance personnel used the qualitative approaches brought forward in 1996 by EN 954-1. I don’t believe that further definition is needed at this point to establish a direct correlation between quantitative and the design requirements for compliance to ISO 13849-1. Trust me, if more details are needed they abound in the four day courses being offered for designers.

On the other hand, is there any direct correlation of quantitative approaches to the risk assessment process? In my opinion, I don’t believe there is any direct correlation. To be brief, I suggest that the risk assessment process is an entirely separate process by which the intended use of the machine, the tasks and hazards, and the level of risk(s) are determined. Within the risk assessment process (for machines in design & legacy machines) it is required that identified hazards must be analyzed and reduced to an acceptable (residual) risk. There are five steps commonly recommended for this effort called the “Hierarchy of Measures” which can be used for the hazard mitigation plan. The implementation of the hazard mitigation plans is completed via design engineering. Most of the available risk assessment approaches use a three factor system for scoring the risk of a hazard - frequency, severity and likelihood. And, this is a qualitative scoring process – not quantitative!

Do you agree that engineering the solution (both new & legacy machines) under ISO 13849-1:2006 is quantitative but not the risk assessment?

Your comments or suggestion are always welcome so please let us know your thoughts. Submit your ideas, experiences, and challenges on this subject in the comments section below. Click on the following text if you don't see a comments box, then scroll down: Machine safety: Confusion amuck, quantitative circuit design versus qualitative risk assessment.

J.B. Titus, CFSERelated articles:

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