Machine safety risk level assessment priority: Possibility, severity, or frequency?

Which factor has the highest priority for assessing hazard risk levels: possibility, severity, or frequency? The ANSI B11.0 – 2010 standard may help.


Are the three factors of possibility, severity, and frequency of equal importance in determining the risk levels for machine safety hazards? Industry safety standards seem to treat them as equal because they don’t address any relative importance. However, what’s your experience?

Doesn’t it seem that both international and domestic standards present these three required factors for assessing risk as independent variables? Although they’re independent, they are also related because when they’re combined they help to determine risk levels of hazards and their related remediation performance requirements. This is shown in the following graph from ANSI B11.0 – 2010. In this case the qualitative process is determining the Performance Level (PL) for the given hazard. 

Using this assessment approach, if you decided that severity (of injury) required a higher priority, would the derived outcome in the risk level be any different? Similarly, would giving the possibility or frequency factors greater priority or lesser priority change the answer? In my opinion, I don’t see it!

Figure D-2: Performance Levels from ISO 13849-1:2006. Reprinted with Permission: ANSI B11.0 – 2010, B11 Standards Inc.

Yet when I talk with users about this issue, they frequently present this example. If severity of harm for a given hazard is “death,” they always give that factor a higher priority (S2) and a higher risk level, which drives the highest circuit performance for machine guarding. The highest circuit performance is PLe, which requires the average probability of dangerous failures per hour of 10-8 to 10-7. PLe means control reliable circuits with redundant components and 24/7 monitoring.

So, here’s the dilemma as I see it: If severity is S2 and frequency and possibility are F1 and P1, respectively, your derived risk level is PLc by ISO 13849-1: 2006 standard requirements. After deciding on S2 and depending on your answers for F1 or F2 and P1 or P2, you could arrive at either PLc, Pld, or PLe, per the graph above. Specifically arriving at PLe by prioritizing severity (S2) is not straightforward. If using the category system, you could likewise arrive at either Cat 1, 2, 3, or 4 by deciding on S2. Perhaps you can prioritize severity by eliminating frequency and possibility and simply defaulting to PLe or Cat 4. But, by eliminating frequency and possibility in your risk analysis, are you in compliance with the standards? Therefore, aren’t all three factors equal in priority?

Does anyone have an answer for this dilemma? Your comments or suggestions are always welcome, so please let us know your thoughts. Submit your ideas, experiences, and challenges on this subject in the comments section below (online).

- J.B. Titus, Certified Functional Safety Expert (CFSE), writes the Control Engineering Machine Safety Blog. Reach him at; Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering,

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