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# Machine safety: Does ISO 13849-1: 2006 weight severity, frequency, and probability equally?

## New quantitative requirements for designing safety-related parts of the control system (SRP/CS) have created many discussions. Even with new requirements from ISO 13849-1, this updated standard begins with the same old qualitative approach to determine the “goal” (Performance Level required- PLr) for any safety function, asking about severity, frequency, and probability.

May 20, 2013

New quantitative requirements for designing safety-related parts of the control system (SRP/CS) have created many related discussions about machine safety.  Yet, even with these new requirements from ISO 13849-1, this updated standard begins with the same old qualitative approach to determine the “goal” (Performance Level required- PLr) for any safety function. The same three questions are still asked; Severity, Frequency and Probability.

EN954-1 came out in 1996 with an amazing way to put more teeth into determining a hazard level and related mitigation solution for any recognized hazard. In so doing we had to analyze each hazard by evaluating the related potential injury by severity, frequency, and probability according to the graph below.

Ten years later in 2006 ISO 13849-1 was updated and released introducing Performance Levels and the requirement to develop the PLr, which I call the goal. To develop the PLr, we again use the qualitative approach by evaluating the related potential injury by severity, frequency, and probability also shown in the graph. There’s a whole lot more we could get into here but let’s keep it focused at the three questions.

What was the criteria for approaching these three questions in their order of severity, frequency and probability? Is severity weighted the most because it’s the first question? Such as; S = 50%, F = 30% and P = 20%? Or is probability asked last because of its greater impact? Such as; S = 25%, F = 35% and P = 40%? Or, does it matter at all? Can all three questions be equally interchanged?

Can anybody provide some insight and background? Has this presented you with any new perspectives? Add your comments or thoughts to the discussion by submitting your ideas, experiences, and challenges in the comments section below.

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Contact: http://www.jbtitus.com for “Solutions for Machine Safety”.

For more than 30 years, J.B. Titus has advised a wide range of clients on machine functional safety solutions, including Johnson & Johnson, Siemens, General Motors, Disney, Rockwell Automation, Bridgestone Firestone, and Samsung Heavy Industries. He holds a Bachelor of Business Administration degree from Oklahoma University in industrial management and an MBA from Case Western Reserve University in marketing and finance. He is a professional member of the American Society of Safety Engineers and is OSHA-certified in machine guarding. Titus is also TUV-certified as a Functional Safety Expert and serves on several American National Standards Institute, National Fire Protection Association, and National Electrical Manufacturers Association national safety and health standards committees. Reach him at jb(at)jbtitus.com and via www.jbtitus.com.