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.

12/07/2012


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 jb(at)jbtitus.com; www.jbtitus.com. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering, mhoske(at)cfemedia.com.

Go Online

www.controleng.com/blogs   

http://www.b11standards.com.

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

See the related posts linked below.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2013 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
The true cost of lubrication: Three keys to consider when evaluating oils; Plant Engineering Lubrication Guide; 11 ways to protect bearing assets; Is lubrication part of your KPIs?
Contract maintenance: 5 ways to keep things humming while keeping an eye on costs; Pneumatic systems; Energy monitoring; The sixth 'S' is safety
Transport your data: Supply chain information critical to operational excellence; High-voltage faults; Portable cooling; Safety automation isn't automatic
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Plant Engineering case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Maintaining low data center PUE; Using eco mode in UPS systems; Commissioning electrical and power systems; Exploring dc power distribution alternatives
Synchronizing industrial Ethernet networks; Selecting protocol conversion gateways; Integrating HMIs with PLCs and PACs
Why manufacturers need to see energy in a different light: Current approaches to energy management yield quick savings, but leave plant managers searching for ways of improving on those early gains.

Annual Salary Survey

Participate in the 2013 Salary Survey

In a year when manufacturing continued to lead the economic rebound, it makes sense that plant manager bonuses rebounded. Plant Engineering’s annual Salary Survey shows both wages and bonuses rose in 2012 after a retreat the year before.

Average salary across all job titles for plant floor management rose 3.5% to $95,446, and bonus compensation jumped to $15,162, a 4.2% increase from the 2010 level and double the 2011 total, which showed a sharp drop in bonus.

2012 Salary Survey Analysis

2012 Salary Survey Results

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.