ISO 13849 – 1 Analysis — Part 1: Start with Risk Assessment

This entry is part 1 of 9 in the series How to do a 13849 – 1 ana­lys­is

I often get ques­tions from cli­ents about how to get star­ted on Functional Safety using ISO 13849. This art­icle is the first in a series that will walk you through the basics of using ISO 13849. Keep in mind that you will need to hold a copy of the 3rd edi­tion of ISO 13849 – 1 [1] and the 2nd edi­tion of ISO 13849 – 2 [2] to use as you go along. There are oth­er stand­ards which you may also find use­ful, and I have included them in the Reference sec­tion at the end of the art­icle. Each post has a Reference List. I will pub­lish a com­plete ref­er­ence list for the series with the last post.

Where to start?

So you have just learned that you need to do an ISO 13849 func­tion­al safety ana­lys­is. You have the two parts of the stand­ard, and you have skimmed them, but you are feel­ing a bit over­whelmed and unsure of where to start. By the end of this art­icle, you should be feel­ing more con­fid­ent about how to get this job done.

Step 1 – Risk Assessment

For the pur­pose of this art­icle, I am going to assume that you have a risk assess­ment for the machinery, and you have a copy for ref­er­ence. If you do not have a risk assess­ment, stop here and get that done. There are sev­er­al good ref­er­ences for that, includ­ing ISO 12100 [3], CSA Z432 [4], and ANSI B11.TR3 [5]. You can also have a look at my series on Risk Assessment.

The risk assess­ment should identi­fy which risks require mit­ig­a­tion using the con­trol sys­tem, e.g., use of an inter­locked gate, a light cur­tain, a two-​hand con­trol, an enabling device, etc.See the MS101 gloss­ary for detailed defin­i­tions. Each of these becomes a safety func­tion. Each safety func­tion requires a safety require­ments spe­cific­a­tion (SRS), which I will describe in more detail a bit later.

Safety Functions

The 3rd edi­tion of ISO 13849 [1] provides two tables that give some examples of safety func­tion char­ac­ter­ist­ics [1, Table 8] and para­met­ers [1, Table 9] and also provides ref­er­ences to cor­res­pond­ing stand­ards that will help you to define the neces­sary para­met­ers. These tables should not be con­sidered to be exhaust­ive – there is no way to list every pos­sible safety func­tion in a table like this. The tables will give you some good ideas about what you are look­ing for in machine con­trol func­tions that will make them safety func­tions.

While you are identi­fy­ing risk reduc­tion meas­ures that will use the con­trol sys­tem for mit­ig­a­tion, don’t for­get that com­ple­ment­ary pro­tect­ive meas­ures like emer­gency stop, enabling devices, etc. all need to be included. Some of these func­tions may have min­im­um require­ments set by Type B2 stand­ards, like ISO 13850 [6] for emer­gency stop which sets the min­im­um per­form­ance level for this func­tion at PLc.

Selecting the Required Performance Level

ISO 13849 – 1:2015 provides a graph­ic­al means for select­ing the min­im­um Performance Level (PL) required for the safety func­tion based on the risk assess­ment. A word of cau­tion here: you may feel like you are re-​assessing the risk using this tool because it does use risk para­met­ers (sever­ity, frequency/​duration of expos­ure and pos­sib­il­ity to avoid/​limit harm) to determ­ine the PL. Risk assess­ment This tool is not a risk assess­ment tool, and using it that way is a fun­da­ment­al mis­take. Its out­put is in terms of per­form­ance level, which is fail­ure rate per hour of oper­a­tion. For example, it is entirely incor­rect to say, “This machine has a risk level of PLc” since we define PLs in terms of prob­able fail­ure rate per hour.

ISO 13849-1 graphical selection tool for determining PLr requirement for a safety function
Graphical Performance Level Selection Tool [1]
Once you have assigned a required Performance Level (PLr) to each safety func­tion, you can move on to the next step: Developing the Safety Requirements Specification.

Book List

Here are some books that I think you may find help­ful on this jour­ney:

[0]     B. Main, Risk Assessment: Basics and Benchmarks, 1st ed. Ann Arbor, MI USA: DSE, 2004.

[0.1]  D. Smith and K. Simpson, Safety crit­ic­al sys­tems hand­book. Amsterdam: Elsevier/​Butterworth-​Heinemann, 2011.

[0.2]  Electromagnetic Compatibility for Functional Safety, 1st ed. Stevenage, UK: The Institution of Engineering and Technology, 2008.

[0.3]  Overview of tech­niques and meas­ures related to EMC for Functional Safety, 1st ed. Stevenage, UK: Overview of tech­niques and meas­ures related to EMC for Functional Safety, 2013.

References


[1]     Safety of machinery — Safety-​related parts of con­trol sys­tems — Part 1: General prin­ciples for design. 3rd Edition. ISO Standard 13849 – 1. 2015.

[2]     Safety of machinery – Safety-​related parts of con­trol sys­tems – Part 2: Validation. 2nd Edition. ISO Standard 13849 – 2. 2012.

[3]      Safety of machinery – General prin­ciples for design – Risk assess­ment and risk reduc­tion. ISO Standard 12100. 2010.

[4]     Safeguarding of Machinery. CSA Standard Z432. 2004.

[5]     Risk Assessment and Risk Reduction- A Guideline to Estimate, Evaluate and Reduce Risks Associated with Machine Tools. ANSI Technical Report B11.TR3. 2000.

[6]    Safety of machinery – Emergency stop func­tion – Principles for design. ISO Standard 13850. 2015.

Series NavigationISO 13849 – 1 Analysis — Part 8: Fault Exclusion”>ISO 13849 – 1 Analysis — Part 8: Fault ExclusionISO 13849 – 1 Analysis — Part 2: Safety Requirement Specification”>ISO 13849 – 1 Analysis — Part 2: Safety Requirement Specification

Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. (http://www.complianceinsight.ca) in Kitchener, Ontario, and is Lead Author and Managing Editor of the Machinery Safety 101 blog.

Doug's work includes teaching machinery risk assessment techniques privately and through Conestoga College Institute of Technology and Advanced Learning in Kitchener, Ontario, as well as providing technical services and training programs to clients related to risk assessment, industrial machinery safety, safety-related control system integration and reliability, laser safety and regulatory conformity.

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