ISO 13849 – 1 Analysis — Part 2: Safety Requirement Specification

This entry is part 2 of 9 in the series How to do a 13849 – 1 ana­lys­is

Developing the Safety Requirement Specification

The Safety Requirement Specification sounds pretty heavy, but actu­ally, it is just a big name for a way to organ­ise the inform­a­tion you need to have to ana­lyse and design the safety sys­tems for your machinery. Note that I am assum­ing that you are doing this in the “right” order, mean­ing that you are plan­ning the design before­hand, rather than try­ing to back-​fill the doc­u­ment­a­tion after com­plet­ing the design. In either case, the pro­cess is the same, but get­ting the inform­a­tion you need can be much harder after the fact, than before the doing the design work. Doing some aspects in a review mode is impossible, espe­cially if a third party to whom you have no access did the design work [8].

If you missed the first instal­ment in this series, you can read it here.

What goes into a Safety Requirements Specification?

For ref­er­ence, chapter 5 of ISO 13849 – 1 [1] cov­ers safety require­ment spe­cific­a­tions to some degree, but it needs some cla­ri­fic­a­tion I think. First of all, what is a safety func­tion?

Safety func­tions include any func­tion of the machine that has a dir­ect pro­tect­ive effect for the work­er using the machinery. However, using this defin­i­tion, it is pos­sible to ignore some import­ant func­tions. Complementary pro­tect­ive meas­ures, like emer­gency stop, can be missed because they are usu­ally “after the fact”, i.e., the injury occurs, and then the E-​stop is pressed, so you can­not say that it has a “dir­ect pro­tect­ive effect”. If we look at the defin­i­tions in [1], we find:


safety func­tion

func­tion of the machine whose fail­ure can res­ult in an imme­di­ate increase of the risk(s)
[SOURCE: ISO 12100:2010, 3.30.]

Linking Risk to Functional Safety

Referring to the risk assess­ment, any risk con­trol that pro­tects work­ers from some aspect of the machine oper­a­tion using a con­trol func­tion like an inter­locked gate, or by main­tain­ing a tem­per­at­ure below a crit­ic­al level or speed at a safe level, is a safety func­tion. For example: if the tem­per­at­ure in a pro­cess rises too high, the pro­cess will explode; or if a shaft speed is too high (or too low) the tool may shat­ter and eject broken pieces at high speed. Therefore, the tem­per­at­ure con­trol func­tion and the speed con­trol func­tion are safety func­tions. These func­tions may also be pro­cess con­trol func­tions, but the poten­tial for an imme­di­ate increase in risk due to a fail­ure is what makes these func­tions safety func­tions no mat­ter what else they may do.

[1, Table 8] gives you some examples of vari­ous kinds of safety func­tions found on machines. The table is not inclus­ive – mean­ing there are many more safety func­tions out there than are lis­ted in the table. Your job is to fig­ure out which ones live in your machine. It is a bit like Pokemon – ya gotta catch ‘em all!

Basic Safety Requirement Specification

Each safety func­tion must have a Performance Level or a Safety Integrity Level assigned as part of the risk assess­ment. For each safety func­tion, you need to devel­op the fol­low­ing inform­a­tion:

Basic Safety Requirement Specification
Item Description
Safety Function Identification Name or oth­er ref­er­ences, e.g. “Access Gate Interlock” or “Hazard Zone 2.”
Functional Characteristics
  • Intended use or fore­see­able mis­use of the machine rel­ev­ant to the safety func­tion
  • Operating modes rel­ev­ant to the safety func­tion
  • Cycle time of the machine
  • Response time of the safety func­tion
Emergency Operation Is this an emer­gency oper­a­tion func­tion? If yes, what types of emer­gen­cies might be mit­ig­ated by this func­tion?
Interactions What oper­at­ing modes require this func­tion to be oper­a­tion­al? Are there modes where this func­tion requires delib­er­ate bypass? These could include nor­mal work­ing modes (auto­mat­ic, manu­al, set-​up, changeover), and fault-​finding or main­ten­ance modes.
Behaviour How you want the sys­tem to behave when the safety func­tion is triggered, i.e., Power is imme­di­ately removed from the MIG weld­er using an IEC 60204 – 1 Category 0 stop func­tion, and robot motions are stopped using IEC 60204 – 1 Category 1 stop func­tion through the robot safety stop input.


All hori­zont­al pneu­mat­ic motions stop in their cur­rent pos­i­tions. Vertical motions return to the raised or retrac­ted pos­i­tions.

Also to be con­sidered is a power loss con­di­tion. Should the sys­tem behave in the same way as if the safety func­tion was triggered, not react at all, or do some­thing else? Consider ver­tic­al axes that might require hold­ing brakes or oth­er mech­an­isms to pre­vent power loss caus­ing unex­pec­ted motion.

Machine State after trig­ger­ing What is the expec­ted state of the machine after trig­ger­ing the safety func­tion? What is the recov­ery pro­cess?
Frequency of Operation How often do you expect this safety func­tion to be used? A reas­on­able estim­ate is needed. More on this below.
Priority of Operation If sim­ul­tan­eous trig­ger­ing of mul­tiple safety func­tions is pos­sible, which function(s) takes pre­ced­ence? E.g., Emergency Stop always takes pre­ced­ence over everything else. What hap­pens if you have a safe speed func­tion and a guard inter­lock that are asso­ci­ated because the inter­lock is part of a guard­ing func­tion cov­er­ing a shaft, and you need to troubleshoot the safe speed func­tion, so you need access to the shaft where the encoders are moun­ted?
Required Performance Level I sug­gest record­ing the S, F, and P val­ues selec­ted as well as the PLr value selec­ted for later ref­er­ence.

Here’s an example table in MS Word format that you can use as a start­ing point for your SRS doc­u­ments. Note that SRS can be much more detailed than this. If you want more inform­a­tion on this, read IEC 61508 – 1, 7.10.2.

So, that is the min­im­um. You can add lots more inform­a­tion to the min­im­um require­ments, but this will get you star­ted. If you want more inform­a­tion on devel­op­ing the SRS, you will need to get a copy of IEC 61508 [7].

What’s Next?

Next, you need to be able to make some design decisions about sys­tem archi­tec­ture and com­pon­ents. Circuit archi­tec­tures have been dis­cussed at some length on the MS101 blog in the past, so I am not going to go through them again in this series. Instead, I will show you how to choose an archi­tec­ture based on your design goals in the next instal­ment. In case you missed the first part of the series, you can read it here.

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.


Note: This ref­er­ence list starts in Part 1 of the series, so “miss­ing” ref­er­ences may show in oth­er parts of the series. Included in the last post of the series is the com­plete ref­er­ence list.

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

[7]     Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems. Seven parts. IEC Standard 61508. Edition 2. 2010.

[8]     S. Jocelyn, J. Baudoin, Y. Chinniah, and P. Charpentier, “Feasibility study and uncer­tain­ties in the val­id­a­tion of an exist­ing safety-​related con­trol cir­cuit with the ISO 13849 – 1:2006 design stand­ard,” Reliab. Eng. Syst. Saf., vol. 121, pp. 104 – 112, Jan. 2014.

Series NavigationISO 13849 – 1 Analysis — Part 1: Start with Risk Assessment”>ISO 13849 – 1 Analysis — Part 1: Start with Risk AssessmentISO 13849 – 1 Analysis — Part 3: Architectural Category Selection”>ISO 13849 – 1 Analysis — Part 3: Architectural Category Selection

Author: Doug Nix

+DougNix is Managing Director and Principal Consultant at Compliance InSight Consulting, Inc. ( 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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