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. Follow me on