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.