Machinery Safety 101

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

This art­icle was updated 2019-07-24. Ed.

Developing the Safety Requirement Specification

The Safety Require­ment Spe­cific­a­tion sounds pretty heavy, but actu­ally, it is just a big name for a way to organ­ize the inform­a­tion you need to have to ana­lyze 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 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.

[0.4] “Code of prac­tice for elec­tro­mag­net­ic resi­li­ence, 1st ed. Steven­age, UK: IET Stand­ards TC4.3 EMC, 2017.

[0.5] “Code of Prac­tice: Com­pet­ence for Safety Related Sys­tems Prac­ti­tion­ers, 1st ed. Steven­age, UK: The Insti­tu­tion of Engin­eer­ing and Tech­no­logy, 2016.


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