Tag Archives: ISO 13849-1 - Page 2

Understanding Risk Assessment

Posted by on January 31, 2011 10 comments

When peo­ple dis­cuss ‘Risk’ there are a lot of dif­fer­ent assump­tions made about what that means. For me, the study of risk and risk assess­ment tech­niques started in 1995. As a tech­nol­o­gist and con­trols designer, I had to some­how wrap my head around the whole con­cept in ways I’d never con­sid­ered. If you’re try­ing to fig­ure out risk and risk assess­ment this is a good place to get started!

What is risk?

From a machin­ery per­spec­tive, ISO 12100:2010 defines risk as:

“com­bi­na­tion of the prob­a­bil­ity of occur­rence of harm and the sever­ity of that harm”

Risk can have pos­i­tive or neg­a­tive out­comes, but when con­sid­er­ing safety, we only con­sider neg­a­tive risk, or events that result in neg­a­tive health effects for the peo­ple exposed.

The risk rela­tion­ship is illus­trated in ISO 12100:2010 Figure 3:


ISO 12100-2010 Figure 3

ISO 12100–2010 Figure 3


Where

R = Risk

S = Severity of Harm

P = Probability of Occurrence of Harm

The Probability of Occurrence of Harm fac­tor is often fur­ther bro­ken down into three sub-​​factors:

  • Probability of Exposure to the haz­ard
  • Probability of Occurrence of the Hazardous Event
  • Probability of Limiting or Avoiding the Harm

How is risk measured?

In order to esti­mate risk a scor­ing tool is needed. There is no one ‘cor­rect’ scor­ing tool, and there are flaws in most scales that can result in blind-​​spots where risks may be over or under-​​estimated.

At the sim­plest level are ‘screen­ing’ tools. These tools use very sim­ple scales like ‘High, Medium, Low’, or ‘A, B, C’. These tools are often used when doing a shop-​​floor inspec­tion and are intended to pro­vide a quick method of cap­tur­ing obser­va­tions and giv­ing a gut-​​feel assess­ment of the risk involved. These tools should be used as a way to iden­tify risks that need addi­tional, detailed assess­ment. To get an idea of what a good screen­ing tool can look like, have a look at the SOBANE Déparis sys­tem.

Every scor­ing tool requires a scale for each risk para­me­ter included in the tool. For instance, con­sider the CSA tool described in CSA Z434:

CSA Z434-03 Table 1As you can see, each para­me­ter (Severity, Exposure and Avoidance) has a scale, with two pos­si­ble selec­tions for each parameter.

When con­sid­er­ing selec­tion of a scor­ing tool, it’s impor­tant to take some time to really exam­ine the scales for each fac­tor. The scale shown above has a glar­ing hole in one scale. See if you can spot it and I’ll tell you what I think a bit later in this post.

There are more than 350 dif­fer­ent scales and method­olo­gies avail­able for assess­ing risk. You can find a good review of some of them in Bruce Main’s text­book “Risk Assessment: Basics and Benchmarks” avail­able from DSE online.

A sim­i­lar, although dif­fer­ent, tool is found in Annex 1 of ISO 13849–1. Note that this tool is pro­vided in an Informative Annex. This means that it is not part of the body of the stan­dard and is NOT manda­tory. In fact, this tool was pro­vided as an exam­ple of how a user could link the out­put of a risk assess­ment tool to the Performance Levels described in the nor­ma­tive text (the manda­tory part) of the standard.

Consider cre­at­ing your own scales. There is noth­ing wrong with deter­min­ing what char­ac­ter­is­tics (para­me­ters) you want to include in your risk assess­ment, and then assign­ing each para­me­ter a numeric scale that you think is suit­able; 1–10, 0–5, etc. Some scales may be inverted to oth­ers, for exam­ple: If the Severity scale runs from 0–10, the Avoidability scale might run from 10–0 (Unavoidable to Entirely Avoidable).

Once the scales in your tool have been defined, doc­u­ment the def­i­n­i­tions as part of your assessment.

Who should con­duct risk assessments?

Lake YogaIn many orga­ni­za­tions, I find that risk assess­ment has been del­e­gated to one per­son. This is a major mis­take for a num­ber of rea­sons. Risk assess­ment is not a solo activ­ity for a ‘guru’ in a lonely office somewhere!

Risk assess­ment is not a lot of fun to do, and since risk assess­ments can get to be quite involved, it rep­re­sents a sig­nif­i­cant amount of work to put on one per­son. Also, leav­ing it to one per­son means that the assess­ment will nec­es­sar­ily be biased to what that per­son knows, and may miss sig­nif­i­cant haz­ards because the asses­sor doesn’t know enough about that haz­ard to spot it and assess it properly.

Risk assess­ment requires mul­ti­ple view­points from par­tic­i­pants with var­ied exper­tise. This includes users, design­ers, engi­neers, lawyers and those who may have spe­cial­ized knowl­edge of a par­tic­u­lar haz­ard, like a Laser Safety Officer or a Radiation Safety Officer. The var­ied exper­tise of the peo­ple involved will allow the com­mit­tee to bal­ance the opin­ion of each haz­ard, and develop a more rea­soned assess­ment of the risk.

I rec­om­mend that risk assess­ment com­mit­tees never be less than three mem­bers. Five is fre­quently a good num­ber. Once you get beyond five, it becomes increas­ingly dif­fi­cult to obtain con­sen­sus on each haz­ard. Also, con­sider the cost. As each com­mit­tee mem­ber is added to the team, the cost of the assess­ment can esca­late exponentially.

Training in risk assess­ment is cru­cial to suc­cess. Ensure that the indi­vid­u­als involved are trained, and that at least one has some pre­vi­ous expe­ri­ence in the prac­tice so that they may guide the com­mit­tee as needed.

When should a risk assess­ment be conducted?


Risk Assessment Lifetime Flow Chart

Risk Assessment in the Lifetime of a Product


Risk assess­ment should begin at the begin­ning of a project, whether it’s the design of a prod­uct, the devel­op­ment of a process or ser­vice, or the design of a new build­ing. Understanding risk is crit­i­cal to the design process. Cost for changes made at the begin­ning of a project are min­i­mal com­pared to those that will be incurred to cor­rect prob­lems that might have been fore­seen at the start. Risk assess­ment should start at the con­cept stage and be included at each sub­se­quent stage in the devel­op­ment process. The accom­pa­ny­ing graphic illus­trates this idea.

Essentially, risk assess­ment is never fin­ished until the prod­uct, process or ser­vice ceases to exist.

What tools are available?

As men­tioned ear­lier in this post, the book ‘Risk Assessment: Basics and Benchmarks” pro­vides an overview of roughly 350 dif­fer­ent scor­ing tools. You can search the Internet and turn up quite a few as well. The key thing with all of these sys­tems is that you will need to develop any soft­ware based tools your­self. Depending on your com­fort with soft­ware, this might be a spread­sheet for­mat, a word pro­cess­ing doc­u­ment a data­base, or some other for­mat that works for your application.

There are a num­ber of risk assess­ment soft­ware tools avail­able as well, includ­ing ISI’s CIRSMA™ and DSE’s DesignSafe. As with the scor­ing tools, you need to be care­ful when eval­u­at­ing tools. Some have sig­nif­i­cant blind spots that may trip you up if you are not aware of their limitations.

Remember too that the out­put from the soft­ware can only be as good as the input data. The old saw “Garbage In, Garbage Out” holds true with risk assessment.

Where can you get training?

There are a few places to get train­ing. Compliance InSight Consulting pro­vides train­ing to cor­po­rate clients and will be launch­ing a series of web-​​based train­ing ser­vices in 2011 that will allow indi­vid­ual learn­ers to get train­ing too.

The IEEE PSES oper­ates a Risk Assessment Technical Committee that is open to the pub­lic as well. See the RATC web site.

The Answer to the Scale Question

The Exposure Scale in the CSA tool has a gap between E1 and E2. Looking at the def­i­n­i­tions for each choice, notice that E1 is less than once per day or shift, while E2 is more than once per hour. Exposures that occur once per hour or less, but more than once per day can­not be scored effec­tively using this scale.

Also, notice the Severity scale: S1 encom­passes injuries requir­ing not more than basic first aid. One com­mon ques­tion I get is “Does that include CPR*?”. This ques­tion comes up because most basic first aid courses taught in Canada include CPR as part of the course. There is no clear answer for this in the stan­dard. The S2 fac­tor extends from injuries requir­ing more than basic first aid, like a bro­ken fin­ger for instance, all the way to a fatal­ity. Does it make sense to group this broad range of injuries together? This def­i­n­i­tion doesn’t quite match with the Province of Ontario’s def­i­n­i­tion of a Critical Injury found in Regulation 834 either.

All of this points to the need to care­fully assess the scales that you choose before you start the process. Choosing the wrong tool can skew your results in ways that you may not be very happy about.

*Cardio-​​Pulmonary Resuscitation

Missing MTTFd data

Posted by on September 13, 2010 Comments Off

What the heck is MTTFd???

When you first start to work through ISO 13849–1, the first thing that will smack you in the head is the plethora of new acronyms. The first one you’ll run into is ‘PL’, of course, since the entire pur­pose of the stan­dard is to aid the designer in deter­min­ing the reli­a­bil­ity Performance Level of the con­trol sys­tem. Shortly after that you’ll find your­self face to face with MTTFd.

MTTFd, or the Mean Time To Failure (dan­ger­ous), is the name given to the expected fail­ure rate per year for a com­po­nent used in a sys­tem that is being ana­lyzed. This rate dif­fers from the straight fail­ure rate for the com­po­nent because it’s lim­ited to the fail­ures that result in a dan­ger­ous fail­ure mode, or that may lead to a haz­ard.

So how do you get this data?

Obtaining MTTFd data for a com­po­nent should be easy for a designer. Component man­u­fac­tur­ers who mar­ket com­po­nents intended for safety appli­ca­tions should pro­vide this data in the com­po­nent spec­i­fi­ca­tions, but there are thou­sands, per­haps mil­lions, of dif­fer­ent com­po­nents being mar­keted today for use in safety sys­tems. Most of the major man­u­fac­tur­ers are already pro­vid­ing this fig­ure, or a fig­ure that can be used to derive MTTFd, B10d, but for many com­po­nents, this data is sim­ply not available.

Here are some ran­domly cho­sen exam­ples of manufacturer’s spec­i­fi­ca­tion sheets that give this data:

Allen-​​Bradley Trojan™ T15 Interlock Switch

Pilz PNOZ X2 (pdf data sheet)

Telemecanique XPS MP Safety Controller (pdf data sheet)

B10d is the num­ber of cycles until 10% of the com­po­nents being tested fail in a dan­ger­ous way. Using fail­ure rate data from the component’s data sheet, it is pos­si­ble to esti­mate B10d from either B10 or T (the appli­ca­tion depen­dent life­time of the com­po­nent). Check out Annex C of the stan­dard if you want to see how this can be done.

But what do you do if the man­u­fac­turer of your favourite con­tac­tor doesn’t pro­vide ANY fail­ure data? Some major man­u­fac­tur­ers still don’t pro­vide any fail­ure rate data at all, some pro­vide expected life­times under spe­cific oper­a­tion con­di­tions. Some pro­vide only EN 954–1:95 data. In the last case, I think this is one of the rea­sons for the EC Machinery Working Group’s deci­sion late last year to extend the tran­si­tion period to ISO 13849–1:07. Need to know more about that decision?

Now what?

Unless you work for a large orga­ni­za­tion, insti­tut­ing a life test­ing pro­gram is not likely to be an option, since you either need a pro­tracted period of time with a few com­po­nents in test, or thou­sands of sam­ples for a short time.

The stan­dard pro­vides the option to use 10 years as a default where no other data is avail­able. 10 years sounds like a long time at first blush, par­tic­u­larly if the planned life­time of the sys­tem involved is 20 years. Typical MTTFd val­ues for high-​​reliability com­po­nents are in the hun­dreds of years, so by com­par­i­son, 10 years is almost noth­ing. Tables are also pro­vided for some kinds of com­po­nents, but the tables are nec­es­sar­ily lim­ited in size, so not every com­po­nent will be listed.

Your only option is to use the data in the stan­dard, or pick up some of the other pub­li­ca­tions that include com­po­nent fail­ure data, like MIL-​​HDBK-​​217F, IEC/​TR 62380 (based on UTE 80810 & RDF 2000), NPRD 95 or IEC 61709 (based on Siemens SN 29500 doc­u­ments). Some of these doc­u­ments may be dif­fi­cult or impos­si­ble to obtain.

The result of this lack of objec­tive data from the com­po­nent man­u­fac­tur­ers is:

  • Conservative results based on the min­i­mum default MTTFd;
  • Potential over-​​design of safety related controls;
  • Increased man­u­fac­tur­ing costs for machine builders;

The rea­sons for this sit­u­a­tion vary by man­u­fac­turer, but ulti­mately it comes down to the cost of life test­ing com­po­nents mul­ti­plied by num­ber of com­po­nents built by each man­u­fac­turer. Typical life tests require load sim­u­la­tors and switch­ing for thou­sands of com­po­nents, as well as data log­ging to trap fail­ures and record rel­e­vant data. In the case of fluid power com­po­nents (pneu­mat­ics and hydraulics), this becomes increas­ingly com­plex. For many com­po­nent man­u­fac­tur­ers, the cost of the life test­ing is pro­hib­i­tive, even though this data is badly needed by their users.

Will we see an improve­ment in the future? The largest con­trols com­po­nent man­u­fac­tur­ers are very likely to pro­vide this data as they have it avail­able, mean­ing as they com­plete test­ing. New designs are much more likely to come with this data ini­tially, while it may be a long time before some of the old stan­dard com­po­nents get time in the life test cell. Until then, lots of com­po­nents will be assigned ’10 years’.

A big thank you to Wouter Leusden for the idea for this post!

Have a thought to share on this topic? Correct an error in the arti­cle? Sound off? Leave a comment!

IEC/​TR 62061–1 Reviewed

Posted by on September 8, 2010 7 comments
This entry is part 2 of 2 in the series IEC/​TR 62061–1

Why You Need to Spend More Cash on Yet Another Document

Standards orga­ni­za­tions pub­lish doc­u­ments in a fairly con­tin­u­ous stream, so for those of us tasked with stay­ing cur­rent with a large num­ber of stan­dards (say, more than 10), the pub­li­ca­tion of another new stan­dard or Technical Report isn’t news — it’s busi­ness as usual. The ques­tion is always: Do we really need to add this to the library?

For those who are new to this busi­ness, hav­ing to pay for crit­i­cal design infor­ma­tion is a new expe­ri­ence. Finding out that it can cost hun­dreds, if not thou­sands, to build the library you need can be overwhelming.

This review aims to help you decide if you need IEC/​TR 62061–1 in your library.

The Problem

As a machine builder or a man­u­fac­turer build­ing a prod­uct designed to be inte­grated into machin­ery, how do you choose between ISO 13849–1 and IEC 62061?

IEC 62061–1 attempts to pro­vide guid­ance on how to make this choice.

History

When CENELEC pub­lished EN 954–1 in 1995, machine builders were intro­duced to a whole new world of con­trol reli­a­bil­ity require­ments. Prior to its pub­li­ca­tion, most machines were built with very sim­ple inter­locks, and no spe­cific stan­dards for inter­lock­ing devices existed. In the years since then, the EN 954–1 Categories have become well known and are applied inside and out­side the EU.

In the inter­ven­ing years, IEC pub­lished IEC 61508. This seven-​​part stan­dard intro­duced the idea of ‘Safety Integrity  Levels’ or SILs. This stan­dard is aimed at process con­trol sys­tems and could be used for com­plex machin­ery as well.

Why the Confusion?

In 2006, IEC pub­lished a machin­ery sec­tor spe­cific stan­dard based on IEC 61508, called IEC 62061. This stan­dard offered a sim­pli­fied appli­ca­tion of the IEC 61508 method­ol­ogy intended for machine builders. The key prob­lem with this stan­dard is that it did not pro­vide a means to deal with pneu­matic or hydraulic con­trol ele­ments, which are cov­ered by ISO 13849–1.

ISO adopted EN 954–1 and reis­sued it as ISO 13849–1 in 1999. This edi­tion of the stan­dard was vir­tu­ally iden­ti­cal to the stan­dard it replaced from a tech­ni­cal require­ments per­spec­tive. EN 954–1/ISO 13849–1 did not pro­vide any means to esti­mate the integrity of the safety related con­trols, but did define cir­cuit archi­tec­tures (Categories B, 1–4) and spoke to the selec­tion of com­po­nents, intro­duc­ing the con­cepts of ‘well-​​tried safety prin­ci­ples’ and ‘well-​​tried com­po­nents’. A sec­ond prob­lem had long existed in addi­tion to this — EN 954–2, Validation, was never pub­lished by CENELEC except as a com­mit­tee draft, so a key ele­ment in the appli­ca­tion of the stan­dard had been miss­ing for five years at the point where ISO 13849–1 Edition 1 was published.

The first cut at guid­ing users in choos­ing an appro­pri­ate stan­dard came with the pub­li­ca­tion of IEC 62061 Edition 1.  Published in 2005, Edition 1 included a table that attempted to pro­vide users with some guid­ance on how to choose between ISO 13849–1 or IEC 62061.

…and then came 2007…

In 2007, ISO pub­lished the Second Edition of ISO 13849–1, and brought a whole new twist to the dis­cus­sion by intro­duc­ing ‘Performance Levels’ or PLs. PLs can be loosely equated to SILs, even though PLs are stated in fail­ures per year and SILs in fail­ures per hour. The same table included in IEC 62061 was included in this edi­tion of ISO 13849–1.

Table 1
Recommended appli­ca­tion of
IEC 62061 and ISO 13849–1(under revision)

(from the Second Edition, 2007)

Technology imple­ment­ing the
safety related con­trol function(s)		ISO
13849–1 (under revision)		IEC 62061
ANon elec­tri­cal, e.g. hydraulicsXNot cov­ered
BElectromechanical, e.g. relays, or
non-​​complex electronics		Restricted to des­ig­nated
archi­tec­tures (see Note 1) and up to PL=e

All archi­tec­tures and up to
SIL 3

CComplex elec­tron­ics, e.g. programmableRestricted to des­ig­nated
archi­tec­tures (see Note 1) and up
to PL=d		All archi­tec­tures and up to
SIL 3
DA com­bined with BRestricted to des­ig­nated
archi­tec­tures (see Note 1) and up
to PL=e		X
see Note 3
EC com­bined with BRestricted to des­ig­nated
archi­tec­tures (see Note 1) and up
to PL=d		All archi­tec­tures and up to
SIL 3
FC com­bined with A, or C com­bined with
A and B		X
see Note 2		X
see Note 3

“X” indi­cates that this item is dealt with by the stan­dard shown in the col­umn heading.

NOTE 1 Designated archi­tec­tures are defined in Annex B of EN ISO 13849–1(rev.) to give a sim­pli­fied approach for quan­tifi­ca­tion of per­for­mance level.

NOTE 2 For com­plex elec­tron­ics: Use of des­ig­nated archi­tec­tures accord­ing to EN ISO 13849–1(rev.) up to PL=d or any archi­tec­ture accord­ing to IEC 62061.

NOTE 3 For non-​​electrical tech­nol­ogy use parts accord­ing to EN ISO 13849–1(rev.) as subsystems.

So how is a machine builder to choose the ‘cor­rect’ stan­dard, if both stan­dards are applic­a­ble and both are cor­rect? Furthermore, how do you assess the reli­a­bil­ity of the safety-​​related con­trols when inte­grat­ing equip­ment from var­i­ous sup­pli­ers, some of whom rate their equip­ment in PLs and some in SILs? Why are two stan­dards address­ing the same topic required? Will ISO 13849–1 and IEC 62061 ever be merged?

The Technical Report

In July this year the IEC pub­lished a Technical Report that dis­cusses the selec­tion and appli­ca­tion of these two key con­trol reli­a­bil­ity stan­dards for machine builders. This guide has long been needed, and pre­cedes a face to face event planned by IEC to bring machine builders and stan­dards writ­ers face-​​to-​​face to dis­cuss these same issues.

The guide, titled IEC/​TR 62061–1 — Technical Report — Guidance on the appli­ca­tion of ISO 13849–1 and IEC 62061 in the design of safety-​​related con­trol sys­tems for machin­ery pro­vides direct guid­ance on how to select between these two standards.

Download IEC stan­dards, International Electrotechnical Commission standards.

Merger

In the intro­duc­tion to the report the TC makes it clear that the stan­dards will be merged, although they don’t pro­vide any kind of a time line for the merger. Quoting from the introduction:

It is intended that this Technical Report be incor­po­rated into both IEC 62061 and ISO 13849–1 by means of cor­ri­genda that ref­er­ence the pub­lished ver­sion of this doc­u­ment. These cor­ri­genda will also remove the infor­ma­tion given in Table 1, Recommended appli­ca­tion of IEC 62061 and ISO 13849–1, pro­vided in the com­mon intro­duc­tion to both stan­dards, which is now rec­og­nized as being out of date. Subsequently, it is intended to merge ISO 13849–1 and IEC 62061 by means of a JWG of ISO/​TC 199 and IEC/​TC 44.

I added the bold face to the para­graph above to high­light the key state­ment regard­ing the even­tual merger of the two doc­u­ments.  If you’re not famil­iar with the stan­dards acronyms, a ‘JWG’ is a Joint Working Group, and a TC is a Technical Committee. TC’s are formed from vol­un­teer experts from indus­try and acad­e­mia sup­ported by their orga­ni­za­tions. So a JWG formed from two TC’s just means that a joint com­mit­tee has been formed to work out the details of the merger. Eventually.

The other key point in this para­graph relates to the replace­ment of Table 1. In the interim, IEC/​TR 62061–1 will be incor­po­rated into both stan­dards, replac­ing Table 1.

Eventually the con­fu­sion will be cleared up because only one stan­dard will exist in the machin­ery sec­tor, but until then, machine builders will need to fig­ure out which stan­dard best fits their products.

Comparing PL’s and SIL’s

The Technical Report does a good job of dis­cussing the dif­fer­ences between PL and SIL, includ­ing pro­vid­ing an expla­na­tion of how to covert one to the other, very use­ful if you are try­ing to inte­grate an SIL rated device into a PL analy­sis or vice-​​versa.

Selecting a Standard

Clause 2.5 gives some solid advice on select­ing between the two stan­dards based on the tech­nolo­gies employed in the design and your own com­fort level in using the ana­lyt­i­cal tech­niques in the two standards.

Another key point is that EITHER stan­dard can be used to ana­lyze com­plex OR sim­ple con­trol sys­tems. Some fans of IEC 62061 have been known to put ISO 13849–1 down as use­ful exclu­sively for sim­ple hard­wired con­trol sys­tems. Clause 3.3 makes it clear that this is not the case. Pick the one you like or know the best and go with that. As an addi­tional thought, con­sider which stan­dard your com­peti­tors are using, and also which your cus­tomers are using. For exam­ple, if your cus­tomers use ISO 13849–1 pri­mar­ily, qual­i­fy­ing your prod­uct under IEC 62061 might seem like a good idea, but may drive your cus­tomers to a com­peti­tor who makes their life eas­ier by using ISO 13849–1. If your com­peti­tors are using a dif­fer­ent stan­dard, try to under­stand the choice before climb­ing on the band­wagon. There may be a com­pet­i­tive advan­tage lurk­ing in being different.

Risk Assessment

Clause 4 speaks directly to the indis­pens­able need to con­duct a method­i­cal risk assess­ment, and to use that to guide the design of the controls.

In my prac­tice, many clients decide that they would pre­fer to choose a con­trol reli­a­bil­ity level that they feel will be more than good enough for any of their designs, and then to ‘stan­dard­ize’ on that design for all their prod­ucts, thereby elim­i­nat­ing the need to thought­fully decide on the appro­pri­ate design for the appli­ca­tion. In other cases, end-​​users may choose to use a ‘stan­dard’ design through­out their facil­ity to assist main­te­nance per­son­nel by lim­it­ing their need to become tech­ni­cally famil­iar with a vari­ety of designs. This is done to speed trou­bleshoot­ing and reduce down time and spares stocks.

The prob­lem with this approach can be that some man­agers believe this approach can elim­i­nate the need to con­duct risk assess­ments, see­ing this as a fruit­less, expen­sive and often futile exer­cise. This is emphat­i­cally NOT the case. Risk assess­ments address much more than the selec­tion of con­trol reli­a­bil­ity require­ments and need to be done to ensure that all haz­ards that can­not be elim­i­nated or sub­sti­tuted are safe­guarded. A miss­ing or badly done risk assess­ment may inval­i­date your claim to a CE mark, or be the land­mine that ends a lia­bil­ity case — with you on the los­ing end.

Safety Requirement Specification (SRS)

Each safety func­tion needs to be defined in detail in a Safety Requirement Specification (SRS). A reli­a­bil­ity assess­ment needs to be com­pleted for each safety func­tion defined in the SRS. This point is dis­cussed in detail in IEC 62061, but is not dealt with in any detail in ISO 13849–1, so IEC/​TR 62061–1 once again bridges the gap by pro­vid­ing an impor­tant detail that is miss­ing in one of the two standards.

If you are unfa­mil­iar with the con­cept of an SRS, each safety func­tion needs to be described with a cer­tain min­i­mum amount of infor­ma­tion, including:

  • The name of safety function;
  • A descrip­tion of the function;
  • The required level of per­for­mance based on the risk assess­ment and accord­ing to either ISO 13849–1 (PLr a to e) or the required safety integrity accord­ing to IEC 62061 (SIL 1 to 3)

Once the safety func­tions are defined and ana­lyzed, each safety func­tion must be imple­mented by a con­trol cir­cuit. The selected PL will drive the design to one or two of the defined ISO 13849–1 archi­tec­tures, and then the com­po­nent selec­tions and other design details will drive the final fail­ure rate and PL. Alternatively, the SRS will drive the selec­tion of IEC 62061 archi­tec­ture (1oo1, 1oo2, 2oo2, etc.) and the rest of the design details will lead to the final fail­ure rate and SIL.

Table 1 in the Technical Report com­pares the levels.

Table 1 – Relationship between PLs and SILs based on the aver­age prob­a­bil­ity
of dan­ger­ous fail­ure per hour

Performance Level (PL)Average prob­a­bil­ity of a dan­ger­ous
fail­ure per hour (1/​h)		Safety integrity level (SIL)
a>= 10–5 to < 10–4No spe­cial safety requirements
b>= 3 x 10–6 to < 10–51
c>= 10–6 to < 3 x 10–61
d>= 10–7 to < 10–62
e>= 10–8 to < 10–73

This table com­bines ISO 13849–1 2007, Tables 3 & 4. No sim­i­lar tables exist in IEC 62061 2005.

Combining Equipment with PLs and SILs

Section 7 of the report speaks to the chal­lenge of inte­grat­ing equip­ment with rat­ings in a mix of PLs and SILs. Until the stan­dards merge and a sin­gle sys­tem for describ­ing reli­a­bil­ity cat­e­gories is agreed on, this prob­lem will be with us.

When design­ing sys­tems using either sys­tem the designer has to deter­mine the approx­i­mate rate of dan­ger­ous fail­ures. In ISO 13849–1, MTTFd is the com­po­nent fail­ure rate para­me­ter, while in IEC 62061, PFHd is the sub­sys­tem fail­ure rate para­me­ter. MTTFd does not con­sider diag­nos­tics or archi­tec­ture, only the com­po­nent fail­ure rate per year, while PFHd does include diag­nos­tics and archti­tec­ture, and it speaks to the sys­tem fail­ure rate per hour. To com­pare these rates, ISO 13849–1 Annex K describes the rela­tion­ship between MTTFd and PFHd for dif­fer­ent architectures.

In the design process only one method can be used, so where equip­ment with dif­fer­ent rat­ings must be com­bined the fail­ure rates must be con­verted to either MTTFd or to PFHd, depend­ing on the sys­tem being used to com­plete the analy­sis. Mixing require­ments within the design of a sub­sys­tem is not per­mit­ted (See Clause 7.3.3).

Fault Exclusions

Fault exclu­sions are per­mit­ted under both stan­dards with some lim­i­ta­tions: up to IEC 62061 SIL 2. No fault exclu­sions are per­mit­ted in SIL 3. Properly jus­ti­fied fault exclu­sions can be used up to PLe. “Properly jus­ti­fied” fault exclu­sions are those that can be shown to be valid through the life­time of the SRP/​CS.

In gen­eral, fault exclu­sions for mechan­i­cal fail­ures of electro­mechan­i­cal devices such as inter­lock devices or emer­gency stop devices are not per­mit­ted, with a few excep­tions given in ISO 13849–2, (See Clauses 7.2.2.4 and 7.2.2.5).

This approach is con­sis­tent with the cur­rent approach taken in Canada, as described in CSA Z432 & Z434. Fault exclu­sions are gen­er­ally not per­mit­ted under ANSI standards.

Worked Examples

Section 8 of the Technical Report gives a cou­ple of worked exam­ples, one done under ISO 13849–1, and one under IEC 62061. For some­one look­ing for a good exam­ple of what a prop­erly com­pleted analy­sis should look like, this sec­tion is the gold at the end of the rain­bow. Section 8.2 pro­vides a good, clear exam­ple of the appli­ca­tion of the stan­dards along with a nice, sim­ple exam­ple of what a safety require­ment spec­i­fi­ca­tion might look like.

Understanding the Differences

One area where pro­po­nents of the two stan­dards often dis­agree is on the ‘accu­racy’ of the ana­lyt­i­cal pro­ce­dures given in the two stan­dards. The Technical Report pro­vides a detailed expla­na­tion of why the two tech­niques pro­vide slightly dif­fer­ent results and pro­vides the ratio­nale explain­ing why this vari­a­tion should be con­sid­ered acceptable.

To Buy or Not to Buy…

At the end of the day, the ques­tion that needs to be answered is whether to buy this doc­u­ment or not. If you use either of these stan­dards, I strongly rec­om­mend that you spend the money to get this Technical Report, if for noth­ing more than the worked exam­ples. Until the two stan­dards are merged, and that could be a few years, you will need to be able to effec­tively apply these approaches to PL and SIL rated equip­ment. This Technical Report will be an invalu­able aid.

It also pro­vides some guid­ance on the direc­tion that the new merged stan­dard will take. Some old argu­ments can be set­tled, or at least re-​​directed, by this document.

Finally, since the TR is to be incor­po­rated in both stan­dards and con­tains mate­r­ial replac­ing that in the cur­rent edi­tions of the stan­dard, you must buy a copy to remain current.

For all of these rea­sons, I would spend the money to acquire this doc­u­ment, read and apply it.

Download IEC stan­dards, International Electrotechnical Commission standards.

Download ISO Standards

If you’ve bought the report and would like to add your thoughts, please add a com­ment below. Got ques­tions? Contact me!

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