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
A	Non elec­tri­cal, e.g. hydraulics	X	Not cov­ered
B	Electromechanical, 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
C	Complex elec­tron­ics, e.g. programmable	Restricted to des­ig­nated archi­tec­tures (see Note 1) and up to PL=d	All archi­tec­tures and up to SIL 3
D	A com­bined with B	Restricted to des­ig­nated archi­tec­tures (see Note 1) and up to PL=e	X see Note 3
E	C com­bined with B	Restricted to des­ig­nated archi­tec­tures (see Note 1) and up to PL=d	All archi­tec­tures and up to SIL 3
F	C 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 (PL_r 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 < 10–4	No spe­cial safety requirements
b	>= 3 x 10–6 to < 10–5	1
c	>= 10–6 to < 3 x 10–6	1
d	>= 10–7 to < 10–6	2
e	>= 10–8 to < 10–7	3

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, MTTF_d is the com­po­nent fail­ure rate para­me­ter, while in IEC 62061, PFH_d is the sub­sys­tem fail­ure rate para­me­ter. MTTF_d does not con­sider diag­nos­tics or archi­tec­ture, only the com­po­nent fail­ure rate per year, while PFH_d 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 MTTF_d and PFH_d 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 MTTF_d or to PFH_d, 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 PL_e. “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!