Machinery Safety Labels: 3 Top Tools for Effective Warnings

This entry is part 4 of 4 in the series Hierarchy of Controls

Machinery Safety Labels

The third level of the Hierarchy of Controls is Information for Use. Safety Labels are a key part of the Information for Use provided by machine build­ers to users and are often the only inform­a­tion that many users get to see. This makes the design and place­ment of the safety labels crit­ic­al to their effect­ive­ness. There is as much risk in the under-​use of safety labels as there is in the over-​use of safety labels. Often, machine build­ers and users simply select gen­er­ic labels that are eas­ily avail­able from cata­logues, miss­ing the oppor­tun­ity to design labels that are spe­cif­ic to the machine and the haz­ards present.

Product Safety and Liability Limitation

If your com­pany man­u­fac­tures machinery that has poten­tial haz­ards asso­ci­ated with its trans­port­a­tion, install­a­tion, use, main­ten­ance, decom­mis­sion­ing and/​or dis­pos­al, you likely have a very strong need to cre­ate effect­ive product safety labels. This task must be done right: product safety labels play an integ­ral role in your company’s product safety and liab­il­ity pre­ven­tion efforts. And that means that people’s lives and your company’s fin­an­cial well-​being are on the line. On that note, it’s import­ant to keep in mind these two factors when it comes to effect­ive safety labels:

  1. If prop­erly designed, they can dra­mat­ic­ally reduce acci­dents. This not only improves a product’s over­all safety record but adds to a company’s bot­tom line by redu­cing product liab­il­ity lit­ig­a­tion and insur­ance costs.
  2. If poorly designed, needed safety com­mu­nic­a­tion does not take place and this can lead to acci­dents that cause injur­ies. With these acci­dents, com­pan­ies face high costs set­tling or fight­ing law­suits because their products lacked “adequate warn­ings.”

With the rise in product liab­il­ity lit­ig­a­tion based on “fail­ure to warn” over the past sev­er­al dec­ades, product safety labels have become a lead­ing focal point in law­suits faced by cap­it­al equip­ment man­u­fac­tur­ers. Let’s look at three best?practice tools for product safety label design. These tools can provide insight to help you cre­ate or improve your safety label strategy in order to bet­ter pro­tect your product users from harm and your com­pany from litigation-​related losses.


As a man­u­fac­turer, you know that your leg­al oblig­a­tion is to meet or exceed the most recent ver­sions of stand­ards related to your product at the time it’s sold into the mar­ket­place. Warning label stand­ards are the first place to turn to when it comes to defin­ing your product safety labels. Up until 1991, there was no over­arch­ing, multi-​industry stand­ard in the U.S., or in the rest of the world, which gave defin­it­ive guid­ance on the prop­er format­ting and con­tent for on-​product warn­ings. In the U.S., that changed nation­ally with the pub­lic­a­tion of the ANSI Z535.4 Standard for Product Safety Signs and Labels in 1991, and inter­na­tion­ally with the pub­lic­a­tion of ISO 3864 – 2 Design Principles for Product Safety Labels in 2004.

As of 2017, Canada does not have a warn­ing label stand­ard. Since Canada imports machinery from the U.S. and the EU, it is quite com­mon to see either ANSI Z535 style labels or ISO 3864 style labels on products. Under Canadian law, neither is more cor­rect. However, Québec has spe­cif­ic require­ments for French lan­guage trans­la­tions, and many CSA stand­ards pre­scribe spe­cif­ic haz­ard warn­ing labels that do not con­form to either ANSI or ISO styles.

Following the design prin­ciples in ANSI Z535.4 or ISO 3864 – 2 will give you a start­ing place for both the con­tent and format choices you have to make for your products’ safety labels, bear­ing in mind the lan­guage require­ments of your jur­is­dic­tion. Note that both of these stand­ards are revised reg­u­larly, every five years or so, and it’s import­ant to be aware of the nuances that would make one format more appro­pri­ate for your product than anoth­er.

Safety label standard ANSI Z535.4 Product Safety Signs and Labels
The ANSI Z535.4 product safety label stand­ard
Safety label standard ISO 3864-2 Graphical symbols - Safety colours and safety signs - Part 2: Design principles for product safety labels.
The ISO 3864 – 2 product safety label stand­ard


From an engin­eer­ing per­spect­ive, your job is to identi­fy poten­tial haz­ards and then determ­ine if they need to be designed out, guarded, or warned about. From a leg­al per­spect­ive, your job is to define what haz­ards are “reas­on­ably fore­see­able” and “reas­on­able” ways to mit­ig­ate risks asso­ci­ated with haz­ards that can­not be designed out. This is where risk assess­ment comes into play.

In today’s world, a product is expec­ted to be designed with safety in mind. The risk assess­ment pro­cess helps you to accom­plish this task. At its most basic level, risk assess­ment involves con­sid­er­ing the prob­ab­il­ity and sever­ity of out­comes that can res­ult from poten­tially haz­ard­ous situ­ations. After identi­fy­ing the poten­tial haz­ards related to your product at every point in its life­cycle, you then con­sider vari­ous strategies to either elim­in­ate or reduce the risk of people inter­act­ing with these haz­ards.

The best prac­tice risk assess­ment stand­ards that exist today (i.e. ANSI Z10, ANSI B11, CSA Z432, CSA Z1002, ISO 12100, ISO 31000, ISO 31010) give you a pro­cess to use to quanti­fy and reduce risks. Using these stand­ards as the basis for a form­al­ized risk assess­ment pro­cess will not only help you to devel­op bet­ter safety labels and a safer product, but it will also provide you with doc­u­ment­a­tion that will help you to show the world that you are a safety-​conscious com­pany who uses the latest standards-​based tech­no­logy to reduce risks. This will be highly import­ant should you be involved in product liab­il­ity lit­ig­a­tion down the road.

From an engin­eer­ing per­spect­ive, your job is to identi­fy poten­tial haz­ards and then determ­ine if they need to be designed out, guarded, or warned about. From a leg­al per­spect­ive, your job is to define what haz­ards are “reas­on­ably fore­see­able” and “reas­on­able” ways to mit­ig­ate risks asso­ci­ated with haz­ards that can­not be designed out. This is where risk assess­ment comes into play.

MIL-STD 882 risk assessment form
A typ­ic­al risk assess­ment scor­ing mat­rix (based on MIL STD 882 as defined in ANSI B11/​ISO 12100 Safety of Machinery – Risk Assessment Annex D)


A large num­ber of machinery man­u­fac­tur­ers sell their products around the globe and when this is the case, com­pli­ance with glob­al stand­ards is a require­ment. The ANSI Z535.4 and ISO 3864 – 2 product safety label stand­ards, and the EU machinery dir­ect­ive place an emphas­is on using well-​designed sym­bols on machinery safety labels so inform­a­tion can be con­veyed across lan­guage bar­ri­ers.

The EU Machinery Directive 2006/​42/​EC requires that all inform­a­tion for use be provided in the offi­cial lan­guages of the coun­try of use. Information for use includes haz­ard warn­ing signs and labels that bear mes­sages in text. Adding sym­bols also increases your labels’ notice­ab­il­ity. The use of sym­bols to con­vey safety is becom­ing com­mon­place world­wide and not tak­ing advant­age of this new visu­al lan­guage risks mak­ing your product’s safety labels obsol­ete and non-​compliant with loc­al, region­al and inter­na­tion­al codes. In ISO 3864 – 2’s latest, 2016 update, a major change in ISO label formats was made: a new “word­less” format that con­veys risk sever­ity was added to the stand­ard. This new label format uses what ISO calls a “haz­ard sever­ity pan­el” but no sig­nal word. It com­mu­nic­ates the level of risk through colour-​coding of the haz­ard sever­ity pan­el. This format option elim­in­ates words – mak­ing trans­la­tions unne­ces­sary.

It should be noted that some­times sym­bols alone can­not con­vey com­plex safety mes­sages. In these cases, text is often still used. When ship­ping to non-​English speak­ing coun­tries, the trend today is to trans­late the text into the lan­guage of the coun­try in which the machine is sold. Digital print tech­no­logy makes this solu­tion much more cost effect­ive and effi­cient than in the past.

Safety label by Clarion Safety Systems on a machine
A typ­ic­al Clarion machine safety label that uses an inter­na­tion­ally format­ted graph­ic­al sym­bol and a format that meets both ANSI Z535.4 and ISO 3864 – 2 design prin­ciples (Design ©Clarion Safety Systems. All rights reserved.)

Concluding Thoughts

The safety labels that appear on your products are one of its most vis­ible com­pon­ents. If they don’t meet cur­rent stand­ards, if they aren’t designed as the res­ult of a risk assess­ment, and if they don’t incor­por­ate well-​designed graph­ic­al sym­bols, your com­pany risks lit­ig­a­tion and non-​conformance with mar­ket require­ments. Most import­antly, you may be put­ting those who inter­act with your machinery at risk of harm. Making sure your product safety labels are up-​to-​date is an import­ant task for every engin­eer respons­ible for a machine’s design.

For more inform­a­tion on effect­ive product safety labelling and resources that you can put to use today, vis­it www​.clari​on​safety​.com. Clarion also offers com­pli­ment­ary safety label assess­ments, where we use our exper­i­ence with the latest stand­ards and best prac­tices to assess your labels and ensure that they’re up-​to-​date in meet­ing today’s require­ments.

Ed. note: Additional Canadian mater­i­al con­trib­uted by Doug Nix.

Digiprove sealCopyright secured by Digiprove © 2017
Acknowledgements: Derek Eversdyke, Clarion Safety Systems, LLC
Some Rights Reserved

Safe Drive Control including Safe Torque Off (STO)

This entry is part 12 of 13 in the series Emergency Stop

Ed. Note: This art­icle was revised 25-​Jul-​17 to include inform­a­tion on safe stand­still.

Safe Drive Control

Variable Frequency Drive for conveyor speed control
Variable Frequency Drive for con­vey­or speed con­trol [1]
Motor drives are every­where. From DC vari­able speed drives and index­ing drives, through AC Variable Frequency drives, servo drives and step­per motor drives, the cap­ab­il­it­ies and the flex­ib­il­ity of these elec­tron­ic sys­tems has giv­en machine design­ers unpre­ced­en­ted cap­ab­il­it­ies when com­pared to basic relay or contactor-​based motor starters. We now have the cap­ab­il­ity to con­trol mech­an­isms using motors in ways that would have been hard to ima­gine at the begin­ning of the indus­tri­al revolu­tion.

Since we are con­trolling machinery, safety is always a con­cern. In the 1990’s when I star­ted design­ing machinery with motor drives, deal­ing with safety con­cerns usu­ally meant adding a suit­ably rated con­tact­or upstream of the drive so that you could inter­rupt power to the drive in case some­thing went wrong. With early servo drives, inter­rupt­ing the sup­ply power often meant los­ing pos­i­tion data or worse, so con­tact­ors were placed between the drive and the motor. This occa­sion­ally caused the drive stage of the servo con­trol­ler to blow up if the switch-​off happened with the motor run­ning and under high load. Motor drive man­u­fac­tur­ers respon­ded by provid­ing con­tact­ors and oth­er com­pon­ents built into their drives, cre­at­ing a fea­ture called Safe Torque Off (STO).

STO describes a state where “The drive is reli­ably torque-​free” [2]. The func­tions dis­cussed in this art­icle are described in detail in IEC 61800 – 5-​2 [3]. The func­tions are also lis­ted in [10, Table 5.2]. Note that only Safe Torque Off and Safe Stop 1 can be used for emer­gency stop func­tions. Safe Torque Off, Safe Stop 1 and Safe Stop 2 can be used for safety-​related stop func­tions ini­ti­ated by a safe­guard­ing device.

If you have been a read­er of this blog for a while, you may recall that I have dis­cussed stop cat­egor­ies before. This art­icle expands on those con­cepts in rela­tion to motor drives and their stop­ping func­tions spe­cific­ally. I’ve also talked about Emergency Stop extens­ively. You might be inter­ested in read­ing more about the e-​stop func­tion in the post “Emergency Stop – What’s so con­fus­ing about that?”

Safe Torque Off (STO)

According to Siemens, “The STO func­tion is the most com­mon and basic drive-​integrated safety func­tion. It ensures that no torque-​generating energy can con­tin­ue to act upon a motor and pre­vents unin­ten­tion­al start­ing.” Risk assess­ment of the machinery can identi­fy the need for an STO func­tion. The devices used for this applic­a­tion are described in IEC 60204 – 1 in clause 5.4 [4]. The design fea­tures for pre­ven­tion of unex­pec­ted start­ing are covered in more detail in EN 1037 [5] or ISO 14118 [6]. If you are inter­ested in these stand­ards, ISO 14118 is in the pro­cess of being revised. A new ver­sion should be avail­able with­in 12 – 18 months.

The STO func­tion oper­ates as shown in Fig.1. The blue line rep­res­ents the drive speed/​velocity, V, on the y-​axis, with time, t, on the x-​axis.

Graph showing motor drive output over time when the STO function is activated.
Figure 1 – Safe Torque Off func­tion [1]
At the begin­ning of the stop­ping pro­cess (orange arrow and dot­ted line), the drive gate pulses are imme­di­ately shut off, remov­ing torque from the motor (i.e., zero torque). The speed of the driv­en equip­ment will drop at a rate determ­ined by the sys­tem fric­tion and iner­tia until stand­still is achieved. The zero torque con­di­tion is then main­tained until the safety func­tion per­mits restart­ing (area out­lined with yellow/​black zebra stripe). Note that drive stand­still may occur if the fric­tion and iner­tia of the sys­tem per­mit, but it is pos­sible that the driv­en equip­ment may coast for some time. You may be able to move the driv­en equip­ment by hand or grav­ity with drive in STO.STO is an uncon­trolled stop [4, 3.56]:

STO is an uncon­trolled stop [4, 3.56]:

uncon­trolled stop
stop­ping of machine motion by remov­ing elec­tric­al power to the machine actu­at­ors
NOTE This defin­i­tion does not imply any oth­er state of oth­er (for example, non-​electrical) stop­ping devices, for example, mech­an­ic­al or hydraul­ic brakes that are out­side the scope of this stand­ard.

The defin­i­tion above is import­ant. Uncontrolled stops are the most com­mon form of stop­ping used in machines of all types and is required as a basic func­tion for all machines. It can be achieved in a num­ber of ways, includ­ing the use of a dis­con­nect­ing device, emer­gency stop sys­tems, and gate inter­lock­ing sys­tems that remove power from machine actu­at­ors.

The concept of an uncon­trolled stop is embod­ied in stop cat­egory 0 [4, 9.2.2]:

stop cat­egory 0 — stop­ping by imme­di­ate remov­al of power to the machine actu­at­ors (i.e., and uncon­trolled stop, see 3.56)

Stop cat­egory 0 is only appro­pri­ate where the machinery has little iner­tia, or where mech­an­ic­al fric­tion is high enough that the stop­ping time is short. It may also be used in cases where the machinery has very high iner­tia, but only for nor­mal stop­ping when coast­ing time is not a factor, not for safety stop­ping func­tions where the time to a no-​motion state is crit­ic­al.

There are a few oth­er stop­ping modes that are often con­fused with STO:

  • Safe Stop 1
  • Safe Stop 2
  • Safe Operating Stop
  • Safe Standstill

Let’s explore the dif­fer­ences.

Safe Stop 1 (SS1)

If a defined stop­ping time is needed, a con­trolled stop­ping func­tion will be required fol­lowed by entry into STO. This stop­ping func­tion is called “Safe Stop 1” (SS1).

SS1 is dir­ectly related to Stop Category 1 [4, 9.2.2]. As described in [4], Stop Category 1 func­tions as fol­lows:

stop cat­egory 1 — a con­trolled stop (see 3.11) with power avail­able to the machine actu­at­ors to achieve the stop and then remov­al of power when the stop is achieved;

A “con­trolled stop” is defined in [4, 3.11]:

con­trolled stop
stop­ping of machine motion with elec­tric­al power to the machine actu­at­or main­tained dur­ing the stop­ping pro­cess

Once the con­trolled stop is com­pleted, i.e., machine motion has stopped, the drive may then be placed into STO (or cat­egory 0 stop). The stop­ping pro­cess is shown in Fig. 2 [7].

Graph showing the reduction of drive speed over time following the beginning of a controlled stopping process.
Figure 2 – Safe Stop 1

The stop­ping pro­cess starts where the orange arrow and dot­ted line are shown. As com­pared to Fig. 1 where the decel­er­a­tion curve is gentle and expo­nen­tial, the act­ive stop­ping peri­od in Fig. 2 is a lin­ear curve from oper­at­ing speed to zero speed. At the blue dot­ted line, the drive enters and stays in STO. The yellow/​black zebra striped area of the curve out­lines the com­plete stop­ping func­tion. This stop­ping meth­od is typ­ic­al of many types of machinery, par­tic­u­larly those with servo driv­en mech­an­isms.

Safe Stop 2 (SS2)

In some cases, the risk assess­ment may show that remov­ing power com­pletely from a mech­an­ism will increase the risk. An example might be a ver­tic­al axis where the motor drive is used to main­tain the pos­i­tion of the tool­ing. Removing power from the drive with the tool raised would res­ult in the tool­ing crash­ing to the bot­tom of the axis in an uncon­trolled way. Definitely NOT the desired way to achieve any kind of stop!

There are a num­ber of ways to pre­vent this kind of occur­rence, but I’m going to lim­it the dis­cus­sion here to the Safe Stop 2 func­tion.

Let’s start with the defin­i­tion [4, 3.11]:

con­trolled stop
stop­ping of machine motion with elec­tric­al power to the machine actu­at­or main­tained dur­ing the stop­ping pro­cess

Wait! This is exactly the same as a stop cat­egory 1, so what is the dif­fer­ence? For that we need to look to [4, 9.2.2]:

stop cat­egory 2 — a con­trolled stop with power left avail­able to the machine actu­at­ors.

The first thing to know about stop cat­egory 2 is that this cat­egory can­not be used for emer­gency stop [4,]. If you have tool­ing where stop cat­egory 2 is the most appro­pri­ate stop under nor­mal con­di­tions, you will have to add an anoth­er means to pre­vent the axis from fall­ing dur­ing the emer­gency stop. This could be a spring-​set brake that is held released by the emer­gency stop sys­tem and is applied when the e-​stop sys­tem removes power from the tool­ing. There are many ways to achieve auto­mat­ic load-​holding besides brakes, but remem­ber, whatever you choose it must be effect­ive in power loss con­di­tions.

As shown in Fig. 3, the oper­a­tion of Safe Stop 2 dif­fers from Safe Stop 1 in that, instead of enter­ing into STO when motion stops, the sys­tem enters Safe Operating Stop (SOS) [8], not STO. SOS is a stop cat­egory 2 func­tion. Full torque remains avail­able from the motor to hold the tool­ing in pos­i­tion. Safe stand­still is mon­itored by the drive or oth­er means.

Graph showing speed reduction to zero, followed by entry into stop category 2.
Figure 3 — Safe Stop 2

Depending on the ISO 13849 – 1 PLr, or the IEC 62061 SILr needed for the applic­a­tion, the drive may not have high enough reli­ab­il­ity on its own. In this case, a second chan­nel may be required to ensure that safe stand­still mon­it­or­ing is adequately reli­able. This can be achieved by adding anoth­er means of stand­still detec­tion, like a second encoder, or a stand­still mon­it­or­ing device. An example cir­cuit dia­gram show­ing this type of mon­it­or­ing can be found in Fig. 4 [10, Fig. 8.37], show­ing a safety PLC and drive used to provide an “inch­ing” or “jog” func­tion.

Circuit diagram for a safe inching mode using a motor drive. Taken from Fig 8.37 in BGIA Report 2/2008e
Figure 4 — Safely lim­ited speed for inch­ing mode – PLd, Cat. 3 [10]
In Fig. 4, the encoders are labelled G1 and G2. Both encoders are con­nec­ted to the safety PLC to provide two-​channel feed­back required for Category 3 archi­tec­ture. G1 is also con­nec­ted to the motor drive for pos­i­tion and velo­city feed­back as needed for the applic­a­tion. Note that this par­tic­u­lar drive also has a con­tact­or upstream, Q1, to provide one chan­nel of the two required for Category 3. The second chan­nel would be provided by the pulse block­ing input on the drive. For more on how this cir­cuit func­tions and how the func­tion­al safety ana­lys­is is com­pleted, see [10].

Safe Operating Stop (SOS)

During a safe oper­at­ing stop (SOS), the motor is brought to a spe­cif­ic pos­i­tion and held there by the drive. Full torque is avail­able to keep the tool­ing in pos­i­tion. The stop is mon­itored safely by the drive. The func­tion is shown in Figure 4 [9].

A graph showing a drive maintaining position following a stop
Figure 5 — Safe Operating Stop

In Fig. 5, the y-​axis, s, rep­res­ents the pos­i­tion of the tool­ing, NOT the velo­city, while the x-​axis rep­res­ents time, t. The start of the pos­i­tion hold­ing func­tion is shown by the orange arrow and dashed line. The peri­od fol­low­ing the green dashed line is the SOS peri­od.

SOS can­not be used for the emer­gency stop func­tion. Under cer­tain con­di­tions it may be used when guard inter­locks are opened, i.e., the guard door on a CNC lathe is opened so that the oper­at­or can place a new work­piece.

There a quite a few addi­tion­al “safe” drive func­tions. For more on these func­tions and how to imple­ment them, see [2] and applic­a­tion data from your favour­ite drive man­u­fac­turer. Reference is also provided in [9, Table 5.2].

Safe Standstill

Safe stand­still is a con­di­tion where motion has stopped and is being mon­itored by a safety-​rated device whose out­put sig­nals are used to con­trol the release of guard lock­ing devices. Safe stand­still is not the same as zero-​speed because zero-​speed can be achieved without the use of safety rated con­trol com­pon­ents and design, while safe stand­still requires both suit­able com­pon­ents and design.

There are a num­ber of ways to achieve safe stand­still. Here are three com­mon approaches [12]:

  1. Rotation sensors
    Sensors includ­ing prox­im­ity sensors, resolv­ers, and encoders can be used to mon­it­or the motion of the drive com­pon­ents. A safe stand­still mon­it­or­ing device is used to when stand­still has occurred.  When a machine has an unstable rest pos­i­tion, a prox­im­ity sensor should be used to ensure the machine is in a safe con­di­tion before the guard lock­ing devices are released.
  2. Back EMF mon­it­or­ing
    Back elec­tro­mot­ive force or Back EMF is the voltage cre­ated in an elec­tric motor due to the rota­tion of the arma­ture in the mag­net­ic field in the motor. This voltage opposes the applied voltage and is approx­im­ately pro­por­tion­al to the rota­tion­al speed of the motor. Back EMF remains after the sup­ply voltage has been removed, allow­ing mon­it­or­ing devices to indir­ectly meas­ure motor speed and stand­still.
  3. Failsafe timer
    Failsafe timers are time delay relays designed for use in safety func­tions. Failsafe timers can be used when the stop­ping per­form­ance of the machinery is con­sist­ent and known.
    Following remov­al of power from the drive motor, the time delay starts. At the end of the time delay, the relay releases the guard lock­ing devices.
    Regular time delay relays can­not be used for this pur­pose, only fail-​safe relays designed to be used in safety func­tions can be used, along with suit­able safety sys­tems design tech­niques like ISO 13849 or IEC 62061.


As you can see, there are sig­ni­fic­ant dif­fer­ences between STO, SS1, SS2, SOS and Safe Standstill. While these func­tions may be used togeth­er to achieve a par­tic­u­lar safety func­tion, some are func­tions of the imple­ment­a­tion of the motor drive, e.g., STO, a func­tion of the design of the motor drive itself, e.g., STO, SS1, SS2, and SOS, or the design of con­trols extern­al to the motor drive, e.g., safe stand­still. The sim­il­ar­it­ies between these vari­ous func­tions can make it easy to con­fuse them. Care needs to be taken to ensure that the cor­rect tech­nic­al approach is used when real­ising the safety func­tion required by the risk assess­ment.


[1]    “Variable Frequency Drives – Industrial Wiki – odesie by Tech Transfer”, Myodesie​.com, 2017. [Online]. Available: https://​www​.myo​desie​.com/​w​i​k​i​/​i​n​d​e​x​/​r​e​t​u​r​n​E​n​t​r​y​/​i​d​/​3​040. [Accessed: 19- Jun- 2017].

[2] “Safe Torque Off (STO) – Safety Integrated – Siemens”, Industry​.siemens​.com, 2017. [Online]. Available: http://​www​.industry​.siemens​.com/​t​o​p​i​c​s​/​g​l​o​b​a​l​/​e​n​/​s​a​f​e​t​y​-​i​n​t​e​g​r​a​t​e​d​/​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​p​r​o​d​u​c​t​-​p​o​r​t​f​o​l​i​o​/​d​r​i​v​e​-​t​e​c​h​n​o​l​o​g​y​/​s​a​f​e​t​y​-​f​u​n​c​t​i​o​n​s​/​p​a​g​e​s​/​s​a​f​e​-​t​o​r​q​u​e​-​o​f​f​.​a​spx. [Accessed: 19- Jun- 2017].

[3]      Adjustable speed elec­tric­al power drive sys­tems – Part 5 – 2: Safety require­ments – Functional. IEC Standard 61800 – 5-​2. 2nd Ed. 2016.

[4]     Safety of machinery — Electrical equip­ment of machines — Part 1: General require­ments. IEC Standard 60204 – 1. 2006.

[5]     Safety of machinery — Prevention of unex­pec­ted start-​up. EN Standard 1037+A1. 2008.

[6]     Safety of machinery — Prevention of unex­pec­ted start-​up. ISO Standard 14118. 2000.

[7]     “Safe Stop 1 (SS1) – Safety Integrated – Siemens”, Industry​.siemens​.com, 2017. [Online]. Available: http://​www​.industry​.siemens​.com/​t​o​p​i​c​s​/​g​l​o​b​a​l​/​e​n​/​s​a​f​e​t​y​-​i​n​t​e​g​r​a​t​e​d​/​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​p​r​o​d​u​c​t​-​p​o​r​t​f​o​l​i​o​/​d​r​i​v​e​-​t​e​c​h​n​o​l​o​g​y​/​s​a​f​e​t​y​-​f​u​n​c​t​i​o​n​s​/​P​a​g​e​s​/​s​a​f​e​-​s​t​o​p​1​.​a​spx. [Accessed: 19- Jun- 2017].

[8]     “Safe Stop 2 (SS2) – Safety Integrated – Siemens”, Industry​.siemens​.com, 2017. [Online]. Available: http://​www​.industry​.siemens​.com/​t​o​p​i​c​s​/​g​l​o​b​a​l​/​e​n​/​s​a​f​e​t​y​-​i​n​t​e​g​r​a​t​e​d​/​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​p​r​o​d​u​c​t​-​p​o​r​t​f​o​l​i​o​/​d​r​i​v​e​-​t​e​c​h​n​o​l​o​g​y​/​s​a​f​e​t​y​-​f​u​n​c​t​i​o​n​s​/​P​a​g​e​s​/​s​a​f​e​-​s​t​o​p​2​.​a​spx. [Accessed: 19- Jun- 2017].

[9]     “Safe Operating Stop (SOS) – Safety Integrated – Siemens”, Industry​.siemens​.com, 2017. [Online]. Available: http://​www​.industry​.siemens​.com/​t​o​p​i​c​s​/​g​l​o​b​a​l​/​e​n​/​s​a​f​e​t​y​-​i​n​t​e​g​r​a​t​e​d​/​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​p​r​o​d​u​c​t​-​p​o​r​t​f​o​l​i​o​/​d​r​i​v​e​-​t​e​c​h​n​o​l​o​g​y​/​s​a​f​e​t​y​-​f​u​n​c​t​i​o​n​s​/​P​a​g​e​s​/​s​a​f​e​-​o​p​e​r​a​t​i​n​g​-​s​t​o​p​.​a​spx. [Accessed: 19- Jun- 2017].

[10]     M. Hauke, M. Schaefer, R. Apfeld, T. Boemer, M. Huelke, T. Borowski, K. Büllesbach, M. Dorra, H. Foermer-​Schaefer, W. Grigulewitsch, K. Heimann, B. Köhler, M. Krauß, W. Kühlem, O. Lohmaier, K. Meffert, J. Pilger, G. Reuß, U. Schuster, T. Seifen and H. Zilligen, “Functional safety of machine con­trols – Application of EN ISO 13849 – Report 2/​2008e”, BGIA – Institute for Occupational Safety and Health of the German Social Accident Insurance, Sankt Augustin, 2017.

[11]     “Glossary”, Schmersalusa​.com, 2017. [Online]. Available: http://​www​.schmersa​lusa​.com/​c​m​s​1​7​/​o​p​e​n​c​m​s​/​h​t​m​l​/​e​n​/​s​e​r​v​i​c​e​/​g​l​o​s​s​a​r​y​.​h​t​m​l#S. [Accessed: 25- Jul- 2017].

[12]     Schmersal Tech Briefs: Safe Speed & Standstill Monitoring. Schmersal USA, 2014.


Special thanks go out to two of my reg­u­lar read­ers for sug­gest­ing this post: Matt Ernst and con­trols­girl, who com­ments fre­quently. Thanks for the ideas and the ques­tions that sparked this post!

ISO 13849 – 1 Analysis — Part 8: Fault Exclusion

This entry is part 9 of 9 in the series How to do a 13849 – 1 ana­lys­is

Fault Consideration & Fault Exclusion

ISO 13849 – 1, Chapter 7 [1, 7] dis­cusses the need for fault con­sid­er­a­tion and fault exclu­sion. Fault con­sid­er­a­tion is the pro­cess of examin­ing the com­pon­ents and sub-​systems used in the safety-​related part of the con­trol sys­tem (SRP/​CS) and mak­ing a list of all the faults that could occur in each one. This a def­in­itely non-​trivial exer­cise!

Thinking back to some of the earli­er art­icles in this series where I men­tioned the dif­fer­ent types of faults, you may recall that there are detect­able and undetect­able faults, and there are safe and dan­ger­ous faults, lead­ing us to four kinds of fault:

  • Safe undetect­able faults
  • Dangerous undetect­able faults
  • Safe detect­able faults
  • Dangerous detect­able faults

For sys­tems where no dia­gnostics are used, Category B and 1, faults need to be elim­in­ated using inher­ently safe design tech­niques. Care needs to be taken when clas­si­fy­ing com­pon­ents as “well-​tried” versus using a fault exclu­sion, as com­pon­ents that might nor­mally be con­sidered “well-​tried” might not meet those require­ments in every applic­a­tion. [2, Annex A], Validation tools for mech­an­ic­al sys­tems, dis­cusses the con­cepts of “Basic Safety Principles”, “Well-​Tried Safety Principles”, and “Well-​tried com­pon­ents”.  [2, Annex A] also provides examples of faults and rel­ev­ant fault exclu­sion cri­ter­ia. There are sim­il­ar Annexes that cov­er pneu­mat­ic sys­tems [2, Annex B], hydraul­ic sys­tems [2, Annex C], and elec­tric­al sys­tems [2, Annex D].

For sys­tems where dia­gnostics are part of the design, i.e., Category 2, 3, and 4, the fault lists are used to eval­u­ate the dia­gnost­ic cov­er­age (DC) of the test sys­tems. Depending on the archi­tec­ture, cer­tain levels of DC are required to meet the rel­ev­ant PL, see [1, Fig. 5]. The fault lists are start­ing point for the determ­in­a­tion of DC, and are an input into the hard­ware and soft­ware designs. All of the dan­ger­ous detect­able faults must be covered by the dia­gnostics, and the DC must be high enough to meet the PLr for the safety func­tion.

The fault lists and fault exclu­sions are used in the Validation por­tion of this pro­cess as well. At the start of the Validation pro­cess flow­chart [2, Fig. 1], you can see how the fault lists and the cri­ter­ia used for fault exclu­sion are used as inputs to the val­id­a­tion plan.

The diagram shows the first few stages in the ISO 13849-2 Validation process. See ISO 13849-2, Figure 1.
Start of ISO 13849 – 2 Fig. 1

Faults that can be excluded do not need to val­id­ated, sav­ing time and effort dur­ing the sys­tem veri­fic­a­tion and val­id­a­tion (V & V). How is this done?

Fault Consideration

The first step is to devel­op a list of poten­tial faults that could occur, based on the com­pon­ents and sub­sys­tems included in SRP/​CS. ISO 13849 – 2 [2] includes lists of typ­ic­al faults for vari­ous tech­no­lo­gies. For example, [2, Table A.4] is the fault list for mech­an­ic­al com­pon­ents.

Mechanical fault list from ISO 13849-2
Table A.4 — Faults and fault exclu­sions — Mechanical devices, com­pon­ents and ele­ments
(e.g. cam, fol­low­er, chain, clutch, brake, shaft, screw, pin, guide, bear­ing)

[2] con­tains tables sim­il­ar to Table A.4 for:

  • Pressure-​coil springs
  • Directional con­trol valves
  • Stop (shut-​off) valves/​non-​return (check) valves/​quick-​action vent­ing valves/​shuttle valves, etc.
  • Flow valves
  • Pressure valves
  • Pipework
  • Hose assem­blies
  • Connectors
  • Pressure trans­mit­ters and pres­sure medi­um trans­ducers
  • Compressed air treat­ment — Filters
  • Compressed-​air treat­ment — Oilers
  • Compressed air treat­ment — Silencers
  • Accumulators and pres­sure ves­sels
  • Sensors
  • Fluidic Information pro­cessing — Logical ele­ments
  • etc.

As you can see, there are many dif­fer­ent types of faults that need to be con­sidered. Keep in mind that I did not give you all of the dif­fer­ent fault lists – this post would be a mile long if I did that! The point is that you need to devel­op a fault list for your sys­tem, and then con­sider the impact of each fault on the oper­a­tion of the sys­tem. If you have com­pon­ents or sub­sys­tems that are not lis­ted in the tables, then you need to devel­op your own fault lists for those items. Failure Modes and Effects Analysis (FMEA) is usu­ally the best approach for devel­op­ing fault lists for these com­pon­ents [23], [24].

When con­sid­er­ing the faults to be included in the list there are a few things that should be con­sidered [1, 7.2]:

  • if after the first fault occurs oth­er faults devel­op due to the first fault, then you can group those faults togeth­er as a single fault
  • two or more single faults with a com­mon cause can be con­sidered as a single fault
  • mul­tiple faults with dif­fer­ent causes but occur­ring sim­ul­tan­eously is con­sidered improb­able and does not need to be con­sidered


#1 – Voltage Regulator

A voltage reg­u­lat­or fails in a sys­tem power sup­ply so that the 24 Vdc out­put rises to an unreg­u­lated 36 Vdc (the intern­al power sup­ply bus voltage), and after some time has passed, two sensors fail. All three fail­ures can be grouped and con­sidered as a single fault because they ori­gin­ate in a single fail­ure in the voltage reg­u­lat­or.

#2 – Lightning Strike

If a light­ning strike occurs on the power line and the res­ult­ing surge voltage on the 400 V mains causes an inter­pos­ing con­tact­or and the motor drive it con­trols to fail to danger, then these fail­ures may be grouped and con­sidered as one. Again, a single event causes all of the sub­sequent fail­ures.

#3 – Pneumatic System Lubrication

3a – A pneu­mat­ic lub­ric­at­or runs out of lub­ric­ant and is not refilled, depriving down­stream pneu­mat­ic com­pon­ents of lub­ric­a­tion.

3b – The spool on the sys­tem dump valve sticks open because it is not cycled often enough.

Neither of these fail­ures has the same cause, so there is no need to con­sider them as occur­ring sim­ul­tan­eously because the prob­ab­il­ity of both hap­pen­ing con­cur­rently is extremely small. One cau­tion: These two faults MAY have a com­mon cause – poor main­ten­ance. If this is true and you decide to con­sider them to be two faults with a com­mon cause, they could then be grouped as a single fault.

Fault Exclusion

Once you have your well-​considered fault lists togeth­er, the next ques­tion is “Can any of the lis­ted faults be excluded?” This is a tricky ques­tion! There are a few points to con­sider:

  • Does the sys­tem archi­tec­ture allow for fault exclu­sion?
  • Is the fault tech­nic­ally improb­able, even if it is pos­sible?
  • Does exper­i­ence show that the fault is unlikely to occur?*
  • Are there tech­nic­al require­ments related to the applic­a­tion and the haz­ard that might sup­port fault exclu­sion?

* BE CAREFUL with this one!

Whenever faults are excluded, a detailed jus­ti­fic­a­tion for the exclu­sion needs to be included in the sys­tem design doc­u­ment­a­tion. Simply decid­ing that the fault can be excluded is NOT ENOUGH! Consider the risk a per­son will be exposed to in the event the fault occurs. If the sever­ity is very high, i.e., severe per­man­ent injury or death, you may not want to exclude the fault even if you think you could. Careful con­sid­er­a­tion of the res­ult­ing injury scen­ario is needed.

Basing a fault exclu­sion on per­son­al exper­i­ence is sel­dom con­sidered adequate, which is why I added the aster­isk (*) above. Look for good stat­ist­ic­al data to sup­port any decision to use a fault exclu­sion.

There is much more inform­a­tion avail­able in IEC 61508 – 2 on the sub­ject of fault exclu­sion, and there is good inform­a­tion in some of the books men­tioned below [0.1], [0.2], and [0.3]. If you know of addi­tion­al resources you would like to share, please post the inform­a­tion in the com­ments!


3.1.3 fault
state of an item char­ac­ter­ized by the inab­il­ity to per­form a required func­tion, exclud­ing the inab­il­ity dur­ing pre­vent­ive main­ten­ance or oth­er planned actions, or due to lack of extern­al resources
Note 1 to entry: A fault is often the res­ult of a fail­ure of the item itself, but may exist without pri­or fail­ure.
Note 2 to entry: In this part of ISO 13849, “fault” means ran­dom fault. [SOURCE: IEC 60050?191:1990, 05 – 01.]

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.

[2]     Safety of machinery – Safety-​related parts of con­trol sys­tems – Part 2: Validation. 2nd Edition. ISO Standard 13849 – 2. 2012.

[3]      Safety of machinery – General prin­ciples for design – Risk assess­ment and risk reduc­tion. ISO Standard 12100. 2010.

[4]     Safeguarding of Machinery. 2nd Edition. CSA Standard Z432. 2004.

[5]     Risk Assessment and Risk Reduction- A Guideline to Estimate, Evaluate and Reduce Risks Associated with Machine Tools. ANSI Technical Report B11.TR3. 2000.

[6]    Safety of machinery – Emergency stop func­tion – Principles for design. ISO Standard 13850. 2015.

[7]     Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems. 7 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.

[9]    Guidance on the applic­a­tion of ISO 13849 – 1 and IEC 62061 in the design of safety-​related con­trol sys­tems for machinery. IEC Technical Report TR 62061 – 1. 2010.

[10]     Safety of machinery – Functional safety of safety-​related elec­tric­al, elec­tron­ic and pro­gram­mable elec­tron­ic con­trol sys­tems. IEC Standard 62061. 2005.

[11]    Guidance on the applic­a­tion of ISO 13849 – 1 and IEC 62061 in the design of safety-​related con­trol sys­tems for machinery. IEC Technical Report 62061 – 1. 2010.

[12]    D. S. G. Nix, Y. Chinniah, F. Dosio, M. Fessler, F. Eng, and F. Schrever, “Linking Risk and Reliability — Mapping the out­put of risk assess­ment tools to func­tion­al safety require­ments for safety related con­trol sys­tems,” 2015.

[13]    Safety of machinery. Safety related parts of con­trol sys­tems. General prin­ciples for design. CEN Standard EN 954 – 1. 1996.

[14]   Functional safety of electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems – Part 2: Requirements for electrical/​electronic/​programmable elec­tron­ic safety-​related sys­tems. IEC Standard 61508 – 2. 2010.

[15]     Reliability Prediction of Electronic Equipment. Military Handbook MIL-​HDBK-​217F. 1991.

[16]     “IFA – Practical aids: Software-​Assistent SISTEMA: Safety Integrity – Software Tool for the Evaluation of Machine Applications”, Dguv​.de, 2017. [Online]. Available: http://​www​.dguv​.de/​i​f​a​/​p​r​a​x​i​s​h​i​l​f​e​n​/​p​r​a​c​t​i​c​a​l​-​s​o​l​u​t​i​o​n​s​-​m​a​c​h​i​n​e​-​s​a​f​e​t​y​/​s​o​f​t​w​a​r​e​-​s​i​s​t​e​m​a​/​i​n​d​e​x​.​jsp. [Accessed: 30- Jan- 2017].

[17]      “fail­ure mode”, 192 – 03-​17, International Electrotechnical Vocabulary. IEC International Electrotechnical Commission, Geneva, 2015.

[18]      M. Gentile and A. E. Summers, “Common Cause Failure: How Do You Manage Them?,” Process Saf. Prog., vol. 25, no. 4, pp. 331 – 338, 2006.

[19]     Out of Control — Why con­trol sys­tems go wrong and how to pre­vent fail­ure, 2nd ed. Richmond, Surrey, UK: HSE Health and Safety Executive, 2003.

[20]     Safeguarding of Machinery. 3rd Edition. CSA Standard Z432. 2016.

[21]     O. Reg. 851, INDUSTRIAL ESTABLISHMENTS. Ontario, Canada, 1990.

[22]     “Field-​programmable gate array”, En​.wiki​pe​dia​.org, 2017. [Online]. Available: https://​en​.wiki​pe​dia​.org/​w​i​k​i​/​F​i​e​l​d​-​p​r​o​g​r​a​m​m​a​b​l​e​_​g​a​t​e​_​a​r​ray. [Accessed: 16-​Jun-​2017].

[23]     Analysis tech­niques for sys­tem reli­ab­il­ity – Procedure for fail­ure mode and effects ana­lys­is (FMEA). 2nd Ed. IEC Standard 60812. 2006.

[24]     “Failure mode and effects ana­lys­is”, En​.wiki​pe​dia​.org, 2017. [Online]. Available: https://​en​.wiki​pe​dia​.org/​w​i​k​i​/​F​a​i​l​u​r​e​_​m​o​d​e​_​a​n​d​_​e​f​f​e​c​t​s​_​a​n​a​l​y​sis. [Accessed: 16-​Jun-​2017].