ISO 13849-1 Analysis — Part 5: Diagnostic Coverage (DC)

This entry is part 5 of 6 in the series How to do a 13849-1 analysis

What is Diagnostic Coverage?

Understanding Diagnostic Coverage (DC) as it is used in ISO 13849-1 [1] is critical to analysing the design of any safety function assessed using this standard. In case you missed a previous part of the series, you can read it here.

In the last instalment of this series discussing MTTFD, I brought up the fact that everything fails eventually, and so everything has a natural failure rate. The bathtub curve shown at the top of this post shows a typical failure rate curve for most products. Failure rates tell you the average time (or sometimes the mean time) it takes for components or systems to fail. Failure rates are expressed in many ways, MTTFD and PFHd being the ways relevant to this discussion of ISO 13849 analysis. MTTFis given in years, and PFHd is given in fractional hours (1/h). As a reminder, PFHd stands for “Probability of dangerous Failure per Hour”.

Three of the standard architectures include automatic diagnostic functions, Categories 2, 3 and 4. As soon as we add diagnostics to the system, we need to know what faults the diagnostics can detect and how many of the dangerous failures relative to the total number of failures that represents. Diagnostic Coverage (DC) represents the ratio of dangerous failures that can be detected to the total dangerous failures that could occur, expressed as a percentage. There will be some failures that do not result in a dangerous failure, and those failures are excluded from DC because we don’t need to worry about them – if they occur, the system will not fail into a dangerous state.

Here’s the formal definition from [1]:

3.1.26 diagnostic coverage (DC)

measure of the effectiveness of diagnostics, which may be determined as the ratio between the failure rate of detected dangerous failures and the failure rate of total dangerous failures

Note 1 to entry: Diagnostic coverage can exist for the whole or parts of a safety-related system. For example, diagnostic coverage could exist for sensors and/or logic system and/or final elements. [SOURCE: IEC 61508-4:1998, 3.8.6, modified.]

That brings up two other related definitions that need to be kept in mind [1]:

3.1.4 failure

termination of the ability of an item to perform a required function

Note 1 to entry: After a failure, the item has a fault.

Note 2 to entry: “Failure” is an event, as distinguished from “fault”, which is a state.

Note 3 to entry: The concept as defined does not apply to items consisting of software only.

Note 4 to entry: Failures which only affect the availability of the process under control are outside of the scope of this part of ISO 13849. [SOURCE: IEC 60050–191:1990, 04-01.]

and the most important one [1]:

3.1.5 dangerous failure

failure which has the potential to put the SRP/CS in a hazardous or fail-to-function state

Note 1 to entry: Whether or not the potential is realized can depend on the channel architecture of the system; in redundant systems a dangerous hardware failure is less likely to lead to the overall dangerous or fail-to- function state.

Note 2 to entry: [SOURCE: IEC 61508–4, 3.6.7, modified.]

Just as a reminder, SRP/CS stands for “safety-related parts of control systems”.

Failure Math

Failure Rate Data Sources

To do any calculations, we need data, and this is true for failure rates as well. ISO 13849-1 provides some tables in the annexes that list some common types of components and their associated failure rates, and there are more failure rate tables in ISO 13849-2. A word of caution here: Do not mix sources of failure rate data, as the conditions under which that data is true won’t match the data in ISO 13849. There are a few good sources of failure rate data out there, for example, MIL-HDBK-217, Reliability Prediction of Electronic Equipment [15], as well as the database maintained by Exida. In any case, use a single source for your failure rate data.

Failure Rate Variables

IEC 61508 [7] defines a number of variables related to failure rates. The lowercase Greek letter lambda, $\lambda$, is used to denote failures.

The common variable designations used are:

$\lambda$ = failures
$\lambda_{(t)}$= failure rate
$\lambda_s$ = “safe” failures
$\lambda_d$ = “dangerous” failures
$\lambda_{dd}$ = detectable “dangerous” failures
$\lambda_{du}$ = undetectable “dangerous” failures

Calculating DC

Of these variables, we only need to concern ourselves with $\lambda_d$, $\lambda_{dd}$ and $\lambda_{du}$. To understand how these variables are used, we can express their relationship as

$\lambda_d=\lambda_{dd}+\lambda_{du}$

Following on that idea, the Diagnostic Coverage can be expressed as a percentage like this:

$DC\%=\frac{\lambda_{dd}}{\lambda_d}\times 100$

Determining DC%

If you want to actually calculate DC%, you have some work ahead of you. Rather than going into the details here, I am going to refer you hardcore types to IEC 61508-2, Functional safety of electrical/electronic/programmable electronic safety-related systems – Part 2: Requirements for electrical/electronic/programmable electronic safety-related systems. This standard goes into some depth on how to determine failure rates and how to calculate the “Safe Failure Fraction,” a number which is related to DC but is not the same.

For everyone else, the good news is that you can use the table in Annex E to estimate the DC%. It’s worth noting here that Annex E is “Informative.” In standards-speak, this means that the information in the annex is not part of the “normative” text, which means that it is simply information to help you use the normative part of the standard. The design must conform to the requirements in the normative text if you want to claim conformity to the standard. The fact that [1, Annex E] is informative gives you the option to calculate the DC% value rather than selecting it from Table E.1. Using the calculated value would not violate the requirements in the normative text.

If you are using IFA SISTEMA [16] to do the calculations for you, you will find that the software limits you to selecting a single DC measure from Table E.1, and this principle applies if you are doing the calculations by hand too. Only one item from Table E.1 can be selected for a given safety function.

Ranking DC

Once you have determined the DC for a safety function, you need to compare the DC value against [1, Table 5] to see if the DC is sufficient for the PLr you are trying to achieve. Table 5 bins the DC results into four ranges. Just like binning the PFHd values into five ranges helps to prevent precision bias in estimating the probability of failure of the complete system or safety function, the ranges in Table 5 helps to prevent precision bias in the calculated or selected DC values.

If the DC value was high enough for the PLr, then you are done with this part of the work. If not, you will need to go back to your design and add additional diagnostic features so that you can either select a higher coverage from [1, Table E.1] or calculate a higher value using [14].

Multiple safety functions

When you have multiple safety functions that make up a complete safety system, for example, an emergency stop function and a guard interlocking function, the DC values need to be averaged to determine the overall DC for the complete system. [1, Annex E] provides you with a method to do this in Equation E.1.

Plug in the values for MTTFD and DC for each safety function, and calculate the resulting DCavg value for the complete system.

That’s it for this article. The next part will cover Common Cause Failures (CCF). Look for it on 20-Mar-17!

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:

[0.2]  Electromagnetic Compatibility for Functional Safety, 1st ed. Stevenage, UK: The Institution of Engineering and Technology, 2008.

References

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.

[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/ifa/praxishilfen/practical-solutions-machine-safety/software-sistema/index.jsp. [Accessed: 30- Jan- 2017].

ISO 13849-1 Analysis — Part 4: MTTFD – Mean Time to Dangerous Failure

This entry is part 4 of 6 in the series How to do a 13849-1 analysis

Functional safety is all about the likelihood of a safety system failing to operate when you need it. Understanding Mean Time to Dangerous Failure, or MTTFD, is critical. If you have been reading about this topic at all, you may notice that I am abbreviating Mean Time to Dangerous Failure with all capital letters. Using MTTFD is a recent change that occurred in the third edition of ISO 13849-1, published in 2015. In the first and second editions, the correct abbreviation was MTTFd. Onward!

If you missed the third instalment in this series, you can read it here.

Defining MTTFD

Let’s start by having a look at some key definitions. Looking at [1, Cl. 3], you will find:

3.1.1 safety–related part of a control system (SRP/CS)—part of a control system that responds to safety-related input signals and generates safety-related
output signals

Note 1 to entry: The combined safety-related parts of a control system start at the point where the safety-related input signals are initiated (including, for example, the actuating cam and the roller of the position switch) and end at the output of the power control elements (including, for example, the main contacts of a contactor)

Note 2 to entry: If monitoring systems are used for diagnostics, they are also considered as SRP/CS.

3.1.5 dangerous failure—failure which has the potential to put the SRP/CS in a hazardous or fail-to-function state

Note 1 to entry: Whether or not the potential is realized can depend on the channel architecture of the system;
in redundant systems a dangerous hardware failure is less likely to lead to the overall dangerous or fail-tofunction
state.

Note 2 to entry: [SOURCE: IEC 61508–4, 3.6.7, modified.]

3.1.25 mean time to dangerous failure (MTTFD)—expectation of the mean time to dangerous failure

Definition 3.1.5 is pretty helpful, but definition 3.1.25 is, well, not much of a definition. Let’s look at this another way.

Failures and Faults

Since everything can and will eventually fail to perform the way we expect it to, we know that everything has a failure rate because everything takes some time to fail. Granted that this time may be very short, like the first time the unit is turned on, or it may be very long, sometimes hundreds of years. Remember that because this is a rate, it is something that occurs over time. It is also important to be clear that we are talking about failures and not faults. Reading from [1]:

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.]

3.1.4 failure— termination of the ability of an item to perform a required function

Note 1 to entry: After a failure, the item has a fault.

Note 2 to entry: “Failure” is an event, as distinguished from “fault”, which is a state.

Note 3 to entry: The concept as defined does not apply to items consisting of software only.

Note 4 to entry: Failures which only affect the availability of the process under control are outside of the scope of this part of ISO 13849.
[SOURCE: IEC 60050–191:1990, 04-01.]

3.1.4 Note 2 is the important one at this point in the discussion.

Now, where we have multiples of something, like relays, valves, or safety systems, we now have a population of identical items, each of which will eventually fail at some point. We can count those failures as they occur and tally them up, and we can graph how many failures we get in the population over time. If this is starting to sound suspiciously like statistics to you, that is because it is.

OK, so let’s look at the kinds of failures that occur in that population. Some failures will result in a “safe” state, e.g., a relay failing with all poles open, and some will fail in a potentially “dangerous” state, like a normally closed valve developing a significant leak. If we tally up all the failures that occur, and then tally the number of “safe” failures and the number of “dangerous” failures in that population, we now have some very useful information.

The different kinds of failures are signified using the lowercase Greek letter $\lambda$ (lambda). We can add some subscripts to help identify what kinds of failures we are talking about. The common variable designations used are [14]:

$\lambda$ = failures
$\lambda_{(t)}$= failure rate
$\lambda_s$ = “safe” failures
$\lambda_d$ = “dangerous” failures
$\lambda_{dd}$ = detectable “dangerous” failures
$\lambda_{du}$ = undetectable “dangerous” failures

I will be discussing some of these variables in more detail in a later part of the series when I delve into Diagnostic Coverage, so don’t worry about them too much just yet.

Getting to MTTFD

Since we can now start to deal with the failure rate data mathematically, we can start to do some calculations about expected lifetime of a component or a system. That expected, or probable, lifetime is what definition 3.1.25 was on about, and is what we call MTTFD.

MTTFD is the time in years over which the probability of failure is relatively constant. If you look at a typical failure rate curve, called a “bathtub curve” due to its resemblance to the profile of a nice soaker tub, the MTTFD is the flatter portion of the curve between the end of the infant mortality period and the wear-out period at the end of life. This part of the curve is the portion assumed to be included in the “mission time” for the product. ISO 13849-1 assumes the mission time for all machinery is 20 years [1, 4.5.4] and [1, Cl. 10].

ISO 13849-1 provides us with guidance on how MTTFD relates to the determination of the PL in [1, Cl. 4.5.2]. MTTFD is further grouped into three bands as shown in [1, Table 4].

The notes for this table are important as well. Since you can’t read the notes particularly well in the table above, I’ve reproduced them here:

NOTE 1 The choice of the MTTFD ranges of each channel is based on failure rates found in the field as state-of-the-art, forming a kind of logarithmic scale fitting to the logarithmic PL scale. An MTTFD value of each channel less than three years is not expected to be found for real SRP/CS since this would mean that after one year about 30 % of all systems on the market will fail and will need to be replaced. An MTTFD value of each channel greater than 100 years is not acceptable because SRP/CS for high risks should not depend on the reliability of components alone. To reinforce the SRP/CS against systematic and random failure, additional means such as redundancy and testing should be required. To be practicable, the number of ranges was restricted to three. The limitation of MTTFD of each channel values to a maximum of 100 years refers to the single channel of the SRP/CS which carries out the safety function. Higher MTTFD values can be used for single components (see Table D.1).

NOTE 2 The indicated borders of this table are assumed within an accuracy of 5%.

The standard then tells us to select the MTTFD using a simple hierarchy [1, 4.5.2]:

For the estimation ofMTTFD of a component, the hierarchical procedure for finding data shall be, in the order given:

a) use manufacturer’s data;
b) use methods in Annex C and Annex D;
c) choose 10 years.

Why ten years? Ten years is half of the assumed mission lifetime of 20 years. More on mission lifetime in a later post.

Looking at [1, Annex C.2], you will find the “Good Engineering Practices” method for estimating MTTFD, presuming the manufacturer has not provided you with that information. ISO 13849-2 [2] has some reference tables that provide some general MTTFD values for some kinds of components, but not every part that exists can be listed. How can we deal with parts not listed? [1, Annex C.4] provides us with a calculation method for estimating MTTFD for pneumatic, mechanical and electromechanical components.

Calculating MTTFD for pneumatic, mechanical and electromechanical components

I need to introduce you to a few more variables before we look at how to calculate MTTFD for a component.

Variables
Variable Description
B10 Number of cycles until 10% of the components fail (for pneumatic and electromechanical components)
B10D Number of cycles until 10% of the components fail dangerously (for pneumatic and electromechanical components)
T10D the mean time until 10% of the components fail dangerously
hop is the mean operation time, in hours per day;
dop is the mean operation time, in days per year;
tcycle is the mean operation time between the beginning of two successive cycles of the component. (e.g., switching of a valve) in seconds per cycle.
s seconds
h hours
a years

Knowing a few details we can calculate the MTTFD using [1, Eqn C.1]. We need to know the following parameters for the application:

• B10D
• hop
• dop
• tcycle

In order to use [1, Eqn. C.1], we need to first calculate nop, using [1, Eqn. C.2]:

We may also need one more calculation, [1, Eqn. C.4]:

Example Calculation [1, C.4.3]

For a pneumatic valve, a manufacturer determines a mean value of 60 million cycles as B10D. The valve is used for two shifts each day on 220 operation days a year. The mean time between the beginning of two successive switching of the valve is estimated as 5 s. This yields the following values:

• dop of 220 days per year;
• hop of 16 h per day;
• tcycle of 5 s per cycle;
• B10D of 60 million cycles.

Doing the math, we get:

So there you have it, at least for a fairly simple case. There are more examples in ISO 13849-1, and I would encourage you to work through them. You can also find a wealth of examples in a report produced by the BGIA in Germany, called the Functional safety of machine controls (BGIA Report 2/2008e) [16]. The download for the report is linked from the reference list at the end of this article. If you are a SISTEMA user, there are lots of examples in the SISTEMA Cookbooks, and there are example files available so that you can see how to assemble the systems in the software.

The next part of this series covers Diagnostic Coverage (DC), and the average DC for multiple safety functions in a system, DCavg.

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:

[0.2]  Electromagnetic Compatibility for Functional Safety, 1st ed. Stevenage, UK: The Institution of Engineering and Technology, 2008.

References

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.

[15]    “The bathtub curve and product failure behavior part 1 of 2”, Findchart.co, 2017. [Online]. Available: http://findchart.co/download.php?aHR0cDovL3d3dy53ZWlidWxsLmNvbS9ob3R3aXJlL2lzc3VlMjEvaHQyMV8xLmdpZg. [Accessed: 03- Jan- 2017].

[16]   “Functional safety of machine controls – Application of EN ISO 13849 (BGIA Report 2/2008e)”, dguv.de, 2017. [Online]. Available: http://www.dguv.de/ifa/publikationen/reports-download/bgia-reports-2007-bis-2008/bgia-report-2-2008/index-2.jsp. [Accessed: 2017-01-04].

Acknowledgements: IEC, ISO and others as cited
Some Rights Reserved

ISO 13849 Analysis — Part 3: Architectural Category Selection

This entry is part 3 of 6 in the series How to do a 13849-1 analysis

At this point, you have completed the risk assessment, assigned required Performance Levels to each safety function, and developed the Safety Requirement Specification for each safety function. Next, you need to consider three aspects of the system design: Architectural Category, Channel Mean Time to Dangerous Failure (MTTFD), and Diagnostic Coverage (DCavg). In this part of the series, I am going to discuss selecting the architectural category for the system.

If you missed the second instalment in this series, you can read it here.

Understanding Performance Levels

To understand ISO 13849-1, it helps to know a little about where the standard originated. ISO 13849-1 is a simplified method for determining the reliability of safety-related controls for machinery. The basic ideas came from IEC 61508 [7], a seven-part standard originally published in 1998. IEC 61508 brought forward the concept of the Average Probability of Dangerous Failure per Hour, PFHD (1/h). Dangerous failures are those failures that result in non-performance of the safety function, and which cannot be detected by diagnostics. Here’s the formal definition from [1]:

3.1.5

dangerous failure
failure which has the potential to put the SRP/CS in a hazardous or fail-to-function state

Note 1 to entry: Whether or not the potential is realised can depend on the channel architecture of the system; in redundant systems a dangerous hardware failure is less likely to lead to the overall dangerous or fail-to-function state.

Note 2 to entry: [SOURCE: IEC 61508–4, 3.6.7, modified.]

The Performance Levels are simply bands of probabilities of Dangerous Failures, as shown in [1, Table 2] below.

The ranges shown in [1, Table 2] are approximate. If you need to see the specific limits of the bands for any reason, see [1, Annex K] describes the full span of PFHD, in table format.

There is another way to describe the same characteristics of a system, this one from IEC. Instead of using the PL system, IEC uses Safety Integrity Levels (SILs). [1, Table 3] shows the correspondence between PLs and SILs. Note that the correspondence is not exact. Where the calculated PFHd is close to either end of one of the PL or SIL bands, use the table in [1, Annex K] or in [9] to determine to which band(s) the performance should be assigned.

IEC produced a Technical Report [10] that provides guidance on how to use ISO 13849-1 or IEC 62061. The following table shows the relationship between PLs, PFHd and SILs.

IEC 61508 includes SIL 4, which is not shown in [10, Table 1] because this level of performance exceeds the range of PFHD possible using ISO 13849-1 techniques. Also, you may have noticed that PLb and PLc are both within SIL1. This was done to accommodate the five architectural categories that came from EN 954-1 [12].

Why PL and not just PFHD? One of the odd things that humans do when we can calculate things is the development of what has been called “precision bias” [12]. Precision bias occurs when we can compute a number that appears very precise, e.g., 3.2 x 10-6, which then makes us feel like we have a very precise concept of the quantity. The problem, at least in this case, is that we are dealing with probabilities and minuscule probabilities at that. Using bands, like the PLs, forces us to “bin” these apparently precise numbers into larger groups, eliminating the effects of precision bias in the evaluation of the systems. Eliminating precision bias is the same reason that IEC 61508 uses SILs – binning the calculated values helps to reduce our tendency to develop a precision bias. The reality is that we just can’t predict the behaviour of these systems with as much precision as we would like to believe.

Getting to Performance Levels: MTTFD, Architectural Category and DC

Some aspects of the system design need to be considered to arrive at a Performance Level or make a prediction about failure rates in terms of PFHd.

First is the system architecture: Fundamentally, single channel or two channel. As a side note, if your system uses more than two channels there are ways to handle this in ISO 13849-1 that are workarounds, or you can use IEC 62061 or IEC 61508, either of which will handle these more complex systems more easily. Remember, ISO 13849-1 is intended for relatively simple systems.

When we get into the analysis in a later article, we will be calculating or estimating the Mean Time to Dangerous Failure, MTTFD, of each channel, and then of the entire system. MTTFD is expressed in years, unlike PFHd, which is expressed in fractional hours (1/h). I have yet to hear why this is the case as it seems rather confusing. However, that is current practice.

Architectural Categories

Once the required PL is known, the next step is the selection of the architectural category. The basic architectural categories were introduced initially in EN 954-1:1996 [12].  The Categories were carried forward unchanged into the first edition of ISO 13849-1 in 1999. The Categories were maintained and expanded to include additional requirements in the second and third editions in 2005 and 2015.

Since I have explored the details of the architectures in a previous series, I am not going to repeat that here. Instead, I will refer you to that series. The architectural Categories come in five flavours:

Architecture Basics
Category Structure Basic Requirements Safety Princple
For full requirements, see [1, Cl. 6]
B Single channel Basic circuit conditions are met (i.e., components are rated for the circuit voltage and current, etc.) Use of components that are designed and built to the relevant component standards. [1, 6.2.3] Component selection
1 Single channel Category B plus the use of “well-tried components” and “well-tried safety principles” [1, 6.2.4] Component selection
2 Single channel Category B plus the use of “well-tried safety principles” and periodic testing [1, 4.5.4] of the safety function by the machine control system. [1, 6.2.5] System Structure
3 Dual channel Category B plus the use of “well-tried safety principles” and no single fault shall lead to the loss of the safety function.

Where practicable, single faults shall be detected. [1, 6.2.6]

System Structure
4 Dual channel Category B plus the use of “well-tried safety principles” and no single fault shall lead to the loss of the safety function.

Single faults are detected at or before the next demand on the safety system, but where this is not possible an accumulation of undetected faults will not lead to the loss of the safety function. [1, 6.2.7]

System Structure

[1, Table 10] provides a more detailed summary of the requirements than the summary table above provides.

Since the Categories cannot all achieve the same reliability, the PL and the Categories are linked as shown in [1, Fig. 5]. This diagram summarises te relationship of the three central parameters in ISO 13849-1 in one illustration.

Starting with the PLr from the Safety Requirement Specification for the first safety function, you can use Fig. 5 to help you select the Category and other parameters necessary for the design. For example, suppose that the risk assessment indicates that an emergency stop system is needed. ISO 13850 requires that emergency stop functions provide a minimum of PLc, so using this as the basis you can look at the vertical axis in the diagram to find PLc, and then read across the figure. You will see that PLc can be achieved using Category 1, 2, or 3 architecture, each with corresponding differences in MTTFD and DCavg. For example:

• Cat. 1, MTTFD = high and DCavg = none, or
• Cat. 2, MTTFD = Medium to High and DCavg = Low to Medium, or
• Cat. 3, MTTFD = Low to High and DCavg = Low to Medium.

As you can see, the MTTFD in the channels decreases as the diagnostic coverage increases. The design compensates for lower reliability in the components by increasing the diagnostic coverage and adding redundancy. Using [1, Fig. 5] you can pin down any of the parameters and then select the others as appropriate.

One additional point regarding Category 3 and 4: The difference between these Categories is increased Diagnostic Coverage. While Category 3 is Single Fault Tolerant, Category 4 has additional diagnostic capabilities so that additional faults cannot lead to the loss of the safety function. This is not the same as being multiple fault tolerant, as the system is still designed to operate in the presence of only a single fault, it is simply enhanced diagnostic capability.

It is worth noting that ISO 13849 only recognises structures with single or dual channel configurations. If you need to develop a system with more than single redundancy (i.e., more than two channels), you can analyse each pair of channels as a dual channel architecture, or you can move to using IEC 62061 or IEC 61508, either of which permits any level of redundancy.

The next step in this process is the evaluation of the component and channel MTTFD, and then the determination of the complete system MTTFD. Part 4 of this series publishes on 13-Feb-17.

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:

[0.2]  Electromagnetic Compatibility for Functional Safety, 1st ed. Stevenage, UK: The Institution of Engineering and Technology, 2008.

References

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. ISO Standard 13849-1. 2015.

[7]     Functional safety of electrical/electronic/programmable electronic safety-related systems. IEC Standard 61508. 2nd Edition. Seven Parts. 2010.