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Entries tagged with: PFDavg

Are You Ready For What’s Coming?

Are You Ready For What’s Coming?

If you’re like me then you’ve been waiting for the new draft of IEC 61511 to be officially issued since its release has been delayed for some time.  However, a draft has been released and can help in preparing end users for what’s to come.  The question…

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Back to Basics 11 – How is SIL Used by an End User?

Back to Basics 11 – How is SIL Used by an End User?

Back to Basics 16 - PFDavg

Back to Basics 16 - PFDavg

PFDavg (the average Probability of Failure on Demand) is the probability that a system will fail dangerously, and not be able to perform its safety function when required. PFDavg can be determined as an average probability or maximum probability over a time period. IEC 61508 and IEC…

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Back to Basics: Failure Rates - FIT

Back to Basics: Failure Rates - FIT

Failures ITime or Failure UnIT

FIT is the number of failures per billion hours for a piece of equipment. 

It is mentioned in both IEC 61508 and IEC 61511 standards as a preferred unit of measurement expressed by 109 hours.

Example: 5 FIT is expressed as 5 failures within 109 hours . 

When you…

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Back to Basics: Failure Rates - λ

Back to Basics: Failure Rates - λ

The Greek symbol lambda, λ, represents failure rates in functional safety, usually expressed in the unit of measurement of FITS.

λ can be expressed as a total failure rate for a device (λT), or it can be broken down into more specific groupings:

  • Safe detected (λSD)
  • Safe undetected (λSU)
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Back to Basics: Failure Rates - λD

Back to Basics: Failure Rates - λD

The Greek symbol  λD represents dangerous failure rates in functional safety, usually expressed in the unit of measurement of FITs, and can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).

λD is the number of dangerous failures per…

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Back to Basics: Failure Rates - λDD

Back to Basics: Failure Rates - λDD

The Greek symbol  λDD is the detectable dangerous failure rate in functional safety expressed in the unit of measurement of FITs which can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).

Lambda DD

λDD is the number of…

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Back to Basics: Failure Rates - λDU

Back to Basics: Failure Rates - λDU

The Greek symbol  λDU is the undetectable dangerous failure rate in functional safety expressed in the unit of measurement of FITs which can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).

λDU is the number of dangerous undetected failures…

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Back to Basics: Failure Rates - λS

Back to Basics: Failure Rates - λS

The Greek symbol  λS represents safe or spurious failure rates in functional safety expressed in the unit of measurement of FITs which can be determined through FMEDAs. (FITs (λ) are failures per billion hours, expressed by 10-9 hours).

λS is the number of safe…

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  • by Dr. William Goble, CFSE
  • Thursday, May 12, 2011
  • Certification

Buy Certified Equipment, Meet the Standard, Right? WRONG!

Certified to SIL 4 - Cycle Test Failure Data is Dangerous

I got a copy of the IEC 61508 certificate for a solenoid valve today from an engineer who thought something was wrong.  Although the certificate was from a well known certification company, the certificate gave a “Dangerous Failure Rate” of 1.7 FITS (1.7 * 10-9 failures per…

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Do you believe the numbers?

In a couple of recent projects and discussions, I have come across something extremely concerning. Engineers are very good at performing accurate calculations, and the PFDavg and PFH computations for SIL performance verification are improving in precision all the time. Unfortunately, there is often such a focus…

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Getting Good Proof Test Coverage Numbers

Getting Good Proof Test Coverage Numbers

Several years ago we recognized that proof test coverage was an important variable that must be considered when doing PFDavg calculations. We ran some models and discovered that the difference between “perfect” proof test coverage (100%) and a very good 90% could mean a whole SIL

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How can I improve my SIL?

How can I improve my SIL?

Click here to read the first entry in this blog series (What is SIL compliance?)

Click here to read the second entry in this blog series (How is SIL Used?)

As we now know, a Safety Integrity Level (SIL) can only be given to a…

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How to Calculate Proof Test Coverage
  • by Dr. William Goble, CFSE
  • Thursday, October 16, 2014
  • Certification

How to Calculate Proof Test Coverage

Most engineers who design and verify safety instrumented functions (SIFs) understand how hard it is to design a manual proof test with high effectiveness (also called high proof test coverage). Those folks who understand that a proof test is not likely to detect all failures never use simplified equations…

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How to Decode an exida IEC 61508 Certificate: Part 2

Click here to read Part 1

Now that you know the step-by-step process of product certification, now let’s take a look at the actual information on the certificate. 

In the left panel (gray background) you will find an exida certification logo. …

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IEC61511 and Failure Rates

IEC61511 and Failure Rates

Most end users I teach in our FSE100 class are not really aware of or fully, understand when we talk about failure rates.  For example, what’s a FIT mean?  For those end users more versed in this, they understand what is meant by a FIT.  Essentially a device can have…

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My Proof Test Coverage is Better Than Yours!

NEVER Use B10 Values for PFDavg Calculations

The machine safety community has defined a number called the “B10 number.”  It is a measure of time where 10% of a population of devices should have failed.  Generally it is a measure of expected end of life or “useful life” as defined by the reliability engineering community.

The…

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OREDA Data vs. FMEDA Data