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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Entries tagged with: PFDavg
- by Loren Stewart, CFSE
- Wednesday, June 26, 2019
- Functional Safety
Back to Basics 11 – How is SIL Used by an End User?
IEC 61508 and IEC 61511 functional safety standards allow Safety Integrity Levels (SIL) to be used four ways by end users:
- To establish risk reduction requirements - How much risk needs/can be removed from the process to achieve a tolerable risk level? (For more…
- by Loren Stewart, CFSE
- Thursday, October 31, 2019
- Functional Safety
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…
- by Loren Stewart, CFSE
- Thursday, March 05, 2020
- Functional Safety
Back to Basics: Failure Rates - FIT
Failures In Time 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…
- by Loren Stewart, CFSE
- Thursday, March 19, 2020
- Functional Safety
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) …
- by Loren Stewart, CFSE
- Thursday, April 02, 2020
- Functional Safety
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…
- by Loren Stewart, CFSE
- Tuesday, June 30, 2020
- Functional Safety
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).
λDD is the number of…
- by Loren Stewart, CFSE
- Thursday, June 04, 2020
- Functional Safety
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…
- by Loren Stewart, CFSE
- Tuesday, August 18, 2020
- Functional Safety
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…
- by Dr. William Goble, CFSE
- Thursday, May 12, 2011
- Certification
Buy Certified Equipment, Meet the Standard, Right? WRONG!
While correcting a CFSP exam, I read a question which went something like this:
If you purchase all IEC 61508 certified equipment for a low demand safety instrumented function, then to meet the requirements of IEC 61511:
A. No other design verification is needed
B.…
- by Dr. William Goble, CFSE
- Thursday, April 07, 2011
- Functional Safety
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…
- by Dr. Eric Scharpf, CFSE
- Tuesday, October 25, 2011
- Functional Safety
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…
- by Dr. William Goble, CFSE
- Thursday, February 09, 2017
- Functional Safety
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…
- by Loren Stewart, CFSE
- Friday, September 25, 2015
- Functional Safety
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…
- 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…
- by Steve Close
- Thursday, October 04, 2012
- Certification
How to Decode an exida IEC 61508 Certificate: Part 2
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. …
- by Steve Gandy, CFSP
- Thursday, August 13, 2020
- Functional Safety
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…
- by Iwan van Beurden, CFSE
- Wednesday, October 23, 2013
- Functional Safety
My Proof Test Coverage is Better Than Yours!
Oh Really?
It must have been the week of the Proof Test Coverage (PTC) questions. In the latest marketing wars between vendors, the Proof Test Coverage has been used as a weapon. Who would have ever thought about using Proof Test Coverage to show that product A…
- by Dr. William Goble, CFSE
- Thursday, November 17, 2011
- Functional Safety
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…
- by Dr. William Goble, CFSE
- Tuesday, August 06, 2013
- Functional Safety
OREDA Data vs. FMEDA Data
A question was posed to me this week about why the OREDA failure rate for a solenoid valve was greater than the Failure Modes, Effects, and Diagnostic Analysis (FMEDA) results from exida for a specific manufacturer’s solenoid valve. The person asking the question was clearly challenging the exida…