I saw and responded to a LinkedIn discussion on this very issue, where someone had asked “if I have a misaligned limit switch that fails dangerously, then is it random or systematic? “. This is an intriguing question because many view human error as being systematic and, whereas, this…
exida explains Blog
Entries tagged with: PFDavg
- by Steve Gandy, CFSP
- Thursday, October 19, 2017
- Functional Safety
Random versus Systematic Faults: What’s the difference?
- by Ted Stewart, CFSP, exidaCSP
- Friday, February 19, 2016
- Certification
Site Safety Index IMPACT on Failure Rates and PFDavg
Performing site audits for numerous customers can be very telling. All sites have their own unique layout, procedures, documentation, and people. Did you know that failure rates differ from site to site? Are you surprised? According to our recent webinar results, the majority of people aren’t, and in fact…
- by Dr. William Goble, CFSE
- Monday, April 11, 2016
- Certification
Site Safety Index™ (SSI) – A Simple Model for Site Variations in Safety
In several of our technical papers over the years, we have noticed that failure rates for the manufacturer, model number are different at different sites, even when the processes were quite similar. When I have mentioned this to end users and manufacturers, often the response is “Yes, I have…
- by Iwan van Beurden, CFSE
- Thursday, August 23, 2012
- Functional Safety
Super-Duper Logic Solver Data; So what?
I am working on a SIL verification project and just had to share some observations.
For this project, the customer decided to choose a logic solver for which no data is listed in the exida Safety Equipment Reliability Handbook (SERH). One could…
- by Denise Chastain Knight, P.E., CFSE, CCPSC
- Thursday, August 27, 2015
- Certification
The Architectural Constraint Blind Side
I did my homework, purchased certified devices, and specified physical redundancy. I expected an uneventful SIL Verification but the assessor is telling me that I have functions failing Architectural Constraints in the sensor and final element groups. How can that be?
Low demand mode Safety Instrumented Function (SIF)…
- by Steve Gandy, CFSP
- Tuesday, August 15, 2023
- Functional Safety
The Importance Of Considering Common Cause with Redundant Systems
Common Cause is one of those variables that is not always considered important when it comes to SIL compliance. And yet, for redundant systems, it can have a significant impact on the PFDavg and Risk Reduction achieved (in Low Demand mode) or PFH (in high or continuous mode). Most of the…
- by Dr. William Goble, CFSE
- Wednesday, February 27, 2013
- Software
The Real PFDavg Variables
I was recently asked how the PFDavg value in an exida FMEDA report was calculated. Those PFDavg values are calculated using the exSILentiaTM program. The person who posed the question was using the equations from Part 6 of IEC 61508 and got a different value. Why?
- by Dr. William Goble, CFSE
- Tuesday, June 07, 2011
- Certification
The Weakest Link
One of the objectives of new performance based standards like IEC 61508 is to allow performance metrics (like PFDavg for low demand systems) to identify the weak links in any proposed design. Obviously, it is not necessary to put effort into design and spend money on areas…
- by Dr. William Goble, CFSE
- Thursday, October 13, 2011
- Functional Safety
Two Billion Hours
When I studied Reliability Engineering in graduate school, I was taught that failure rate data came from field failure studies. I assumed that operating companies always kept accurate records when a piece of equipment failed. I also assumed that someone would investigate the failures and assign a root cause.…
- by Dr. William Goble, CFSE
- Monday, October 29, 2018
- Certification
Using SILSafeData to Check Your Failure Data Source
www.SILSafeData.com is a complimentary resource that contains the upper and lower bounds failure rates for many categories of automation equipment, as well as the methodology used by exida to derive the numbers.
Many of you have asked how to…
- by Loren Stewart, CFSE
- Friday, September 18, 2015
- Functional Safety
What is SIL compliance?
The Safety Integrety Level (SIL) of a product is determined by three things:
- The Systematic Capability Rating
- The Architectural Constraints for the element
- The PFDavg Calculation for the product
The Systematic Capability Rating
Systematic Capability is established by having your quality management system audited per IEC 61508.…
- by Ted Stewart, CFSP, exidaCSP
- Thursday, January 14, 2016
- Certification
WHY are Failures Occurring at a Different Rate from Our Other Site?
Site audits have shown many differences in how things are done from site to site. This impacts many variables in a safety instrumented system!
A key metric for process industry designs is called average Probability of Failure on Demand (PFDavg). After several studies of many field failure and proof…
- by Steve Gandy, CFSP
- Thursday, June 22, 2017
- Functional Safety
Why Run-to-Fail Is Not A Good Maintenance Strategy For SIS
One of the fundamentals of the Probability of Failure on Demand (PFDavg) calculations in SIL verification analysis, is the concept of a constant failure-rate during useful life of the equipment. Probabilistic calculations assume that the failure rate of the devices used in a Safety Instrumented System (SIS)…
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