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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Entries tagged with: Silsafe
- by Loren Stewart, CFSE
- Tuesday, November 19, 2019
- Functional Safety
Back to Basics 17 - PFH (Probability of dangerous Failure per Hour)
PFH (Probability of dangerous Failure per Hour) is the probability that a system will fail dangerously, and not be able to perform its safety function when required. PFH can be determined as a probability or maximum probability over a time period of an hour. IEC 61508 and
- by Loren Stewart, CFSE
- Tuesday, February 25, 2020
- Functional Safety
Back to Basics: Failure Rates
Failure rates are the number of failures per unit time for a piece of equipment which are usually assumed to be a constant value. They can be broken down into several categories, such as safe and dangerous, detected and undetected, and independent/normal and common cause. Failure rates are often…
- 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, October 21, 2021
- Functional Safety
FAQ: More Failures with Actuated Valves than what published failure rates indicate?
During a recent exida webinar we received the following question:
We tend to see more failures with actuated valves, than what manufacturers published failure rates would indicate! Any reason?
There are several reasons. Some manufacturers publish data based on field return data where they classify failures caused by customers…
- 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 Dr. William Goble, CFSE
- Thursday, September 19, 2019
- Functional Safety
My Final Element Field Failure Data says 150 FITS and OREDA is 2000 FITS
I got an email saying that the exida's www.SILSafeData.com minimum failure rates were way too high. The email went on to say that his REAL field failure data showed a result of 150 FITS for a remote actuated valve assembly. The lowest SILSafeData limits for clean service, Class IV leakage,…
- by Loren Stewart, CFSE
- Tuesday, December 22, 2015
- Functional Safety
SILSafe Data Website
exida has launched the web site www.silsafedata.com. SILSafe Data is a web site listing a number of product categories used in process control and the expected range of failure rates for process industry applications. The use of realistic and application appropriate failure rate data has a significant…
- 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 Dr. William Goble, CFSE
- Thursday, January 24, 2019
- Certification
Who gives exida the authority to publish SILSafeData?
SILSafeData contains upper and lower bound ranges of dangerous undetected failure rates for many automation devices. It was created by exida using statistical analysis of FMEDA failure rate predictions. The component database used in the FMEDAx tool is based on…