# exida explains Blog

## Back to Basics 01 - Functional Safety

In the following series of blogs, we'll go back to basics and run down everything you need to know to get started in functional safety.  We'll start with some more general terms and descriptions and make our way to more advanced material.

### 1. Functional Safety

Functional safety means the…

## Back to Basics 02 - Safety Integrity Level (SIL)

In the following series of blogs, we'll go back to basics and run down everything you need to know to get started in functional safety.  We'll start with some more general terms and descriptions and make our way to more advanced material.

## Back to Basics 03 - Safety Instrumented Function (SIF)

In the following series of blogs, we'll go back to basics and run down everything you need to know to get started in functional safety.  We'll start with some more general terms and descriptions and make our way to more advanced material.

### 3. SIF

SIF – Safety Instrumented Function…

## 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…

## Back to Basics 21 – The B10 Method

The B10 method uses cycle test data to predict failure rates.

A cycle test is done on a set of products (>20) until 10% of the units under test fail. The number of cycles until failure is called the B10 point.

The B10 number of cycles is converted to a…

## Back to Basics 22 – Cycle Testing

A cycle test is done on a set of products (>20) until 10% of the units under test fail.

The number of cycles is converted to a time period by knowing the cycles per hour in any particular application.

A failure rate is calculated by dividing the 10% failure…

## 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…

## Back to Basics: Failure Rates - FITS

Failures ITime or Failure UnIT

FITs 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 10-9 hours.

Example: 5 FITs is expressed as 5 failures every 10-9 hours (5x10-9).

When you…

## 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)

## 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…

## 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…

## 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…

## 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…

## Do You Really Know What You Know (When it Comes to Functional Safety)?

You don’t’ really know what you know until you have to explain it (or teach it) to someone else.

When I’m asked about some of the technical aspects of functional safety, I have to stop and ask myself “What Do I Know About This?”  I’m not the kind…

## Duty of Care (Too Good to Be True Failure Rate Data)

I have received several calls lately to our Australia / New Zealand office about whether it is acceptable to use published failure rates that seem too good to be true.

The person calling is usually doing a SIL verification calculation for an operating plant or for an…

• by Dr. William Goble, CFSE
• Tuesday, March 05, 2019
• Certification

## exida Failure Rates are Higher Than TÜV Italia. Who Is Right?

An owner-operator engineer compared the failure rates for two similar products from an exida certificate with those from a TÜV Italia certificate.  exida numbers were an order of magnitude higher.  Who is right?

There are some relevant fundamental facts in the field of Reliability Engineering:

1. Failure rate prediction methods depend…

## Field Data Validation with Predictive Analytics

Who cares about field failure data? Why are we even here?

### IEC 61511 – Fundamental Concepts

The fundamental concepts from our functional safety standards are the probabilistic performance based design.  Many of you know that this was terribly controversial when this was first proposed. Even to this day, there…

## 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…

## 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,…

## The exida FMEDA Process - Accurate Failure Data for the Process Industries

In this blog, I will talk about the FMEDA method and how it can generate realistically accurate failure rate data.

The first question we have to ask is “why do you need failure rate data ?”

One of the fundamental concepts in today’s functional safety standard, IEC 61508 and…

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