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:
Failures In Time 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…
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…
The bow tie methodology, originally developed by ICI in the late 1970’s, has seen increasing industry adoption over the last 20 years. Bow tie diagrams build on the “Swiss Cheese Model” of hazard escalation and causation popularized by James Reason. They provide a powerful means for visualizing how a…
What is Stiction?
Stiction is the resistance to the start of motion usually measured as the difference between the external force being applied in order to overcome the static friction and the force to maintain movement between the two contacting or working surfaces.
It can…
You may be wondering why this question is being asked? Isn’t it obvious that systematic issues are important and need to be considered? It may be that some of you reading this blog may not even understand what is meant by systematic issues. In which case, it may surprise you to know…
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…
The “ISO 26262 Road vehicles – Functional safety” specification defines the concepts of “Independence”, “Interference” and other terms which can be a little confusing. This discussion will try to make the concepts clearer.
Let’s talk about “Independence” and associated concepts first. “Independence” is a concept used within Dependent…
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…
Safe Failure Fraction (SFF) is defined as the ratio of the average rate of safe failures plus dangerous detected failures of the subsystem to the total average failure rate of the subsystem. It is defined for a single channel (no redundancy, 1oo1).
It is a measurement of the likelihood of…
Route 2H is one of two Architectural constraints options made available in the standards IEC 61508-2 and IEC 61511. Route 1H . Both Route 1H and Route 2H are limitations that impose the hardware selected to implement a safety-instrumented function, regardless of the performance calculated for a subsystem.
What exactly is Route 2H…
Route 1H is one of two Architectural constraints options made available in the standards IEC 61508-2 and IEC 61511. Route 1H . Both Route 1H and Route 2H are limitations that impose the hardware selected to implement a safety-instrumented function, regardless of the performance calculated for a subsystem.
Route 1H is…
We all know that an FMEDA is only as good as the assumptions made regarding typical design engineering practices and the database used for the failure rates. Here is the catch, not all products are designed and manufactured per the same rules! Many manufacturers boast that their…
The question of whether companies follow good site safety practices is an interesting one. Firstly, what do we mean by good site safety practices? I think most companies believe they follow good site safety practices but do they really? Are they really measuring and tracking this?
The basis of having good site…
New reports out now are claiming that an aging, failed elbow pipe appears to be the initial cause of a June 21 fire and subsequent explosions that injured five people at the largest oil refinery on the East Coast, federal investigators said.
The U.S. Chemical Safety and Hazard Investigation…