Pointers on MTTF, MTBF, MTTFd and Availability

Some common questions have started coming up as customers increasingly design their safety systems to SIL or PL. Both of these design methods require reliability data on the components that make up the safety system. There are many acronyms floating around and some common misconceptions about their definitions, here are some explanations that may help you out:

MTTF – Mean Time To Failure
As the name suggests, this metric is the average time until a component fails, based on reliability data or testing results.

MTBF – Mean Time Between Failures
This metric is sometimes assumed to be equal to the MTTF. However the average time between failures also includes the MTTR (Mean Time To Repair) thus:
If the component has a very long expected life compared to the MTTR, then the MTTF and MTBF will be very similar.

The relationship between these values determines the availability of the component:
Availability = MTTF/MTBF
As availability approaches 1, the device is operational more. The smaller the MTTR, in relation to the life of the component, the closer the availability gets to an ideal value of 1.

What's the difference between MTTFd and MTTF?
So what about the value MTTFd? Is this the same as MTTF? The answer is no, MTTFd only considers dangerous failures of the component. 

For example: If an E-Stop contact needs to open to initiate a safe stop, MTTFd will only consider the failures that cause the contact to remain closed. However MTTF would consider failures that cause the contact to remain open or closed. 

In general, if you can source one of MTTFd or MTTF, but you require the other value, there is a relationship that can be used to calculate the metric you require:
Hopefully this clears up any confusion you have about theses reliability metrics.

Published: 19 August 2013