Created on October 21, 2016

It has been so long after the last time I published my Safety Buzz. In this occasion, I’d like to share you regarding AVAILABILITY term, which is commonly used by vendors (mainly for instrumentation). Since I was a process engineer, seems like I had a wrong concept about this. So here it is.

When you need to buy PLC, you will find vendors propose with 99% or 99.99% of availability. Something with endless of nine (9) sounds impressive. But, do we really will have a good PLC? What is actually this really means?

In the middle of preparation for EC-50 certification, I finally could find the answer in Safety Instrumented System – Design, Analysis, and Justification by Paul Gruhn. Here is the explanation.

“99.99% of availability could mean:

The system could have a nuisance trip once a month and be down for only 4.3 minutes


It could have a nuisance trip once a year and be down for 53 minutes


It could trip once every 10 years and be down for 88 hours.”

There is no exact meaning of availability. Then can we justify our PLC just from the availability data? The answer is exactly no. We actually just want to know how often it’s going to happen instead of how long it would take.

Then ISA SP84 committee introduced the new term of, the better term representing nuisance trip, which is Mean Time To Failure, spurious (MTTFsp).

Hopefully, this explanation gives you better understanding about AVAILABILITY.

Any comments or further discussion, don’t hesitate to contact me.

Let’s learn and grow together.

Ir. Ainun Margiansyah Putri, SFS
Process Safety Engineer


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