Page 44 - RCM - A practical Guide_V1

RCM - A Practical Guide

Relationship to reliability
MTBF
The MTBF is, as the name suggests, the mean time between failures of an asset type. It’s an average.
To calculate it you need to know:
• How old is the asset type in terms of operation?
• How many assets of the asset type have failed?
Using the example of 25 item failures in 6500 asset type operating hours, the MTBF is calculated:
6500/25 = 260 operating hours
Calculating task intervals
Again, RCM is about choices, there are two equations that can be used to calculate a FF interval. The
RCM guidance and standard that you are following the study will doubtless detail how the formulae
came about - this handbook won’t go into detail regarding the origin of the formulae but, instead, will
introduce the topics and difficulties with carrying them out.
The availability method:

Usually used for those failure modes that are not contrary to safety (or anything else important).
What do we need to know?

• The acceptable unavailability of the protective function (Utive)
• The MTBF of the protective function (Mtive)

FFI = 2 X Utive X Mtive

The rigorous method:

Usually used for those failure modes that are considered contrary to safety (or anything else
important) because, as the name suggests, it applies a little more rigour to the calculation and takes
into account the probability of multiple failure.

What do we need to know?

• The MTBF of the protected function (Mted)
• The MTBF of the protective function (Mtive)
• The accepted probability of multiple failure (Mmf)
FFI = 2 X Mtive X Mted
Mmf

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