Page 43 - RCM - A practical Guide_V1

RCM - A Practical Guide

Criteria
For FF maintenance to be selected certain criteria must be met:
• The loss of function must be Hidden
• Predictive maintenance (OC) must not be appropriate
• Preventive maintenance (HT) must not be appropriate
• There must be a clearly defined failure condition
• The failure must be detectable
Testing with caution
If you’re asking whether or not you can test to identify failure it means that you’ve answered no to OC
and HT options which, in turn means, that the failure characteristics are to be unusable to predict or
too random to prevent. Usually we’re looking here at asset types following Pattern F of the six graphs.
When we’re trying to identify a failure, the last thing that we want is to induce other failures either
through unnecessary disturbance of equipment or excessively conservative frequencies. Remember,
these failures are random and that means that they may be just as likely to fail when you flick the
switch to test as they are during their normal day.
FF task interval
The interval for FF maintenance is related to a required availability of the function (decided by the
organisation) and to its reliability (mean time between failure (MTBF)). A FF task is only worth doing if
it achieves the required availability of the hidden function.
Relationship to availability
FF maintenance is trying to identify that a function is unavailable to allow the rectification and
restoration of that functionality. In that way, it is an attempting to ensure the required availability of
the function.
An organisation, as stated, usually decides the required availability for categories of failure
consequences. As a guide (taken from aviation):

Consequence type Required availability (%)

Safety, environmental or other important reason 99.5 - 99.99
Other 95

Once the required availability of a protective function is known it is possible to calculate the
acceptable unavailability. This acceptable unavailability figure is used in the calculation of the task
interval.
Acceptable availability (Utive) = 1 - Availability

E.g. for 99.5% availability, Utive = 1 - 99.5% = 0.5% (or 0.005).

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