Page 34 - RCM - A practical Guide_V1

RCM - A Practical Guide

Pump 1 has failed for whatever reason, the machine will grind to halt, and most importantly not
perform its function. The operator will notice that (evident) and activate Pump 2.
But…how does the operator know that Pump 2 is in a working condition?
If Pump 1 continues to operate normally then the operator may never know that Pump 2 is sat in a
failed state, while going about their normal duties.
The functional failures that will not be noticed by the operator are deemed hidden.

Using the failure effects
We now know whether or not the operator will be aware of the loss of function but does that matter
to us (for whatever reason).
Invariably, RCM is employed in risk averse environments so the first option of this second step to
discovering in what way the failure matters, is usually to decide whether the failure is contrary to
safety. The question will be in two slightly different forms:

Evident - Does the failure cause a loss of function (or secondary damage)
that could have a direct adverse effect on safety?
Hidden - Does the failure by itself or in combination with another failure
have an adverse effect on safety?

(Or words to the effect of)
If there is a risk that any failure will affect safety RCM dictates that every effort must be made to
reduce the probability of occurrence consequences to an acceptable level.
Assessing whether there is an intolerable risk of the failure mode occurring is far more difficult. It boils
down to an organisations approach to risk. In most instances, any failure mode whose failure effects
identify risk of death or serious injury would be categorised as safety and addressed further by the
RCM process.
Other considerations
Most RCM decision diagrams will contain at least four task selection legs as a minimum. Two each for
both hidden and evident functional failures - 1 leg for those with safety consequences and the other
for those without safety consequences.
If there are more things than safety that are important to a business, then the tree can be tailored to
add additional task selection legs.
The figure below shows a simplified, tailored decision tree (evident side) where additional legs have
been added, in order, for different priorities.

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