When people are discussing the frequency of predictive
(or condition-based) maintenance tasks, one often hears
either – sometimes both – of the following statements:
• it doesn't fail so often, so we don't need to check it so often
• we need to check more critical plant more often than less
critical plant.
In both cases, the speakers are wrong.
The frequency of predictive maintenance tasks has nothing to do with the frequency of failure and nothing to do with
the criticality of the item. The frequency of any form of
condition-based maintenance is based on the fact that most
failures do not occur instantaneously, and that it is often
possible to detect the fact that the failure is occurring
during the final stages of deterioration.