Maintenance Q&As

What are the dangers of using the mean time between failure (MTBF) metric?

Answered May 09 2019

The dangers of using the mean time between failure (MTBF) metric are in the detail – or lack thereof. Remember that the MTBF metric is taken by dividing the total uptime duration by the number of failures, or in formula form MTBF = total uptime / # of breakdowns. When MTBF is presented, since it is essentially an averaged value, you cannot instantly ascertain whether the metric is being influenced more by the uptime duration or by the number of breakdowns.

It is good practice to be mindful of the following aspects of using MTBF:

1) Across multiple assets
When using MTBF to characterize the reliability of multiple assets, a clear understanding of how MTBF values vary must be established. For example, given 10 electric motors that run for 10 hours each, experiencing a total of 10 breakdowns, will result in the MTBF as MTBF = [(10 motors)*(10 hours per motor)/10 breakdowns] = 10. Note that MTBF assumes that failure rates are constant. For this example, a perfect representation of MTBF would be if each of the 10 motors broke down once over the whole duration. However, the same MTBF = 10, would be obtained if only 1 of the motors broke down 10 times, which is a rather exaggerated example of how taking the average might not be reflective of the actual state.

2) Definition of breakdown
An important aspect of using MTBF as a metric, is being clear on what constitutes a “breakdown”. The definition should be clearly stated across all personnel so that the metric can be consistently measured. For example, will a nationwide blackout that causes stoppages be considered a breakdown? Do minor operator errors that cause less than a minute of stoppage be considered a breakdown? If the team agrees on the definition of a breakdown, then deviations can be averted as measurements are consistently carried out across the plant.

3) Definition of uptime
As the other component of the MTBF metric, the definition of uptime should also be stated clearly. Do warm-up and cool-down times count as part of uptime? Or would you only include time when production outputs are constant, i.e. steady-state operations?

MTBF used as a reliability metric
MTBF used as a reliability metric is a useful tool to gain insights if used correctly and consistently. The important caveat is that all components are clearly defined. Definitions and calculations should be properly aligned across the team.

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