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Understanding MTBF (Mean Time Between Failures)

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MTBF is defined as the point in time at which 1/e devices on average will still be operational (1/e is approximately 37%)

Furthermore, MTBF specifically excludes wear-out factors. A fan’s MTBF may be dozens of years but it will invariably wear-out in approximately 3 years. A power supply with an MTBF of 40,000 hours does not mean that the power supply should last for an average of 40,000 hours. According to the theory behind the statistics of confidence intervals, the statistical average becomes the true average as the number of samples increase. An MTBF of 40,000 hours, or 1 year for 1 module, becomes 40,000/2 for two modules and 40,000/4 for four modules.

Finally, MTBF does not directly tell what the expected lifetime of a unit is.
eg: MTBF 174 years for a device, the probability of survival is:
Time Probability of Survival
1 year 99.43%
5 years 97%
10 years 94%
20 years 89%
30 years 79%
..... .....
174 years 36.8%

Some customers are more interested in a B10 reliability rating (the time at which 90% of the units are expected to survive).
For a 174 year MTBF, the B10 rating is about 15 years.
B10 rating is a measure of time where 10% of a population of devices should have failed

In summary:
  • MTBF is not the reliability
  • MTBF does not include wear-out
  • MTBF is calculated at 40C with 50% electrical stress (for every 10C rise, lifetime is approximately halved)
  • MTBF is not accepted by all customers (some regard it as a meaningless number, both absolutely and relatively)
MTBF values what Digi provides for our devices will have Digi Quality Level & Historical Quality Level values and are derived using the Bellcore RQGR Method I Parts Count Method. The Quality Level is defined on the MTBF Help Page .
Digi Quality level is based on the components FIT(Failures In Time) information that is provided by the manufacturer or estimated by the Digi engineer based on the FIT value of similar parts.
Historical quality level is based on more empirical data on the components that have been observed over time.

Each component has a base FIT number – and with the optimal quality level (III), this FIT number can be used directly. At other (lower) quality levels, the FIT number is multiplied by a factory between 1 and 3 (thereby degrading the MTBF number).

For example, if a manufacturer simply purchases components on the open market, has no specific quality process (incoming inspection, vendor review, …), the FIT number would be multiplied by 3.

Historically, Digi used the worst case number (3 – associated with Quality Level 0). In reviewing Digi's quality process, we decided that the quality process in place (Digi does incoming inspection, regularly review vendors, etc), entitled us to use a different quality level. This resulted in an improvement in the calculated MTBF. Digi customers should use the ‘Digi Quality Level’.




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