MTBF is an acronym for Mean Time Between Failures.

MTBF ratings are a measure of the reliability of a product. It is important to keep in mind that reliability and lifetime are not the same thing. Reliability is a measure of how many failures can be expected to occur in a group of units over the lifetime of the product. An MTBF rating of 3million hours means that you can expect one failure for every 3million ballast hours of operation. A product cannot be expected to last longer than its design lifetime regardless of how high its MTBF rating is.

The most common application of MTBF ratings is to calculate maintenance costs. For example if 1,000 ballasts with an MTBF rating of 3 million hours have been installed into a system or several systems and are operating an average of 24 hours per day, 365 days a year, how many failures are expected in one year?


Rounding up to the nearest integer yields 3 ballast failures in one year.

MTBF ratings only apply to a statistically significant population of units. The larger the number of units in operation the more accurate the MTBF calculations will be.

Another clarification that should be made is that not all methods for developing the MTBF ratings are equal. The most accurate type of MTBF rating is “demonstrated MTBF”. Demonstrated MTBF ratings are developed by analyzing the failures for a group of units that has been operating in actual field service over an extended period of time (typically several years). Consequentially, demonstrated MTBF is a statement of historical fact.