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What is MTBF?
MTBF is an abbreviation for Mean Time Between Failures.

MTBF is a measure of how reliable a product is. MTBF is usually given in units of hours; the higher the MTBF, the more reliable the product is.

For electronic products, it is commonly assumed that during the useful operating life period the parts have constant failure rates, and part failure rates follow an exponential law of distribution. In this case, the MTBF of the product can be calculated as:

MTBF = 1/(sum of all the part failure rates)

and the Reliability (R), which is the probability that the product will work for some time T without failure, is given by:

R(T) = exp(-T/MTBF)

Thus, for a product with an MTBF of 500,000 hours, and an operating time of interest of 5 years of 24x7 (43,800 hours):

R = exp(-43800/500000) = 0.916127

which says that there is an 91.6% probability that the product will operate for the 5 years without a failure, or that 91.6% of the units in the field will still be working at the 5 year point. Thus, if you have 1000 units in the field, this calculation estimates that 916 will still be working (84 will have failed) at the 5 year point. Note: the Reliability calculation assumes replacement upon failure.

The MTBF figure for a product can be derived in various ways: lab test data, actual field failure data, or prediction models (such as Telcordia SR-332 or MIL-HDBK-217). The RelCalc for Windows software can help you do your MTBF prediction.

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