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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 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 250,000 hours, and an operating time of interest of 5 years of 24x7 (43,800 hours):

R = exp(-43800/250000) = 0.839289

which says that there is an 83.9% probability that the product will operate for the 5 years without a failure, or that 83.9% of the units in the field will still be working 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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