MTBF / MTTF / MTTR

MTBF / MTTF / MTTR

1. Definitions

   • Mean time to fail (MTTF)  - a measure of time between first turn on and the first service interruption Mean time to repair ( MTTR ) - a measure of time between service interruption and service restoration Mean time between failure (MTBF) - a measure of time between two failures MTTF is also used for systems which can not be repaired and therefore need to be replaced

2. Relation

   • MTBF = MTTR + MTTF

3. reliability versus availability

   • IEEE-90 - states : - Reliability is the ability of a system or component to perform it's required functions under stated conditions for a specified period of time -Availability is the degree to which a system or component is operational and accessible when required for use MTBF is a theoretical model anddoes not equate to service life Some MTBF modesl do not differentiate between availability and reliability 

4. MTBF, any value?

   • Which MTBF? - HALF ( Highly accelerated life testing) - Markov - FMECA ( Failure mode, effects and criticality ) - RBD (Reliability block diagram) which assumption set? - different assumptions for similar MTBF standard could still lead to skewed results both methodology and assumption set chosen by the vendor is unknown. Hence comparison of MTBF is not always effective