Mean Time To Failure (MTTF) is defined as which of the following?

• A. The average time expected until a non-repairable failure occurs.
• B. The average time required to repair a failed component.
• C. The time between successive failures.
• D. The total uptime of the system.

Answer: (C)

Distinguish time to first failure from time between failures. MTTF is the average operational time until a non-repairable component fails for the first time. In systems that can be repaired and put back into service after a failure, the relevant metric is MTBF, the average time between successive failures. So the statement that describes MTTF is: the average time until a non-repairable failure occurs. The other ideas correspond to MTTR (time to repair), MTBF (time between failures), or total uptime, which are different concepts. For example, a disposable light bulb has an expected life equal to its MTTF, while a repairable machine’s reliability metric would be the MTBF—the time between failures.