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Mean Time To Failure(MTTF)

Reliability Engineering
mechanicalelectricalinstrumentation

Definition

The average time from the start of operation until the first failure of a non-repairable item, or the average time between replacement and next failure for items that are replaced rather than repaired. MTTF is technically correct for non-repairable components (bearings, seals, filters, lamps) while MTBF applies to repairable systems. In practice, the terms are often used interchangeably.

Context

MTTF is displayed alongside MTBF in Reliatic's reliability overview. For components that are replaced on failure (not repaired), MTTF provides the expected service life. This drives preventive replacement strategies — replacing components at an interval based on MTTF to avoid unplanned failures.

Formula

MTTF = Total Operating Time of All Units / Number of Failures (for non-repairable items)

Examples

  1. Mechanical seals on a critical pump fleet have an MTTF of 14,600 hours (approximately 20 months), informing a preventive replacement strategy at 12,000 hours (80% of MTTF).
  2. LED indicator lamps have an MTTF of 50,000 hours, far exceeding the MTTF of 2,000 hours for the incandescent lamps they replaced.

Applicable Standards

  • IEC 60300-3-1
  • ISO 14224:2016

Related Terms

Mean Time Between Failures(MTBF)

The average time between consecutive failures of a repairable system or component, calculated as the total operating time divided by the number of failures during that period. MTBF is the primary reliability metric for repairable systems and represents the expected time of continuous operation between failure events. Higher MTBF indicates better reliability.

Mean Time To Repair(MTTR)

The average time required to repair a failed item and restore it to operational status, measured from the point of failure detection to the point of verified restoration. MTTR includes fault diagnosis, logistics (obtaining parts and tools), physical repair, testing, and recommissioning. It does not include administrative delays or waiting time unless specifically defined as Mean Time To Restore.

Failure Rate(Lambda)

The average number of failures per unit of time for a population of items, typically expressed as failures per hour, per year, or per million hours. Failure rate (lambda) is the reciprocal of MTBF for constant-failure-rate systems. It is a statistical average measure, distinct from the instantaneous hazard rate, and is most accurate when the failure rate is approximately constant (beta near 1).

Reliability

The probability that an item will perform its required function under stated conditions for a specified period of time. Reliability is a quantitative measure expressed as a probability (0 to 1) or percentage. It is a function of the failure rate and time: for constant failure rate systems, R(t) = e^(-lambda*t). Higher reliability means lower probability of failure over the same period.

B-Life

The time by which a specified percentage of a population is expected to have failed, derived from the Weibull distribution parameters. B10 life is the time by which 10% of the population has failed (90% survival); B50 is median life (50% survival); B90 is the time by which 90% have failed. B-life values are used to set preventive replacement intervals and warranty periods.

Where this appears in Reliatic

This concept is used in the Reliability Analytics module.