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Effective Failure Rate of "n" Active Redundant Units, with "m" Required for Success (with Repair)

This tool calculates the effective failure rate of "n" active online units, where "m" are required for successful operation. The calculation implements Equation 1 shown on page 90 of the United States Air Force Rome Laboratory Reliability Engineer's Toolkit (1993). It takes into account repair of any failed units and assumes that these units are repaired within the mean corrective maintenance time (Mct) entered below. See this page for additional details on time inputs (Mct) for system restoration. The effective mean time between failure (MTBF), sometimes referred to as the mean time between critical failure (MTBCF), is the reciprocal of the effective failure rate.

Reliability Block Diagram:

Reliability Block Diagram showing n active units with m units required for success

Calculation Inputs:

Number of units required for mission success (m):
Total number of active on-line units (n):
Mean corrective maintenance time (Mct), hours:
Decimal places:

Featured Reference:

Introduction to Reliability Engineering
Introduction to Reliability Engineering

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  1. United States Air Force Rome Laboratory Reliability Engineer's Toolkit (1993).
  2. MIL-HDBK-338, Electronic Reliability Design Handbook.
  3. Klion, Jerome, A Redundancy Notebook, Rome Air Development Center, RADC-TR-77-287, December 1987.
  4. Reliability Modeling and Prediction, MIL-STD-756B, November 1981.
  5. McGregor, Malcolm A., Approximation Formulas for Reliability with Repair, IEEE Transactions on Reliability, Volume R-12, Number 4, December 1963.
  6. Bazovsky, Igor, Reliability Theory and Practice.