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Effective Failure Rate of Two Active Redundant Units, with Different Failure and Repair Rates

This tool calculates the effective failure rate of two active on-line units, where "1 of 2" units are required for success. Each unit may have a different failure and/or repair rate. The calculation implements Equation 2 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 unit mean corrective maintenance time (Mct) entered below. See this page for additional details on time inputs (Mct) for system restoration.

Reliability Block Diagram:

Reliability Block Diagram showing two active units with different failure and repair rates.  1 of 2 required for success.

Calculation Inputs:

Unit A mean corrective maintenance time (MctA), hours:
Unit B mean corrective maintenance time (MctB), hours:
Decimal places:

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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.