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Effective Failure Rate of One Standby Offline Unit with "n" Active On-line Units Required for Success. (without Repair)

This tool calculates the effective failure rate of a configuration that has a single off-line standby unit ("cold spare") with "n" active on-line units required for success. The off-line spare unit is assumed to have a failure rate of zero while it is in a non-operating state. All on-line units are assumed to have equal failure rates. Upon failure of one of the on-line units, the standby off-line unit is switched in with a probability of "P," the probability that that the switching mechanism works properly. For perfect switching, P=1. The calculation implements Equation 6 shown on page 90 of the United States Air Force Rome Laboratory Reliability Engineer's Toolkit (1993).

Reliability Block Diagram:

Reliability Block Diagram showing one cold standby spare unit with n active on-line units, all of which are required for success. No repair is possible.

Calculation Inputs:

Number of active on-line units (n):
Probability that switch will work correctly (P, range 0 to 1.0):
Decimal places:

Featured Reference:

Reliability Engineering: Theory and Practice
Reliability Engineering: Theory and Practice

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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.