Redundancy – the number one way to increase reliability

The U.S. military defines redundancy in MIL-STD-721 as follows:

  • Redundancy – The existence of more than one means for accomplishing a given function. Each means of accomplishing the function need not necessarily be identical.
  • Active Redundancy – That redundancy wherein all redundant items are operating simultaneously.
  • Standby Redundancy – That redundancy wherein an alternative means of performing the function is not operating until it is activated upon failure of the primary means of performing the function.

Active redundancy is when you have multiple components running all of the time but if one fails you can still operate until the failed component is fixed. The performance might be degraded but the function is feasible. An example would be having two engines. If one engine fails, you can still control you boat and get back to port at a degraded speed and with less maneuverability. This type of configuration is about two times the reliability of the series components.

With standby redundancy, only one of the components operates at a time. The “standby” component is there just for the case where the other fails. For example, having an electric fuel pump installed and ready to be activated when the mail fuel pump fails. Standby redundancy is the most reliable configuration. It is 99 percent reliable after the redundant components pass into the middle part of the bathtub reliability curve. Remember, if a component has never been used its reliability is low due to infant mortality. It is important to verify all standby redundant components so they move beyond the early part of the bathtub curve. In the case of the redundant fuel pump having a means of using it from time to time would provide that verification that the pump is viable.

Your goal is to balance reliability with practicality and cost. Some redundancy is critical and inexpensive. Other items are expensive and non critical. Focus first on the critical items and make them redundant when possible.

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