Practical models for determining standby redundancy levels

The top-down approach to determining required standby redundancy to improve reliability and to extend mean time between failures is discussed. Simple analytical expressions are derived to capture the essence of the problem. In cases where the values of the parameters can be estimated based on experience with similar systems, the method should allow reasonable early estimates of required standbys. When prior information is not available, required values of the parameters can be determined and used as design requirements. The simple expressions are amenable to programming on pocket calculators and personal computers, allowing rapid evaluation of alternatives during early phases of system design