Benefit Analysis of Concurrent Redundancy Techniques

The use of redundancy in commercial systems can bring two improvements: 1) more reliable systems less subject to disruption of service due to failures, and 2) a reduction in maintenance and outage costs. This paper is concerned with evaluating the reduction in maintenance and outage cost. We introduce a figure of merit, the cost reduction (CR), to show the cost advantages of redundancy. Using the CR concept, a benefit analysis is carried out for Warm Stand-By (WSB) and Triple Modular Redundancy (TMR). Maintenance and disruption of service are assumed to be the main sources of operating cost and only a policy of corrective maintenance is considered. Results are expressed as a function of the ratio between the outage cost rate and the maintenance cost rate (ro/rm) and the conditions are stated for which 1) WSB and TMR are economically attractive with respect to a non-redundant system, and 2) TMR is economically better than WSB. It is shown that when designing for benefit; 1) maintainability is the main feature when outage costs and maintenance cost have the same order of magnitude. 2) coverage factor is the main feature when disruption of service has a appreciable cost consequence.