Static transfer switches have been applied to AC power systems to improve the reliability and availability of AC power for computers and other electronic loads. There has been a growing need to provide fault tolerance to protect against power system failures, whether in the public utility system, in a UPS system, or in the building power distribution system. Examples of power system failures are UPS system failure, battery failure, circuit breaker trip, insulation failure, short circuit, misoperation of controls, and operator error. Protecting against such "fast" power system failures requires a commensurately fast switch to prevent upset of the electronic loads. Static transfer switches (STSs) in the power distribution system can provide the fast switching required to protect electronic loads against AC power failures as well as facilitate transfers between two AC power systems for maintenance without shutdown. To achieve protection against AC power system failures requires proper application of the STS. A fundamental requirement for a good STS application is the availability of two independent AC power sources that are synchronized, nominally available, and have a very low probability of simultaneous failure. This paper explores what makes a good static transfer switch application, and presents several good and bad STS applications.
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