Automatic transfer switches (ATSs) are by far the most commonly used form of transfer switches for emergency or legally required standby power systems.

Brian A. Rener, PE, LEED AP, M+W Group, Chicago

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ATSs traditionally consist of switches but may also use a collection of motor-operated circuit breakers, or more recently may be solid-state “static transfer switches” if rated per UL1008.

ATSs contain voltage and frequency monitors that have settings when the primary power source has dropped out (typically 75% to 95% of normal levels) and when it has returned (typically 85% to 98% of normal levels). When the power has dropped below the preset level, the ATS will transfer to the alternate power source within a normally selectable time of 0 to 6 secs. Delays to retransfer back to normal power are provided from 0 to 30 min. This delay to retransfer is desirable to ensure that the normal power source has fully returned and is stable before retransferring. If the ATS is using a normally off-line power source, such as an emergency generator, the ATS will send a start signal to the generator.

ATSs must be able to handle the anticipated full load current continuously, 24 hours a day, seven days a week for an anticipated minimum life of at least 20 years. Current ratings range from 30 to 4000 amps. Typical ratings include 30, 40, 70, 80, 100, 150, 225, 260, 400, 600, 800, 1000, 1200, 1600, 2000, 3000, and 4000 amps. The engineer should select a transfer switch equal to or greater than the calculated continuous current.

A transfer switch must be capable of withstanding and closing into the available fault current at its location in the system until the overcurrent protection device upstream clears the fault. The engineer should determine the available fault current at the transfer switch location and the ratings of the overcurrent protection devices to be used with the ATS. Methods for calculating short circuits and applying protective devices can are found in ANSI/IEEE Standard 242-2001, Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems. In addition, UL1008, Standard for Safety Automatic Transfer Switches, lists minimum requirements and testing for ATSs including withstand ratings and coordination ratings with overcurrent protection. The engineer should be familiar with the contents of these publications before specifying and applying ATSs.