First, introducing a delay is most useful for transferring circuits that primarily supply motor loads at facilities that can tolerate short power interruptions. Nevertheless, large motors can require more than 10 seconds to slow and stop. If the motor is part of a life safety system (for example, a smoke control system in a hospital or high-rise building), it may not be possible to introduce the delay and then restore power to the motor within the 10-second maximum time permitted by Article 700 of the National Electric Code®. This application would require a different approach.
Second, the duration of the delay must be developed on a case-by-case basis after considering the nature of the motor(s) located downstream of a transfer switch. Facility-specific delay recommendations should be provided by the designer of the power distribution system or another qualified electrical professional after consulting residual voltage decay information from motor and load equipment manufacturers.
Third, transfer delays should not be confused with transfer switch timing delays. The latter refers to various settings used in transfer switch controllers to ensure that transfers occur only when needed. For more information, review the ASCO Power Technologies Technical Brief entitled Elements of Time Delays and White Paper entitled Timing Delays for ATS Transition Modes.
Momentarily Parallel Two Power Sources
Closed Transition Transfer Switches close on the contacts of the alternate power sources before releasing the contacts of the original power source. When a Closed Transition Transfer Switch senses that both power sources are present, acceptable, and in synchronism, it parallels the power sources for an instant, typically for less than 100 milliseconds. Transfers of any kind of load, including motor and inductive loads, thus proceed in a “bumpless” manner without any power interruption. This can be the best option for:
• life-safety motor loads that must receive backup power within prescribed timeframes
• sensitive, high-value, digital, or electronic loads downstream of a transfer switch
• facilities seeking to minimize any momentary effects from transfer switching operations
Conditions for closed transition transfer require that both power sources are present, exhibit a voltage differential of five percent or less, and exhibit a frequency differential of less than two Hertz. Figure 7 shows the output voltage during closed transfer switching operations. For more information, review the ASCO Power Technologies publication entitled Transferring Loads with Zero Power Interruption.