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Elements of Time Delays

In a prior Tech Brief, ASCO Power Technologies described the roles of essential time delays invoked during a load transfer cycle between two power sources. The following sections describe elements of those delays and how they are used to avoid nuisance transfers in backup power systems.

Avoiding Unnecessary Transfers

When deciding whether to transfer power, it is important to avoid switching activity that may actually be unnecessary. That is because transfers can result in (depending on power system configuration) momentary power disruptions that affect sensitive loads and their operations. Nuisance transfers also result in unnecessary wear-and-tear on electrical equipment and engine-generator sets. Avoiding nuisance switching is the best way to avoid these effects.

Assessing Stability of a Normal Power Source

Automatic Transfer Switches (ATSs) continually monitor their connected power source to verify acceptable voltage and frequency. If an ATS controller detects an absence of voltage on the normal source, it initiates a series of operations to transfer electrical load to the emergency power source. (For more information, review the ASCO document entitled Basic Control of Automatic Transfer Switches.)

Several phenomena can trigger unnecessary transfer. For example, a momentary drop in voltage could result from a fault current or a lightning strike. When a large fault occurs, line voltage can drop. If voltage drops below a preset threshold, an ATS controller could initiate a transfer. However, if an overcurrent protection device clears the fault before transfer occurs, the transfer becomes unnecessary. 

Likewise, if a lightning strike produces an instant voltage fluctuation that includes a decrease to a level below a pre-set threshold, the transfer again becomes unnecessary if voltage recovers in a matter of milliseconds. Consequently, applying a delay allows a controller to monitor the persistence of the condition – if a voltage drop or frequency aberration is persistent, then the need to transfer load to an alternate power source is confirmed.

Two Elements of Transfer Delays

Two elements are required to invoke a delay: (1) a triggering value, and (2) a set time interval. Their relationship is described in the following scenarios.

In the first example, voltage is the triggering value. The system designers and managers desire to transfer to an emergency power when voltage on the normal source drops below 75 percent of the nominal circuit voltage. On retransfer to the normal source, a higher pickup voltage value of 90 percent is selected to avoid nuisance transfer to a potentially unstable normal source.

In addition, designers may determine that the facility and its operations can tolerate voltage fluctuations in these ranges for up to x seconds without undue impact in equipment and business operations. A time delay of x seconds is assigned to delays for normal-to-emergency and emergency-to-normal transfers.
The effect of these settings on normal-to-emergency transfers is seen in Figure 1 above. There, the normal source provides power at full nominal voltage until t1, when voltage dips briefly. In this instance, the voltage remains above the dropout voltage and recovers quickly. Because the voltage did not cross the dropout voltage threshold, transfer is not initiated, and an unnecessary transfer is avoided. Another voltage dip at t2 falls below the dropout voltage, but remains there for an insufficient time before recovering, again avoiding a nuisance transfer. At t3, the voltage drops and remains below the dropout voltage until the x second delay expires. Because the voltage condition is persistent, the ATS starts the genset and transfers to the emergency source.
When considering retransfer, recall that the emergency source is providing power (shown in red) while the normal source (shown in green) is coming back online, as shown in Figure 2 above. At t5, voltage appears on the normal source. Because it fails to reach the pickup voltage, transfer is not initiated, and an unnecessary transfer is avoided. At t6, voltage reappears above the pickup voltage, but remains for an insufficient time before dropping. Transfer is not initiated to again avoid a nuisance transfer. At t7, the voltage reappears and remains above the pickup voltage until the x second delay expires. Because the voltage condition is persistent, the ATS retransfers to the normal source at t8, and backup power is no longer needed.


Transfer delays involve both absolute (voltage, frequency) and temporal (time-based) settings. Satisfying both criteria helps ensure that transfer actions will occur only when needed. By using pickup voltage settings that are higher than dropout voltage settings, controllers assess voltage trends to avoid nuisance transfers of load, increasing reliability and reducing wear-and-tear on backup system equipment.

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