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Common Rack Automatic Transfer Switch Scenarios and Questions


This article discusses common Rack Automatic Transfer Switch (ATS) Scenarios and Questions and provides configuration recommendations.

Product Line

Rack Automatic Transfer Switches including the following models only:
  • AP7721
  • AP7722A
  • AP7723
  • AP7724
  • AP7730
  • AP7731
  • AP7732
  • AP7750A
  • AP7752
  • AP7753

  • All firmware versions
  • All Rack ATS models listed above


As the Rack Automatic Transfer Switches are complicated and involved products, we have provided some common scenarios and questions along with recommendations and general information on functionality.
With these models, please ensure that after analyzing the situation, the latest available ATS firmware has been applied and instructions followed - INFORMATION BULLETIN - Upgrading Rack ATS Firmware (AP77XX) - potential to permanently damage ATS during download.


Please review the scenarios and questions below for additional information.

When both sources are available and stable, the ATS will switch back to its preferred source after a 30 second wait period. Attempts to continually switch the unit prior to the completion of the 30 second period could result in loss of load or damage to the ATS. Anytime an anomaly is detected within this sampling period, the 30 second timer will be reset.

The following is a list of potential causes of ATS to source switching when the upstream UPS goes to battery or when power appears to be present at the ATS input:

  1. Switching of the ATS when the UPS goes to or from "on battery" mode
The switching of the ATS occurs as a result of an internal source synchronization when the UPS transfers from mains power to battery power. The UPS creates a frequency shift to speed up or slow down the frequency in order to align the inverter in phase with the sync source chosen. This frequency shift actually creates a deviation in the way the ATS measures the voltage coming from the UPS. This voltage deviation can be large enough for the ATS to interpret this deviation as a voltage drop resulting in the switching from the preferred source to the secondary source. Having two UPS's powered by the same circuit feeding an ATS can also cause output issues. An ATS is designed to supply redundancy, and if both UPS's transfer to battery at the same time due to an input issue, either the transfer time the UPS takes from "online" operation to "on battery", or the frequency shift of two UPS's switching to battery at the same time, can be seen as a complete interruption of power by an ATS. It is not recommended to use 2 UPS's on the same input circuit as the sources of an ATS.
  1. Dropping the load when the ATS switches sources while both appear to have power
The tripping of the circuit breakers during the switching process has been determined to be the result of switching between two sources that are out of phase with each other producing a voltage potential that exceeds the rating of the internal relays responsible for switching from one power source to the other. ATS firmware v3.5.7 and higher aims to resolve this issue and will help prevent the ATS from tripping the upstream circuit breakers.

Here are some recommended actions that can be put into place to reduce the probability of the ATS switching when the UPS transfers to battery mode:

  1. Revise the voltage transfer range from medium to wide, revise frequency deviation from +/-2 Hz to +/-3 Hz on the ATS:
This will increase the amount of voltage and frequency deviation allowed prior to switching, reducing the probability of switching during this event.
  1. Set the sensitivity preference from high to low:
This will allow more time for the frequency synchronization to occur before the ATS takes action and switches.
  1. Set the source preference from source A to none on the ATS:
If neither source is preferred, this is the ideal setting. This will avoid switching back to Source A when the two UPSs are most likely to be out of sync with each other.


Using the Rack ATS with a Generator

APC recommends placing a UPS between the generator and the ATS input. The UPS will condition the input voltage to the ATS to prevent thrashing.  A second UPS should be used between the power utility and the other input of the ATS. This UPS will power the load while the generator starts and stabilizes. The UPS should be sized to allow time for the generator to be started and the time needed for the UPS attached to the generator to operate normally. While the generator is starting, the generator voltage and frequency are not stable. If the ATS is connected directly to the generator, the ATS can switch to the generator input before the generator has stabilized. The load of the ATS will cause the generator voltage to dip. This dip can also shift the frequency of the voltage. Either of these issues can cause the ATS to switch to the other input.

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