ATS/UPS Voltage Drop

Headshot of author Collin Wheeler, PE
Collin Wheeler, PE

Electrical back-up systems are designed with an automatic transfer switch (ATS), as well as an uninterrupted power supply (UPS) on systems in which engineers do not want the electric power to blink. Both systems have many settings on how to react to incoming power levels, such as the allowable voltage drop before the ATS or UPS reacts to correct the problem. A UPS will correct a voltage drop of 85 percent of normal voltage by the internal electronic of a rectifier. If voltage drops lower than 85 percent, the batteries in the UPS help correct the output voltage. An ATS, on the other hand, will react to low voltage by starting the emergency generators and transferring power from the utility to the emergency generator.

On a recent job, our engineers found it best that the settings in the UPS and ATS were coordinated with each other. There was a concern that if the UPS was using the batteries and depleting their amp/hour capacity, an actual power outage would drain the UPS batteries to zero. The protected system would lose all power until the emergency power system started up, resulting in at least ten seconds of outage. We recommend that the voltage drop for the ATS is set to slightly higher than the UPS – in our case, it was 86 percent voltage drop.

If you’re planning a project involving an electrical back-up system, get in touch to see how HAWA can help.

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