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What Is a Transfer Switch?

By Geisha A. Legazpi
Updated May 17, 2024
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A transfer switch, which may be either an automatic or manual transfer switch, is an electrical switch that changes the power source of a load. Transfer switches are usually electromechanical, although there are also electronic switches. Automatic transfer switches have logic controls that make decisions based on the nature of the power outage.

To demonstrate the transfer switch, a power load such as a laptop charger may be plugged into commercial power. When the commercial power is interrupted for some reason, a person may plug the charger into a portable generator running on gasoline. The action of transferring the power plug from the commercial power into the generator output is the transfer, while the person is the automatic transfer agent. It is possible to wire the setup using a double-pole-double-throw switch, which can be placed in one position then the other to transfer the load from one power source to the other. This is the equivalent of a transfer switch.

A power backup system usually requires a transfer switch. When commercial power is interrupted for some reason, the transfer switch changes to receive power from the generator. The generator takes time to start, and there are a few minutes delay before a transfer. Other systems have a battery backup where the direct current (DC) supplied power is uninterrupted.

Open transition transfer switches open the circuit from the previous power source before connecting to the new power source. Closed transition makes a new circuit before breaking the other. The choice between open and closed is the short- to long-term characteristic of the load and the nature of the alternating current (AC) or DC power deemed as alternative sources of power. For instance, an air-conditioning unit may be left without power for a few minutes, while a computer server with a critical mission cannot be left without power for any length of time.

Soft loading prevents the overloading of the backup power source. Instead of transferring the entire load into the backup, soft loading may leave portions of the load de-energized. It may also delay the energization of the entire power load.

A generator transfer switch (GTS) usually switches the load between generator output and commercial power. It takes a few minutes for a generator to start and stabilize the output voltage and frequency, thus the GTS relies on logic control circuitry to affirm correct voltage and frequency before switching. Generators may be connected in parallel if there is a way to synchronize the output voltage phases.

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