An automatic transfer switch consists of an electrical power control device that reroutes electricity under certain power conditions. It enables the safe switching of power from one source to another. Consequently, this ensures a safe and reliable transfer of power while minimizing the risks of fire, damage, or back feeding of power to the utility lines.

An ATS ensures continuity of power to the load. However, the time between loss of power in the utility line and the time the load is connected to the alternative power source remains critical in some applications. For example, a power loss for a few seconds is unacceptable in some hospitals or with emergency equipment. In some of these installations, an intermediate power supply is utilized. These include the battery-based inverter or UPS power to fill the gap between the loss of power and connection to the generator power.

Advanced ATS devices may have the ability to detect the voltage dips characteristic of power failures and automatically start the generator before the utility power goes off completely.

Consumers need to ensure that the ATS will have adequate capacity to handle the generator power. Things to consider include the voltage rating, continuous current rating, and type of load. In addition, consider the mode of transfer, desired transfer time delay, ATS style, and more.

The four main categories of ATS devices are defined by the following transition modes.

Open-transition: The open transaction mode of operation disconnects the primary power before connecting the power from the secondary source. This break-before-make operation means a short duration when there is no connection to either of the power sources.

Fast closed-transition: The ATS is based on the make-before-break switching operation. It parallels the two supplies for about 100 milliseconds after which it disconnects one of the sources. This operation ensures that the load is never disconnected from power.

Soft closed-transition: The soft closed-transition transfer synchronizes generator power with the utility line and then transfers the load from the utility to the generator. When transferring the load back to the utility, follow a similar process.  In this case, the generator output slowly decreases until completely off.

Sub-cycle transfer: These are based on semiconductor switches and are the fastest automatic transfer switches. Upon detecting a power failure, the ATS transfers the load to the other source within the first one-quarter of the electrical cycle. This ensures no power interruption to the load. Sub-cycle ATS devices are the most complex and expensive compared to those using mechanical switches.

Since consumers may not have all the necessary technical knowledge to choose the correct application, it can be a good idea to hire a qualified electrical contractor to select the correct sizing and installation.

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