Electricity 101Energy Supply

Automatic Transfer Switch for Generator Connection

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.

How the automatic transfer switch works

Automatic Transfer SwitchThe automatic transfer switch (ATS) detects power failures or restorations. It automatically transfers the load from one source to the other seamlessly. For example, once it detects a power failure in the utility line, the ATS initiates procedures for starting the alternative source such as a backup generator. It then waits until the generator voltage and frequency stabilizes.  Finally, it transfers the load from the failed utility line to the generator.

At the end of the operation, it physically disconnects the circuit from the mains and connected it to the generator or other backup power source. A small interval exists between the time of disconnection from one source and the moment the other source is connected.

The ATS continues monitoring the utility line until it senses that power has been restored. Once the utility power stabilizes, the ATS automatically transfers the load back to the utility and disconnects the generator. It then waits for the generator to cool by running on no-load before turning off the engine.

Sensing the voltage and frequency variations

The ATS device transfers the load to the generator or any other source once it verifies that the source has reached the preset minimum voltage or frequency. It largely relies on the frequency and voltage sensors. They provide the control circuit with the line information and hence the ability to detect a failure or partial drop in the supply voltage of the primary line.

A three phase ATS monitors all the phases and responds once a voltage on any of the phases drops below a preset value.

Time delay

Automatic Transfer SwitchA time delay feature prevents the device from making the changeover due to false alarms, such as when the supply voltage dips momentarily. Without the delay, the Automatic Transfer Switch continues on starting the generator and transferring the load unnecessarily. The setting differs from one device to the other as well as the application. Typical time delays range between 0 and 6 seconds.

The ATS monitors the power until stability occurs for the changeover. The time varies between zero and 30 minutes. However, the ATS overrides the delay setting and connect the load to the restored power in case the generator or alternative source such as the solar power becomes unstable.

The ATS requires additional time to allow the generator to cool down before turning off the engine. The cooling happens by allowing the generator to run without a load.

Automatic transfer of critical loads

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.

Considerations when selecting an automatic transfer switch

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.

Mode of transfer operation

Automatic Transfer SwitchThe 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.

Style of the Automatic Transfer switch

Commonly used styles include:
• Standard switches: The ATS used with its sub-panel
• Load centers that combine the ATS and associated subpanel in one circuit. Suitable for small individual loads of up to a maximum of between 8 and 16 circuits
• Service disconnect ATS: for large generators with ratings of up to 200 A and usually placed between the electrical panel and the meter

Engaging electrical experts

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.

Electrical distributors, such as those working with USESI, have a wide range of expertise. They understand the principles behind generators, transfer switches, and associated switch-gear. As such, these pros help customers make the right decisions, whether planning to upgrade existing generators or buy a new one. In addition, most electrical distributors offer both qualified in-house installers and relationships with reliable third party contractors.

This ensures that the equipment installation meets all electrical codes, insurance, and warranty requirements.


Switching power between different parallel sources involves a lot of monitoring and control. For example, once an outage occurs, the consumer, operators must ensure the safety of switching back on the utility once power resumes. Making such a decision in a manual operation presents a big challenge. An ATS helps do this automatically and safely. Please visit USESI to learn more.