Automatic Transfer Switches

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Automatic Transfer Switches

What is an Automatic Transfer Switch?

Low voltage automatic transfer switch assemblies provide a reliable means to transfer essential load connections between primary and alternate power sources. Data centers, hospitals, factories, and a variety of other facility types that require continuous or near-continuous availability typically use an emergency (alternative) power source such as a generator or backup utility when their normal (primary) power source is unavailable.

How does an automatic transfer switch work?

An Automatic Transfer Switch (ATS) is an automatic, intelligent circuit breaker device controlled by dedicated control logic. The main purpose of an ATS is to ensure the continuous supply of electrical energy from one of two power sources to a connected load circuit (electrical equipment – lights, motors, computers, etc.).
The control logic or automatic control is typically microprocessor based and constantly monitors the electrical parameters (voltage, frequency) of primary and AC power sources. If the connected power source fails, the ATS automatically transfers (switches) the load circuit to the other power source (if present). Typically, most automatic transfer switches will default to connecting to the primary power source (utility) and will only connect to the alternate power source (motor generator, backup utility) when necessary (failure of primary source) or requested ( operator command).

A typical transfer sequence includes:

  1. The normal power source fails.
  2. The transfer switch transfers the load to the backup power source when the power from the generator or backup power supply is stable and within prescribed voltage and frequency tolerances. Depending on the needs and preferences of a facility, the transfer occurs automatically or is initiated manually.
  3. The transfer switch returns the load from the backup power source to the normal power source when power is restored. The retransfer process is automatic or manually initiated.

What ATS arrangements are available?

Two power sources


The standard transfer switch configuration includes a power supply service and generator for normal and backup power sources. This system arrangement is typically referred to as an emergency generator system. The single generator shown may be multiple motor-generator sets operating in parallel.


This use case uses two supply sources that provide redundancy in the distribution system and allow for quick recovery of service to the load if an upstream equipment failure occurs. The two sources can be independent of each other, requiring the provision of two electrical services, or they can come from a single electrical service distributed via redundant paths within the facility.


Transfer switches are sometimes used between two generator sets for primary power use, often in remote installations. In order to share the running time equally, the output power is periodically exchanged between the generator sets.

Three power sources


Critical installations with an emergency standby generator system often include provisions for a second generator connection to serve as a redundant back-up to be used in inclement weather or during scheduled maintenance on the first generator. As shown, in some cases the first generator is permanently installed on site while the second generator is a portable roll-up type deployed when needed.


This configuration enhances redundancy with a dual utility arrangement and includes an emergency standby generator source. As shown, the generator can be designed for use by a single transfer switch or for multiple transfer switches using a priority control system.

Utility—Utility—Generator 2
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