An Introduction To Automatic Transfer Switches

Author : Zhejiang Geya Electrical Co. Ltd | Published On : 06 Sep 2021

Automatic Transfer Switches: What are they?

An automatic transfer switch assembly can reliably transfer load connections between primary and alternate sources of electrical power. Many facilities requiring continuous or near-continuous uptime, such as data centers, hospitals, factories, and many others, use an emergency (alternative) power source, such as a generator or a backup utility feed, in case their normal (primary) power source becomes unavailable. There are various automatic transfer switch manufacturers  to buy these switches from.

How does a transfer switch work automatically?

An automatic transfer switch (ATS) is a self-acting, intelligent power switching device controlled by the specialized control logic. The main purpose of an ATS is to ensure the continuous delivery of electrical power from one of two power sources to a connected load circuit (electrical equipment such as lights, motors, computers, etc.).

Control logic or automatic controllers are usually microprocessor-based and constantly monitor the electrical parameters (voltage, frequency) of primary and alternate sources of power. In the event that the power source fails, the ATS will transfer (switch) the load circuit to another (existing) power source. 

In general, automatic transfer switch are typically set to connect to the primary power source (utility) by default, only connecting to alternate power sources (engine-generator, backup utility) when required (primary power source failure) or requested (operator command).

A Transfer Switch Typically Includes:

  • A normal utility power source fails.
  • If the generator or backup utility feed is stable and within prescribed voltage and frequency tolerances, the transfer switch shifts the load to the emergency power source. Depending on the facility's needs and preferences, the transfer process is self-acting or manually initiated.
  • When utility power is restored, the transfer switch transfers the load from the emergency power source to the normal power source. Usually, the re transfer process is self-acting or manually initiated.

What are the ATS arrangements?

Different arrangements are possible using two power sources and three power sources.

There are two power sources

  • Utilities-Generator

In a typical transfer switch configuration, there is an electric utility service and a generator for normal and emergency power. Usually, emergency standby generators are referred to as this system arrangement. The single generator shown may be several engine-generator sets operating simultaneously.

  • Utility-Utility

Two utility sources are employed in this use case, providing the distribution system with redundancy and allowing for quickly restoring power to the load in the event of failure of the upstream equipment. They can come from two independent electric sources, requiring the public utility to provide dual electric services, or they can come from a single electric service distributed through redundant paths within the facility.

Types of ATS transitions:

With transfer switches, loads can be switched between normal and emergency power sources. An important aspect in determining the kind of transition that should be performed pertains to the specific functions a given load performs and their impact on safety or security.

  • Open transition

A break-before-make transition is an open transition. Transfer switches disconnect from one power source before connecting to another. In addition to open transitions, there are also open in-phase transitions.

  • Closed transition

Closed transitions are transfers where you make a decision before making a change. In order to break its connection with the first power source, the transfer switch connects to a second power source. Since there is no gap between disconnection and connection, downstream loads receive power continuously throughout the transfer process.