What is an Automatic Transfer Switch : Working & Its Applications

A transfer switch is used to switch an electrical load in between two power sources like main & back up. These switches allow the secure connection otherwise disconnection of different power sources to an electric load. But, it is significant to correctly isolate the power sources once they are not used & make sure that changes from one power supply to another are performed in a secure & controlled manner. Transfer switches are available in two types automatic and manual transfer switches. Manual switches operate by an operator whereas automatic switches operate automatically whenever they detect one of the power sources has gained power or lost. So this article discusses an overview of one of the types of automatic transfer switch or ATS.


What is an Automatic Transfer Switch?

A switch that is used to transfer power supply automatically from its main source to a backup power source once it detects a power outage within the main source is known as an automatic transfer switch or ATS. Whenever a power outage occurs within the main power system, then this switch invokes a standby power source like a UPS or uninterruptable power supply. The automatic transfer switch symbol is shown below.

ATS Symbol
ATS Symbol

Once the automatic transfer switch is connected to both primary & backup power sources, then it acts as an electrical relay to serve as a mediator between equipment as well as the power supplies.

Automatic Transfer Switch Working Principle

The working principle of an automatic transfer switch is to switch electrical energy between both primary & backup power sources automatically without human interference. This switch is an essential component in an EPSS (emergency power supply system).

These switches include a set of relays as well as solid-state devices that are mounted to check & control the incoming voltage. This switch functions like a monitor if the incoming supply is smooth & above a particular level. Once the voltage drops then it is detected by a solid-state relay device & produces a signal to begin the generator.

An ATS switch plays a key role in connecting the standby generator to your home. Generally, the flow of electricity is from the utility to the main electrical system of your home, where it gives power to all your circuits. In the entire electrical system, this switch works like the brain. Once this switch is connected, then they can switch automatically between utility power & generator power. Whenever this switch detects a power failure at home, then it supplies generator power to your home, after that it detects the power when it is restored & gives utility power back to your home.

Automatic Transfer Switch Circuit Diagram

The wiring diagram of the automatic transfer switch for a single-phase power generator is shown below. This wiring diagram is very simple and has relatively less cost. The required components of this wiring mainly include 1PCs magnetic contactor, 2PCs MCB, cables for line, and wiring control.

Automatic Transfer Switch Wiring Diagram
Automatic Transfer Switch Wiring Diagram

Working

Once the power supply from the public utility is turned ON, then the flow of current toward the contactor & coil contactor will be turned ON. After that, the NO terminal will be connected & supply electricity from the public utility to the load at home, while the NC terminal is disconnected.

Whenever the power from the public utility goes out, the electricity flow toward the coil contractor will be cut off, so that the contractor doesn’t work, the NC terminal will connect and the NO terminal will disconnect again.

Once the magnetic contactor activates, then the NC terminal is connected automatically, the power will flow through the NC terminal from the generator and the flow of electricity from the generator will supply to the load at home. This automatic transfer switch (ATS) simply operates to change power from generator to generator or from public utility supply to a power generator.

Types of Automatic Transfer Switches

Automatic transfer switches are available in three types compact ATS, CB ATS, and contactor ATS where each type is discussed below.

Compact ATS

The compact automatic transfer switch is also called a change-over switch which is used for breaking short-circuits currents. The modern automatic transfer switches will reduce the time taken for device installation within the panel while achieving a high range of reliability because of the limited no. of components to be connected. These switches are normally the most expensive.

Compact ATS
Compact ATS

Circuit Breaker ATS

The CB automatic transfer switches are used for making, breaking & withstanding short circuit currents. Generally, these switches are connected by numerous ACBs or MCCBs with an installer. These accessories must be used within the circuit. This type of switch mainly includes two interlocked CBs, thus simply one breaker can be closed at any time. As compared to contactor ATS, these switches are more expensive.

Circuit Breaker ATS
Circuit Breaker ATS

Contactor ATS

This kind of automatic transfer switch is used for making & withstanding short-circuit currents but not used for breaking short-circuit currents. Generally, these switches depend on 3-pole or 4-pole low voltage contactors. The contactor type ATS has a simple design, mechanically seized & operated electrically. As compared to the circuit breakers, it operates very fast and also decreases transfer time. Generally, these switches are the most economical.

Contactor ATS
Contactor ATS

How an Automatic Transfer Switch detects a Power Loss ?

The automatic transfer switch will detect a loss of power by monitoring both incoming and outgoing lines. The incoming line is monitored by a voltage sensor that detects whether or not there is enough voltage coming into the system to operate safely. If there is not enough voltage, an alarm will sound and the automatic transfer switch will begin to switch over to the standby generator.

The outgoing line is monitored by a frequency sensor that detects whether or not there is enough frequency coming out of the system to operate safely. If there is not enough frequency, an alarm will sound and the automatic transfer switch will begin to switch over to the standby generator

Automatic Transfer Switch Vs Static Transfer Switch

The difference between an automatic transfer switch and a static transfer switch includes the following.

Automatic Transfer Switch Static Transfer Switch
An automatic transfer switch is an intelligent & self-acting power switching device. A static transfer switch is intelligent and automatic and fixed switching equipment.
The main function of an ATS is to transfer the power supply automatically from its main power source to a backup power source once it detects a power outage within the primary source. The main function of STS is to transmit critical loads in between two separate AC power sources without any interruption.
ATS switches are available in two types circuit breaker & contactor. STS switches are available in two types single phase & three-phase.
STS has no break within supply throughout the transfer. ATS has a short ms break within supply throughout the transfer.

Advantages and Disadvantages

The advantages of an automatic transfer switch include the following.

  • The main benefit of this switch is to transfer the power of the load to the generator automatically.
  • This switch includes in-built sensors for detecting any fluctuations within the frequency of the main power supply & voltage.
  • These switches have a time delay mechanism, which attempts to guard both the generator device &your electric circuits against the fluctuations within power.
  • These switches are reliable, simple, and secure.
  • These switches are required for uninterrupted electrical supply.
  • These types of switches are ideal for efficiently switching to generator power

The disadvantages of automatic transfer switch include the following.

  • The main drawback of this switch is that there is a high error margin for many establishments.
  • These types of switches are expensive as compared to manual switches.
  • These switches require more maintenance as compared to manual switches.

Applications

The applications of automatic transfer switch include the following.

  • These switches are used for protecting telecommunication & data networks, industrial processes & critical installations like financial transaction centers & health care facilities.
  • These switches are used to transfer electrical power between two power sources like primary utility & secondary backup power source.
  • These types of switches are also used in other electricity switching applications like utility to utility, the generator to generator & three-source systems.
  • ATS is used in Hospitals, schools, medical buildings, malls, restaurants, malls, warehouses, factories, etc.

How much does an Automatic Transfer Switch/ATS costs ?

The cost of an automatic transfer switch depends on the size and type of the unit, as well as where it’s installed. For example, a small residential transfer switch can cost as little as $200 while a large commercial one might cost more than $1,000. The average price is around $500.

Thus, this is all about an overview of an automatic transfer switch – working with applications. The control mechanism of these switches is most complicated. This switch has the ability to switch & control between the two power sources in less time like a minimum of seconds. It also helps in beginning a standby generator automatically whenever there is a power outage from the main supply. Here is a question for you, what is a manual switch?

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