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Difference Between an Automatic Transfer Switch (ATS) and Switchgear

Switching and Distribution

Transfer Switches and Switchgear
Emergency, prime, or continuous power supplied by diesel generators are common within industrial settings. Once power is generated, it must be distributed throughout the facility. 

Some industries only require generator power for emergency lighting and other emergency systems. Facilities that have critical or life support systems utilize a more advanced distribution system. 

Smaller and less advanced power generation and distribution systems can contain:
  • Generator
  • Automatic Transfer Switch
  • Switchgear (depending on size)
  • Circuit Breaker(s)
Larger more advanced power generation and distribution systems can contain:
  • Generator(s) 
  • Automatic Transfer Switch(s)
  • Automatic and Manual Switchgear (for low, medium or high voltage)
  • Uniterruptible Power Supply - from small ones to Battery Banks and Rotary UPS Systems
  • Electrical components that are remotely accessed
  • Remote alarm and monitoring capability
This article provides definitions for the Automatic Transfer Switch (ATS) and types of switch gear. 

Automatic Transfer Switch (ATS)

Automatic Transfer Switch Configuration Example
When utility power is lost, the automatic transfer switch (ATS) commands generator startup. When the generator is ready, it switches to emergency power. 

Single Generator Configurations
It is important to remember that circuit breakers remain closed until they trip due to an overload or short circuit condition. 

When power fails:
  1. The automatic transfer switch commands the generator to start.
  2. When generator is ready to supply power, the ATS switches emergency power to the load.
  3. Once utility power is restored ATS switches to utility power and commands generator shutdown.
Multiple Generator Configurations
Some sites have buildings that are located far enough apart to prevent paralleling operations during loss of power. These sites often employ the use of multiple generators.

The figure to the right demonstrates an example. The installation configuration of the generators does not allow parallel operation even if the generator control panel has paralleling software installed. The sequence of automatic transfer switch events is basically the same as the single generator configuration. When utility power is lost to the site, each individual building's ATS performs its duties and emergency power is brought online until power is restored. With this style of installation, it is possible that one building on the site loses utility power and the remaining has power. Only the generator for the building with failed utility power would initiate.

As an ASCO Reseller, Generator Source offers a wide variety of ATS products. To get more technical details on transfer switch technology, NEC Requirements,  maintaining transfer switches and more ASCO has an extensive library of technical whitepapers worth checking out.

Electrical Switchgear

Electrical Switchgear in Plant
The first switchgear used was in power station applications. This consisted of knife switches mounted on panels constructed of marble or asbestos. However, as current and voltage needs escalated, the open knife switch concept became a safety hazard. The knife switches advanced to enclosed circuit breakers to prevent shock. 

Residential emergency generator applications today use the circuit breaker panel as the switch gear. In this simple application, the automatic transfer switch supplies power to the circuit breaker in the panel in the home when power is lost. This is an example of the simplest switchgear application.

The next level of switchgear is used for industries that do not have mission critical power requirements. Often power is supplied to individually-designated circuit breakers. Many of these style circuit breakers are equipped with an LED to indicate power. Examples of types of systems powered are:
  • Emergency Lighting - Lighting for exit walkways and exit signs throughout building
  • Hazardous Ventilation - Exhaust ventilation for rooms that contain toxic chemical storage
  • Communications - Phone system energized or some phones with the system
  • Alarm and Monitoring - Fire monitoring and electronic door systems

Hospitals, aerospace facilities, data centers and other industries that require continuous power when utility power is interrupted employ more advanced switchgear  systems. These systems are custom designed by power enginners for the facility. Large installations with critical power requirements can have a control room.

The control room is the center hub for electrical power distribution. Multiple emergency generators can be paralleled and placed on-line in the event
of a brown out or complete loss of power. Power distribution panels can be high voltage and have sub-panels equipped with step-down transformers
connected for low voltage operation. An example of a typical power loss is listed below:

  1. All utility power is lost to multiple generator automatic transfer switches. Alarms are sounded for power loss.
  2. ATS commands the generators to be started.
  3. When generators are up to speed and rated power, ATS switches to emergency generator power.
  4. Switchgear and generator software allow for automatic generator paralleling. 
  5. Switchgear distributes power to designated circuits.
  6. In-house or remote operator energizes circuits that are vital and de-energizes non-essential loads (depends on current operations of facility).
  7. When power is restored ATS switches to utility power and generators are shutdown. 
Each facility offers different needs in emergency standby power needs.  Switchgear requirements get even more complex based on the type of system required.  To see a good example of the various types of components found in medium voltage switchgear watch the illutrated video below on Siemens NXAir product.

If you have any additonal questions on ATS or switchgear requiremnts for your backup power system contact us today.


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