This website requires certain cookies to work and uses other cookies to help you have the best experience. By visiting this website, certain cookies have already been set, which you may delete and block. By closing this message or continuing to use our site, you agree to the use of cookies. Visit our updated privacy and cookie policy to learn more.
This Website Uses Cookies
By closing this message or continuing to use our site, you agree to our cookie policy. Learn More
This website requires certain cookies to work and uses other cookies to help you have the best experience. By visiting this website, certain cookies have already been set, which you may delete and block. By closing this message or continuing to use our site, you agree to the use of cookies. Visit our updated privacy and cookie policy to learn more.
ES Magazine logo
search
cart
facebook twitter linkedin youtube
  • Sign In
  • Create Account
  • Sign Out
  • My Account
ES Magazine logo
  • Home
  • Magazine
    • Current Issue
    • Digital Edition
    • Ad Index
  • Products
  • News
  • Sectors
    • Colleges/ Universities
    • Commercial
    • Government/ Institutional
    • Health Care
    • Hospitality
    • K-12
    • Mission Critical
  • Topics
    • Building Automation
    • Cooling & Chillers
    • Design/ Construction Process
    • Heating & Boilers
    • IAQ
    • Life Safety
    • Motors & Drives
    • Pumps & Flow Controls
    • Retrofits
    • Ventilation
  • Columns
    • Back2Basics
    • Building Automation Column
    • Case In Point
    • Commissioning
    • Editor's Note
    • Facility Files
    • IAQ: A Physician's View
    • Tomorrow's Environment
  • Multimedia
    • Videos
    • Podcasts
    • Webinars
    • Video Spotlights
    • Interactive Spotlights
    • Sneak Peek Spotlight
    • White Paper eBlast
    • Case Study eBlasts
  • Resources
    • eNewsletters
    • Continuing Education
    • White Papers
    • Industry Events
    • ES Store
    • Classifieds
    • Custom Content & Marketing Services
    • Market Research
  • Today's Boiler
    • Today’s Boiler Digital Edition
  • Directory
    • SourceBook
    • Take a Tour
  • Contact
    • Contact Us
    • Advertise
Home » Current Affairs: The Heart of an Emergency and Standby Power System
Columns

Current Affairs: The Heart of an Emergency and Standby Power System

December 18, 2003
Timothy J. Coyle, P.E.
Reprints
No Comments
The automatic transfer switch (ATS) is charged with detecting loss of normal power, signaling the standby source to start, and transferring the load, and is critical to successful operation. We will start by examining ATS basics and then discuss special features and application considerations.

Operation

A simplified schematic of a three-phase ATS is shown in the figure here. Two sets of solenoid- or motor-operated contacts are provided. In normal operation, the contacts labeled "N" are closed and the "E" contacts are open, connecting the load terminals to the normal source. When normal power is lost and the emergency source is available, the "N" contacts open and the "E" contacts close, transferring the load to the emergency source. In general, only one set of contacts is closed at a time to prevent connecting the normal and emergency sources.

Integral controls sense the loss of normal power (usually the utility) and cause the switch to transfer when the emergency source (usually a standby generator) reaches acceptable voltage and frequency.

Three-phase sensing of normal source volt-age is used to detect undervoltage or phase-loss conditions. When any phase voltage remains outside of an adjustable range for a preset length of time, contacts will close causing the standby generator to run. When acceptable voltage and frequency is sensed on the emergency source terminals, the switch transfers.

Upon return of an acceptable normal source, a time delay is initiated to ensure the source is stable. If voltage remains within the acceptable range for the delay period, usually 15 to 30 sec, the switch retransfers. Any excursion from the acceptable range during this period will reset the timer. If the transfer switch is serving motor loads or elevators, other control functions are required to prevent damaging transients when transferring between two energized sources. We will discuss these features next month.

After retransfer, the engine control contact remains closed and the engine continues to run at no load for a short period of time. This cool-down time delay is adjustable and typically set at 5 min.

Ratings

Like other electrical control equipment, the basic ratings of an ATS are voltage, cur-rent, and short-circuit withstand capability. The voltage rating must equal the system nominal voltage. In most cases, manufacturers do not construct the power components differently for different voltage levels and the contact structure itself is capable of being used at any voltage up to the maximum rating offered. Control circuits, however, are designed to operate at specific terminal voltage and a switch rated for 208/120 V may not be used without modification at 480/277 V even though the contact construction and insulation is identical to that of a switch rated for the higher voltage.

The current rating of an ATS is the value that it can carry continuously within its design temperature rise at an ambient of 40°C. If the expected ambient is higher, consult the manufacturer regarding both derating of the switch and the capability of the controls to function properly. The ATS is required by standards to be capable of making and breaking its rated current during transfer.

The short circuit withstand rating of an ATS must equal or exceed the fault current available from the system. Two different ratings are available, depending on test method.

A switch tested with specific fuses or circuit breakers is assigned a rating that applies only when the upstream protective device is of that manufacturer and catalog number. Switches are also tested for a three-cycle (48 millisecond) rating, which applies regardless of the type of upstream protection provided. The latter ratings are typically lower, and thus are preferred for design when the actual upstream device may be unknown.

Construction

Most ATSs are designed for this specific application with contacts and operating mechanisms optimized for frequent switching. Others are assembled from standard molded-case circuit breakers provided with external motor-operators. Each manufacturer will be happy to provide you with a list of the advantages of its products' construction and the disadvantages of the other approach. I won't get into the middle of that argument except to say that in general the customized designs are considered to have an advantage in switching duty, and the circuit breaker types to have an advantage in short circuit withstand capability. Both types must pass the same tests to be listed by UL as automatic transfer switches.ES

es-subscribe

Recent Articles by Timothy Coyle, P.E.

Motors: Terms Of Service, Part 2

Utility Interconnection With On-Site Generation

Terms Of Service: Motor Fundamentals

Size Matters: Trends In Standby Power Systems For Health Care Facilities

Calculating Backup Power Supply Requirements

Timothy Coyle, an electrical engineer with 35 years of experience, specializes in planning, analysis, and design of critical power systems in institutional and utility facilities. He is a senior electrical engineer with Karges-Faulconbridge, Inc. Email him at tjcoyle@kfi-eng.com.

Related Articles

Current Affairs: Outsmart the Outage (December 2000)

Current Affairs: Have An Earthly Idea

Current Affairs: Power Quality Part 2 (September 2000)

Current Affairs: Power Quality - Part 3 Vfd/Motor Interaction

Related Products

The Integrative Design Guide to Green Building: Redefining the Practice of Sustainability

Uncomplicating The Heat Pump: Electrical System Troubleshooting DVD

Uncomplicating The Heat Pump: Refrigeration & Air Flow Systems DVD

Engineered Systems May 2019 Issue

Related Directories

Kohler Power Systems

John Deere Power Systems

Indeck Power Equipment Co.

You must login or register in order to post a comment.

Report Abusive Comment

Subscribe For Free!
  • Print & Digital Edition Subscriptions
  • eNewsletters
  • Online Registration
  • Subscription Customer Service

More Videos

Popular Stories

Dynamic Data Center Workloads Require a Dynamic Cooling Solution

Dynamic Data Center Workloads Require a Dynamic Cooling Solution

Protecting Pharmacists

Protecting Pharmacists: New USP 800 Standards Focus on Safety in Handling Hazardous Drugs

The Dangers of Neglecting Regular Boiler Maintenance

The Dangers of Neglecting Regular Boiler Maintenance

A2L Nov 2019 Figure 1

A2L Refrigerants: Safely Addressing Flammability Concerns

Jeff Edwards

Jeffrey W. Edwards Presented with ASPE Award of Merit

ES-20toWatch-winners250


Todays boiler

Events

December 30, 2030

Webinar Sponsorship Information

For webinar sponsorship information, visit www.bnpevents.com/webinars or email webinars@bnpmedia.com.

View All Submit An Event

Poll

Favorite Engineered Systems columns

Which Engineered Systems column is your favorite:
View Results Poll Archive

Products

The Green Energy Management Book

The Green Energy Management Book

Learn from our experts how to evaluate job opportunities, market your services, sell a Walk-through Survey, target areas for an Energy Audit, calculate energy savings, do retrofit work, and win continuing contracts for retrofit work.

See More Products

ES Kohler webinar


ES_2018Top10States_360

Engineered Systems Magazine

Engineered Systems December 2019

2019 December

Check out the December 2019 edition of Engineered Systems: Discussion of the "next generation" of building controls, the health impact of indoor particulate matter, evidence-based decision-making in the built environment and much more!
View More Create Account
  • Resources
    • ES Glossary
    • HVAC glossary
    • New Products
    • Mechanical Group
    • Partners
    • List Rental
    • Privacy Policy
    • Survey And Sample
  • Want More
    • Connect
  • Contact

Copyright ©2019. All Rights Reserved BNP Media.

Design, CMS, Hosting & Web Development :: ePublishing