Transfer switches in emergency systems

Automatic transfer switches (ATSs) are by far the most commonly used form of transfer switches for emergency or legally required standby power systems.

04/17/2012


ATSs traditionally consist of switches but may also use a collection of motor-operated circuit breakers, or more recently may be solid-state “static transfer switches” if rated per UL1008.

ATSs contain voltage and frequency monitors that have settings when the primary power source has dropped out (typically 75% to 95% of normal levels) and when it has returned (typically 85% to 98% of normal levels). When the power has dropped below the preset level, the ATS will transfer to the alternate power source within a normally selectable time of 0 to 6 secs. Delays to retransfer back to normal power are provided from 0 to 30 min. This delay to retransfer is desirable to ensure that the normal power source has fully returned and is stable before retransferring. If the ATS is using a normally off-line power source, such as an emergency generator, the ATS will send a start signal to the generator.

ATSs must be able to handle the anticipated full load current continuously, 24 hours a day, seven days a week for an anticipated minimum life of at least 20 years. Current ratings range from 30 to 4000 amps. Typical ratings include 30, 40, 70, 80, 100, 150, 225, 260, 400, 600, 800, 1000, 1200, 1600, 2000, 3000, and 4000 amps. The engineer should select a transfer switch equal to or greater than the calculated continuous current.

A transfer switch must be capable of withstanding and closing into the available fault current at its location in the system until the overcurrent protection device upstream clears the fault. The engineer should determine the available fault current at the transfer switch location and the ratings of the overcurrent protection devices to be used with the ATS. Methods for calculating short circuits and applying protective devices can are found in ANSI/IEEE Standard 242-2001, Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems. In addition, UL1008, Standard for Safety Automatic Transfer Switches, lists minimum requirements and testing for ATSs including withstand ratings and coordination ratings with overcurrent protection. The engineer should be familiar with the contents of these publications before specifying and applying ATSs.


Rener is manager, electrical platform leader and manager of quality assurance with M+W Group, Chicago. He is a member of the Consulting-Specifying Engineer editorial advisory board. This article is dedicated to his late father, Pierre J. Rener, who taught him and many others about effective communication, dedication to one’s profession, and the importance of education and lifelong learning. 



No comments
Consulting-Specifying Engineer's Product of the Year (POY) contest is the premier award for new products in the HVAC, fire, electrical, and...
Consulting-Specifying Engineer magazine is dedicated to encouraging and recognizing the most talented young individuals...
The MEP Giants program lists the top mechanical, electrical, plumbing, and fire protection engineering firms in the United States.
High-performance buildings; Building envelope and integration; Electrical, HVAC system integration; Smoke control systems; Using BAS for M&V
Pressure piping systems: Designing with ASME; Lab ventilation; Lighting controls; Reduce energy use with VFDs
Smoke control: Designing for proper ventilation; Smart Grid Standard 201P; Commissioning HVAC systems; Boilers and boiler systems
Case Study Database

Case Study Database

Get more exposure for your case study by uploading it to the Consulting-Specifying Engineer case study database, where end-users can identify relevant solutions and explore what the experts are doing to effectively implement a variety of technology and productivity related projects.

These case studies provide examples of how knowledgeable solution providers have used technology, processes and people to create effective and successful implementations in real-world situations. Case studies can be completed by filling out a simple online form where you can outline the project title, abstract, and full story in 1500 words or less; upload photos, videos and a logo.

Click here to visit the Case Study Database and upload your case study.

Protecting standby generators for mission critical facilities; Selecting energy-efficient transformers; Integrating power monitoring systems; Mitigating harmonics in electrical systems
Commissioning electrical systems in mission critical facilities; Anticipating the Smart Grid; Mitigating arc flash hazards in medium-voltage switchgear; Comparing generator sizing software
Integrating BAS, electrical systems; Electrical system flexibility; Hospital electrical distribution; Electrical system grounding
As brand protection manager for Eaton’s Electrical Sector, Tom Grace oversees counterfeit awareness...
Amara Rozgus is chief editor and content manager of Consulting-Specifier Engineer magazine.
IEEE power industry experts bring their combined experience in the electrical power industry...
Michael Heinsdorf, P.E., LEED AP, CDT is an Engineering Specification Writer at ARCOM MasterSpec.