Understanding transfer switch operation

Consulting engineers should understand transfer switch construction, performance requirements, selection criteria, and desired operation to ensure that critical systems and equipment are supplied with reliable backup power when needed.

12/03/2015


This article is peer-reviewed.Learning objectives: 

  • Explain basic transfer switch operation.
  • Describe types of standby systems and transfer switch requirements.
  • Compare types of transfer switches and their operations.
  • Evaluate transfer switch construction and performance requirements. 

When utility power is interrupted, power system failure is not an option for many facilities. Standby power systems have many components, including transfer switches that must be designed correctly. During power transitions, transfer switch timing and sequence is critical to ensure proper system operation. Consulting engineers must understand transfer switch types, timing requirements, ratings, and the types of standby systems where transfer switches are used to transfer to backup power. The basis of this article is NFPA 70-2014: National Electrical Code (NEC) unless otherwise noted.

Basic transfer switch operation

Transfer switches are responsible for transitioning electrical power from the primary source to a secondary source in the event of primary source interruption, maintenance, or failure. The primary source most commonly consists of the utility service. The secondary source typically consists of the backup or emergency power source. The sequence of operation typically occurs as follows:

  1. The primary source is interrupted or fails.
  2. When the secondary source is stable and within voltage and frequency tolerances, the transfer switch transitions to the secondary power source. This transition can occur automatically or manually.
  3. When the primary source is restored and stabilized, the transfer switch transitions back to the primary source and resumes under normal operation. This transition back to the primary source can occur automatically or manually.


Figure 1: The diagram shows a common configuration of a single generator and three ATSs. The separate transfer switches segregate the standby loads. Courtesy: JBA Consulting EngineersStandby system types

Standby system types include emergency systems, legally required standby systems, optional standby systems, critical operations power systems (COPS), and systems that support health care facilities (see Figure 1).

Emergency systems (NEC Article 700): Emergency systems are defined by the NFPA as "intended to automatically supply illumination, power, or both, to designated areas and equipment in the event of failure of the normal supply or in the event of accident to elements of a system intended to supply, distribute, and control power and illumination essential for safety to human life." These systems may include fire detection and alarm systems, elevators, fire pumps, and egress lighting.

Transfer equipment, including transfer switches, are required to be automatic, identified for emergency use, and approved by the authority having jurisdiction (AHJ). Transfer equipment shall be designed and installed to prevent inadvertent, simultaneous connection of primary and secondary supplies of power. Transfer equipment shall supply only emergency system loads. Power must be transferred to the secondary source in 10 sec or less.

Legally required standby systems (NEC Article 701): Legally required standby systems are defined by the NFPA as "intended to automatically supply power to selected loads (other than those classed as emergency systems) in the event of failure of the normal source." These systems may include heating and refrigeration systems, communications systems, ventilation and smoke removal systems, and other processes that, when stopped in the event of primary source interruption, could create hazards or hamper rescue or firefighting operations.

Transfer equipment, including transfer switches, are required to be automatic, identified for standby use, and approved by the AHJ. Transfer equipment shall be designed and installed to prevent inadvertent, simultaneous connection of primary and secondary supplies of power. Power must be transferred to the secondary source in 60 sec or less.

Optional standby systems (NEC Article 702): Optional standby systems are defined by the NFPA as "intended to supply power to public or private facilities or property where life safety does not depend on the performance of the system." These systems may include data processing and communication systems, and mission critical systems that are not legally required by the AHJ.

Transfer equipment, including transfer switches, for optional standby systems are not restricted to the same requirements as emergency and legally required system transfer equipment. However, transfer equipment shall be designed and installed to prevent inadvertent, simultaneous connection of primary and secondary supplies of power. There are no code requirements for power to be transferred to the secondary source within a certain time frame.

Critical operations power systems (COPS) (NEC Article 708): Interruptions or outages to designated critical operations areas may negatively impact national security, economy, public health, or safety. The requirement to comply with NEC Article 708 is provided by any governmental agency having jurisdiction or by a facility providing documentation establishing the necessity for such a system. These systems may include power systems, HVAC, fire alarm, security, and communications in these areas. NFPA 1600-2013: Standard on Disaster/Emergency Management and Business Continuity Programs contains further information on this topic.

Transfer equipment, including transfer switches, are required to be automatic and identified for standby use. Transfer equipment shall be designed and installed to prevent inadvertent, simultaneous connection of primary and secondary supplies of power.

Figure 2: The diagram shows an open-transition transfer switch operation. The break-before-make operation ensures the switch will disconnect from the primary source before connecting to the secondary source. Courtesy: JBA Consulting Engineers


Health care facilities (NEC Article 517): Essential electrical systems for hospitals consist of the emergency system and equipment system to supply a limited amount of lighting and power essential for life safety and effective hospital operation when the normal service is disconnected. The number of transfer switches used "shall be based on reliability, design, and load considerations" in accordance with NEC Article 517.30(B)(4). Each branch on the emergency system and equipment system, respectively, shall have one or more transfer switches to serve the system loads. However, one transfer switch shall be permitted in a facility with a maximum demand on the essential electrical system of 150 kVA or less. NFPA 99-2015: Health Care Facilities Code has additional requirements for transfer switch operation and features.

Transfer switch types

Transfer switch types include open-, closed-, fast closed-, soft closed-transition, and bypass/isolation.

Open-transition transfer switches: Open-transition transfer is commonly described as "break-before-make." This means that the transfer switch disconnects from the primary source before establishing the connection to the secondary source (see Figure 2). There is a short-duration electrical system outage during this transition. In addition, open transition, by design, does not allow paralleling of the two sources at the same time. Open-transition transfer switches are the most commonly used type. They are less expensive than other options.


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TED , TN, United States, 01/28/16 11:16 AM:

good job. Could use a little more info on safely splitting the emergency power side from utility.
Bruce , MN, United States, 02/02/16 05:27 PM:

Very clear and helpful!
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