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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.

By Ryan Ishino, PE, RCDD, LEED AP, JBA Consulting Engineers, Irvine, Calif. December 3, 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.

Closed-transition transfer switches: Closed-transition transfer is commonly described as "make-before-break." This means that the transfer switch creates a connection to the secondary source while connected to the primary source (see Figure 3). When the connection to the secondary source is established, the primary source will disconnect. This enables a continuous source of supply to the electrical system as the two sources are paralleled together. Paralleled (or interconnected) sources shall comply with NEC Article 705: Interconnected Electric Power Production Sources, which addresses basic safety requirements related to parallel operation of generators and normal/primary sources (commonly the utility service).

Figure 3: The diagram shows a closed-transition transfer switch operation. The make-before-break operation ensures the switch will connect to the secondary source while connected to the primary source. Courtesy: JBA Consulting Engineers

Closed-transition switches transfer when both sources are synchronized in phase, voltage, and frequency. The length of the synchronization period where both sources are paralleled is usually limited by the utility company’s interconnect agreement and requirements.

Fast closed-transition transfer switches: Fast closed-transition transfer switches use a momentary paralleling of sources (typically less than 100 msec) using a control system similar to the open-transition transfer switch system. Some fast closed-transition transfer switches use passive synchronization to sense the phase relationship between the two live sources (in the event of paralleling) and allow interconnection of sources when they are synchronized. This is considered passive because there is no direct control over the generator frequency, and the sources are paralleled for such a short duration of time. Although the intent is to not parallel the sources for an extended amount of time, utility service providers commonly require reverse-power relay protection to protect their systems from sustained paralleled operation. Fast closed-transition switches are more expensive than open-transition switches but less expensive than soft closed-transition switches.

Soft closed-transition transfer switches: Soft closed-transition transfer switches use an automatic synchronizer to enable the generator to synchronize with the utility service and transfer the loads. The transfer time may vary from seconds to several minutes, usually depending on requirements of the utility provider. During this process, there is a sustained duration of paralleled operation between the two sources. As such, voltage and frequency disruptions are typically reduced due to the gradual transfer of loads between sources. Similar to fast closed-transition switches, utility service providers commonly require a higher level of relay protection and approvals to implement this type of system.

Bypass/isolation transfer switches: As the name implies, bypass or isolation capabilities may be provided to the transfer systems listed above to bypass the primary transfer switch components without interrupting power to the facility. The secondary switching component offers inherent redundancy and increases reliability. This enables transfer switch maintenance without shutting down equipment as well. Bypass/isolation capabilities are typically specified for highly critical equipment or continuous loads where a loss of power is detrimental to the performance of the facility or operations.

However, bypass/isolation capabilities typically add a significant cost to the system and add additional space requirements to accommodate the extra equipment.

Figure 4: The photo shows paralleling switchgear equipment that serves a large hospitality venue in Las Vegas. The property used paralleling switchgear to combine electrical capacity from multiple generators while providing load management and transfer. CParalleling switchgear

Paralleling switchgear is typically used to combine multiple power sources (commonly two or more generators) and connect to a common bus to use the aggregate capacity of the sources (see Figure 4). The power sources must be synchronized where the frequency, voltage, phase angle, and phase rotation are within prescribed limits and the sources can be paralleled together. Paralleling switchgear may use motorized circuit breakers and programmable logic controllers to operate and prioritize distribution loads, and, as such, this configuration may not require stand-alone transfer switches to transfer loads. However, some AHJs may require separate transfer switches for loads dictated by NEC Articles 700 and 701 for complete segregation of systems; the AHJ should be consulted for acceptable and approved system configurations. Load maintenance and priorities can be established to ensure that highest priorities, such as emergency, legally required, and optional standby loads, are provided with standby/backup power within specified time frames to comply with transfer requirements.

Nonessential loads can also be provided with priority steps in the event there is additional available capacity on the backup system after the aforementioned higher priorities have been addressed. Paralleling switchgear systems typically can be programmed to provide many sophisticated functions, such load shed, generator off-loading, utility soft transfers, high-level metering, and load maintenance functions. However, these functions can increase the costs significantly and usually require a higher level of technical operations personnel to maintain them over their lifetime.

Transfer switch operation

Transfer switch operation occurs based on the initiation and transfer processes. The initiation process is what identifies that the transfer needs to occur. This event may consist of a loss of, or inconsistent voltage from, the primary source. The transfer is the process of shifting load from the secondary or alternate source, and vice versa.

Automatic: In automatic mode, the transfer switch controller manages the entire process, and initiation begins when the controller senses a loss of the primary source. The controller monitors the source voltage and sends a command to the generators to run when the voltage falls below a preset limit for a prescribed time frame. The controller also monitors the secondary source voltage and frequency, and when these values are within acceptable limits, the switch transfers the load from the primary to the secondary source. When the primary source has been reestablished for a prescribed time to ensure stability, the switch may automatically transfer the load back to the primary source. Most critical and life safety loads require automatic operation as defined by the NEC.

Nonautomatic: In nonautomatic mode, the transfer switch is manually initiated by an operator and then an internal device within the switch equipment operates the transfer switch by electric operation. The operator has the ability to determine when to initiate the load transfer, but the actual transfer operation is electrically actuated.

Manual: In manual mode, the entire process is completed manually by an operator. There is not typically a controller, voltage-sensing equipment, or electrical mechanism used to operate the load transfer. Manual switches are the most basic types of transfer switch and are common in noncritical facilities or applications.

Transfer switch construction, performance requirements

Codes and standards, such as UL 1008-8: Transfer Switch Equipment, UL 1008A-1: Standard for Medium-Voltage Transfer Switches, and NFPA 110-2016: Standard for Emergency and Standby Power Systems provide transfer switch construction and performance requirements.

UL 1008: UL 1008 is the most commonly applied and adopted standard to address transfer switch construction and testing in the U.S. This standard applies to automatic, manual or nonautomatic, closed transition, hybrid, fire pump transfer switches, bypass/isolation switches, and several others. However, this standard does not specifically address switches rated higher than 600 V.

UL 1008 includes requirements that address construction and performance of the transfer switch equipment. The construction requirements within the standard include, but are not limited to, the enclosure equipment, field and internal wiring of the components, and installation of the equipment.

The performance and testing requirements include, but are not limited to, withstand and closing rating, overvoltage/undervoltage, overload, temperature, and endurance tests. These tests and construction requirements demonstrate that the transfer switch equipment should be dependable and reliable when the transfer operation is necessary.

UL 1008A: Similarly, UL 1008A contains requirements for automatic, nonautomatic, and manual transfer switches with operation higher than 750 V and up to 46 kV. The requirements within the standard cover the transfer switch and associated control devices and relays.

NFPA 110: NFPA 110 is commonly applied and adopted within the United States.Chapter 6 of the standard includes requirements for transfer switch equipment. Requirements in the standard related to automatic transfer switch (ATS) features specify that their capabilities must include:

  • Electrical operation and mechanical holding
  • Transfer and retransfer of the load automatically
  • Visual annunciation when not in automatic mode.

Chapter 6 also requires that "the transfer switch shall be capable of withstanding the available fault current at the point of installation." In addition, "the transfer switch shall have a continuous current rating and interrupting rating for all classes of loads to be served." The electrical rating of the transfer switch shall be sized appropriately for the total connected load.

NFPA 110 includes requirements for source monitoring, such as undervoltage sensing devices, to monitor all ungrounded lines of the primary source, voltage- and frequency-sensing equipment to monitor one ungrounded line, and to ensure that transfer to the secondary source is inhibited until voltage and frequency are within specified limits.

There are time-delay devices are provided to delay the transfer process to avoid nuisance starting and load transfer based on transient power dips or momentary disturbances to the primary source. An adjustable time-delay device shall be provided to delay the load-transfer process to avoid excessive system voltage drop for use where the failure of operation of equipment could result in injury or loss of human life. In addition, another adjustable time-delay device (with automatic bypass) shall be provided to delay transfer from the secondary source (typically the emergency power supply) to the primary source and to allow the primary source to restabilize.

Staying in power

Understanding transfer switch construction and performance requirements, in addition to selecting the proper transfer switch types and desired operation for the specific standby systems served, will help ensure that critical systems and equipment are supplied with reliable backup power when they are needed.


Ryan Ishino is the associate director of electrical for the Orange County office at JBA Consulting Engineers in Irvine, Calif. He has experience in multiple market sectors including hospitality, commercial, health care, education, and central plant design.