Transfer switches: Which configuration is right for your system?

When it comes to picking the right transfer switch for a facility, engineers need to consider many aspects such as system installation, operation modes, and switching mechanisms to help prevent downtime in the event of a power outage.

By Charlie Hume July 5, 2017

Many commercial and industrial facilities require continuous uptime to maintain business continuity in the event of a power outage. For this reason, these facilities rely on electrical distribution equipment such as transfer switches to safely transition electrical power between normal and emergency power sources.

Not all transfer switches are alike, however. The sheer number of available options and configuration modes can be daunting for an engineer while designing a system. Because of that, engineers need to understand the configurations available to determine what is correct for the application’s needs when implementing transfer switch technology. 

Common system installation types

Engineers first need to understand their system installation type to determine the best transfer switch. The National Electrical Code (NEC) defines four categories for transfer switches: emergency systems, legally-required systems, critical operations power systems, and optional standby systems. 

Emergency systems

Emergency systems supply, automatically distribute, and control electricity used by systems essential to life safety during fires and similar disasters. They include fire detectors, alarms, emergency lights, elevators, public safety communication systems, and ventilation systems. They are often found in hotels, theaters, arenas, and hospitals. And they are regulated by a municipal, state, federal, or other government agency. They require the transfer of power from the normal to emergency power source to be completed within 10 seconds. 

Legally-required systems

Like emergency systems, legally-required systems are government-regulated, but they are designed to automatically supply power to a selected set of regulated tools that are not classified as emergency systems. These systems serve functions such as critical heating, refrigeration, communication, ventilation, and lighting that could create hazards or interfere with rescue or firefighting operations if electrical power is unavailable. Power transfers between normal and emergency sources must be complete within 60 seconds.

Critical operations power systems (COPS)

These systems supply, distribute, and control electricity in designated control areas when a normal power source fails. These include HVAC, fire alarm, security, communication, signaling, and other services that a government agency has deemed important to national security, the economy, or public health and safety. 

Optional standby systems

Operational standby systems are not required to function automatically during power failures. They supply power to loads with no direct impact on health or life safety. These systems are most common in commercial buildings, farms, and residences. 

Understanding transition types

There are two basic ways transfer switches can transition loads between normal and emergency power sources: open or closed. The specific functions performed by a given load and the importance of those functions to safety or security play an important role in determining which kind of transition is required. 

Open transition

An open transition is a "break before make" transfer, meaning the transfer switch breaks its connection to one power source before making a connection to the other. For some time between disconnection and connection, neither the normal power source nor the emergency source is providing electricity to downstream loads. There are two kinds of open transition: open delayed and open in-phase. 

Open delayed transition

In an open delayed transition, the transfer switch pauses in-between disconnecting from one power source and connecting to the other. That delay typically lasts either a specific, pre-set amount of time, or however long it takes the load voltage to drop below a pre-specified level. 

Open in-phase transition

With open in-phase transitions, an automatic controller uses built-in intelligence to execute an open transition at the precise moment it expects the normal and emergency power sources to be synchronized in phase, voltage, and frequency. If synchronization doesn’t occur within that time span, some transfer switches have the ability to default automatically to a delayed transition that serves as a failsafe. 

Closed transition

A closed transition is a "make before break" transfer, meaning the transfer switch makes a connection to the new power source before breaking its connection to the old one. Because there is no gap between disconnection and connection, downstream loads receive continuous power throughout the transfer process. Switches configured for closed transitions usually transfer power automatically as soon as both power sources are closely synchronized in phase, voltage, and frequency. The overlap period during which both sources are simultaneously connected, or "paralleled," usually lasts no more than 100 ms to comply with local utility interconnect requirements. 

Switching mechanisms

The switching mechanism is the part of a transfer switch that is physically responsible for carrying the rated electrical current and shifting the load connection from one power source to another. Low-voltage switching mechanism technology comes in two basic varieties: contactor type and circuit breaker type. Circuit breaker switching mechanisms can be further divided into two sub-types: molded case and power case. 

Contractor switching mechanisms

Contractor switching mechanisms are the most common and affordable. These mechanisms often are constructed as a double-throw switch where an electrical operator opens one set of power contacts while closing a second set. In an open transition design, a mechanical interlock often is employed to prevent simultaneous closure of both contact sets. In a closed transition design, the mechanical interlock is absent. Contactor switching mechanisms are designed to support all three transition types: open delayed, open in-phase, and closed. However, these mechanisms don’t include integral overcurrent protection, so the power contacts are not self-protecting. 

Molded case switching mechanisms

Molded case switching mechanisms are used routinely for closing and interrupting a circuit under both normal and abnormal conditions. They are capable of supporting a mechanically-operated, over-center toggle or a motor operator. When configured for use in a transfer switch, a pair of molded case switches are operated through an interlocking mechanical linkage, which can be driven manually or automatically. These mechanisms provide a compact, cost-effective and service entrance-rated solution, as they eliminate the need for additional upstream protective devices. 

Power case mechanisms

Power case mechanisms are larger, faster, and more powerful than molded case mechanisms. The two-step stored energy technology they use can be operated mechanically and electrically, and some models feature integral overcurrent protection similar to what typically is found in molded case designs. Their high interrupt rating also makes power case mechanisms a good fit for applications vulnerable to large fault currents. 

Operation modes

Power transfers involve two processes: initiation and operation. Initiation is what starts the transfer, while operation is what completes it. Most transfer switches can support multiple operation modes through the addition of configurable options. 

Manual mode

In manual mode, initiation and operation are performed manually, typically by pushing a button or moving a handle, giving the operator maximum control of the transfer. An advantage of manual operation is, with molded case or power case designs, transfers can occur under load as a failsafe if the automatic controller sustains damage or becomes inoperable. 

Non-automatic mode

In non-automatic mode, the operator manually initiates a transfer by pressing a button or rotating a switch that causes an internal electromechanical device to electrically operate the switching mechanism. This device allows transitions to be completed more rapidly than they would with manual mode.

Automatic mode

Automatic mode involves the transfer switch controller completely managing initiation and operation, which can reduce delay time compared to manual and non-automatic mode. Initiation is triggered when the automatic controller senses an unavailability or loss of source power and operation typically is performed by an electric solenoid or motor. While this mode completes the transfer in the shortest time and isn’t dependent upon a human operator, automatic transfer switches tend to cost more than devices that operate only in manual or non-automatic mode. 

Bypass isolation mode

Bypass isolation mode allows users to service transfer switches safely without compromising availability. Traditional transfer switches feature a single switching mechanism, but bypass isolation transfer switches include dual-switching mechanisms that provide redundancy for critical applications. The primary switching mechanism handles day-to-day distribution of electrical power to the load, while the secondary switching mechanism serves as a backup. During repair or maintenance procedures, a technician can bypass power around the primary mechanism through the secondary mechanism to ensure that critical loads remain powered without interruption. 

Determine the right configuration for a facility

Transfer switches support multiple operation modes and transition types, and feature a range of different switching mechanisms. To determine the best fit for their facility, engineers should take the time to understand all the different aspects of transfer switch configurations and make their choices based on unique application needs.

By understanding the configurations available and choosing the right switch for the specific requirements, control engineers can help keep a facility up and running in the event of an outage or power loss, positioning them as essential contributors to the business’ bottom line.

Charlie Hume is a product line manager, automatic transfer switches, at Eaton. In his role, he is responsible for overseeing development and delivery of Eaton’s transfer switch solutions, including breaker-based and contactor-based switches. Edited by Chris Vavra, production editor, Control Engineering, CFE Media,


Key Concepts

  • Engineers need to understand what is correct for their unique needs when implementing transfer switch technology.
  • Factors engineers need to consider include system installation, transition types, switching mechanisms, and operation modes.
  • Picking the right transfer switch technology will keep a facility up and running in the event of an outage or power loss.

Consider this

What other factors should be considered when selecting a transfer switch for a facility?