What is a Transfer Switch? Types and Applications

What is a transfer switch? 

Transfer switches are devices that allow the safe connection or disconnection of  different sources of electricity to an electric load. Many homes and businesses are equipped with a standby generator that is used in the event of a power outage. Many university campuses and factories also have their own on-site power plants, but occasionally switch to the local grid for power. Some industrial users have sophisticated electrical systems with multiple on-site generators and grid access points that can connected to different electrical loads. In all cases, it is important to properly isolate the electricity generation sources when they are not in use and ensure that transitions from one supply to another are conducted in a safe and controlled manner.  

Broadly speaking, transfer switches are required in three categories of use cases. Transfer switches can be used to switch between grid service and a generator; between different generators; or between different grid service entrances. 

Transfer switch application types 

Utility-to-generator transfer switches 

Transfer switches prevent electricity travelling in the wrong direction, such as from a home generator into the power grid. This means utility workers can be sure that they will not get a shock from someone’s home generator when they are working on overhead lines. This is one of the reasons why most electric codes mandate the use of a transfer switch when a residential generator is connected to the home’s electrical panel.  

It’s not just home generators that require a transfer switch. Many types of businesses, industries and government services also rely on power generators, which also require a transfer switch. In some applications, specific requirements need to be catered for when installing a generator and transfer switch. For example, a data center’s backup power supply must become instantly available in the event of a power outage. More on this later.

Utility-to-utility transfer switches 

Utility-to-utility transfer switches are applicable when a facility is equipped with multiple utility service feeds. A utility-to-utility transfer switch is used when electric consumers within the facility require the option to switch from one service to the other. A simple use case would be one where a commercial or industrial building has two electrical meters but a single electrical system. Depending on which tenant is using the building at any given time, the building can be switched from one meter to the other, ensuring that each tenant is only responsible for their own electrical consumption. 

Generator-to-generator transfer switches 

Generator-to-generator transfer switches are needed at locations equipped with more than one on-site power supply. Typically, a generator-to-generator transfer switch is required when a home is equipped with solar panels and a generator. If the solar panels and the generator are not electrically isolated, the AC inverter associated with the solar panels may attempt to feed power back to the generator, creating a hazardous and potentially damaging situation for both the generator and the inverter. The transfer switch allows the homeowner to switch power between the solar panels and the generator, keeping the two systems electrically separated. Generator-to-generator transfer switches are also required at large industrial facilities with multiple on-site gensets. Depending on the facility’s electrical needs at any given time, different generators may be activated or deactivated. A mine, for example, may have variable power needs depending on which shafts are active and require ventilation at any given time, and may run a different set of generators accordingly.  

Transfer switches are, in summary, required in a large variety of applications and are therefore available in many different sizes and configurations to suit every possible use case. When selecting a transfer switch for a certain use case, multiple criteria need to be considered, including the voltage, current and number of phases of the application, whether the switch has a safety-critical role, the duration of service interruption that the load can tolerate, the environment in which the switch will be placed, and more.  


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Types of transfer switches 

One key consideration is how the switch is triggered. There are three main types of triggering mechanisms: manual, non-automatic and automatic.  

What are manual transfer switches? 

Manual switches, as their name indicates, need to be operated manually—usually by throwing a lever on the side of the transition switch. The lever moves a connector inside the switch enclosure from one position connecting to one power source, to another position connecting to the other power source. Think of a railroad worker pulling levers to move tracks at railroad junctions. Manual switches are relatively simple, with no or few electronic parts liable to fail, and are cheaper than automated switches. Manual switches, however, are prohibited in certain safety-critical applications where it is necessary to quickly switch power sources whether a human is present or not. 

What are non-automatic transfer switches? 

The next level up in terms of complexity is a non-automatic electrical push-button switch, which does the same job as a lever, but uses an electronically operated mechanical device to trigger the switch. The button can either be situated on the transfer switch itself or it can be located somewhere else, for example in a control room. Non-automatic switches are convenient when the transfer switch is difficult to access, or when there are multiple transfer switches to operate. The decision of when and whether the press the button, however, remains in human hands.   

What are automatic transfer switches? 

Automatic switches have the most sophisticated control mechanisms. They include a smart controller, which decides, independently of human intervention, when to activate the switch. An automatic switch may, for example, detect a power outage, automatically start up a standby generator, and make the switch to accept power from the generator. Then, when service is restored, the switch may automatically switch back and shut down the generator, all without any human intervention. The controller can be programmed by its owner to react automatically within preset parameters to any changes on the grid, or to any custom triggers.

Types of transfer switch transitions 

Another important consideration when selecting a transfer switch is how the transition between one source and another are made. Transitions can be either ‘open’ or ‘closed’. With certain types of load, selecting the right type of transition can be crucial for safety. 

What is open transition? 

Open transitions are often described as ‘break before make’ transitions because the load is disconnected from the old power source before it is connected to the new source. Open transitions can be either ‘open delayed’ or ‘open in-phase’. In a delayed transition, there is a set delay between the instant that the load is disconnected and the instant it is reconnected to the new source. Switches with delayed transitions are often used with inductive loads, such as transformers and large electric motors, which can otherwise generate electrical disturbances when rapidly reconnected to a non-synchronized source. In a way, delayed transition switches work like a manual transmission in a motor vehicle. You press the clutch pedal to disengage the transmission while shifting gears and, if the vehicle is in motion, you would shift to neutral until stopping before shifting to reverse. 

In some cases, the brief loss of power experienced by the load during the switching delay can be undesirable . In an office building or in a home, it would cause computers not protected by an uninterruptible power supply to shut down or reboot. At an air traffic control center, for example, computers rebooting in an untimely way would be highly unwelcome. This is where ‘open in-phase’ transitions come in. To carry out an in-phase transition, the transfer switch waits until the phase and voltage of the old and new sources match precisely, and then switches the load to the new source. The transition takes around 30 to 50 milliseconds and, from the load’s perspective, occurs without any appreciable interruption in the power supply. In-phase transition switches require a digital microcontroller to ensure that transitions occur when the sources are synchronized and therefore cannot be operated manually.  



What is closed transition? 

When strictly no interruption in power is acceptable, closed transition switches may be an option. Closed transitions are ‘make before break’ transitions, meaning that the transfer switch establishes a connection to the new power source before disconnecting the old power source. The transfer switch waits to do so, as is the case with open in-phase transitions, until the precise instant when both sources are in phase. Closed transitions, like open in-phase transitions, are only possible when the old power source can be maintained until the transition is complete. When power in the old source goes out unexpectedly, the transition switch may revert to a delayed transition. Closed transition switches, in addition, can be subject to specific requirements by the electric utility, resulting in the need for additional safeguards and documentation.  

Other types of features are available for transfer switches designed for certain specific use cases.  

What are service Entrance Rated transfer switches? 

The National Electric Code requires a way to disconnect the electrical service where it enters a building. In homes, this requirement is usually satisfied by the presence of a main circuit breaker in the electrical panel. Among other benefits, the main circuit breaker provides overcurrent protection to the home’s entire electrical system. This way, if a faulty electrical device in the home causes a short circuit, it will trip the breaker and cut power to the house, rather than start a fire. Where there is a main circuit breaker in the panel, a regular transfer switch can be placed downstream of the electrical panel. This is usually the case when a generator is installed in existing construction. In new construction, it can be cost-effective to install a Service Entrance Rated transfer switch, which incorporates a main circuit breaker. When a Service Entrance Rated transfer switch is placed between the main meter and the electrical panel, no additional main breaker is needed at the electrical panel. Additionally, because the switch is placed before the main panel, it can be installed outside, requiring one fewer penetration into the home and making the connection to the generator easier. As a result, selecting a Service Entrance Rated transfer switch can reduce overall costs, even though the switch may be slightly more expensive than a non-Service Entrance Rate switch. 

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What are bypass isolation transfer switches? 

Bypass isolation transfer switches are specialized switches that can be inspected and tested without interruption to the power supply. This feature is made possible by the presence of a second switch circuit within the transfer switch. When one switch circuit is being inspected, power is diverted through the other circuit. The presence of a second circuit also provides a level of redundancy which ensures that the switch continues to function should the main circuit fail. Bypass isolation transfer switches are more complex and more expensive than single circuit switches, but, because they are more reliable and ensure the absence of disruption to the power supply, they are required by code in many healthcare settings and other mission-critical applications.  

Selecting the right transfer switch for your needs can be complicated. Electrical codes provide helpful guidance but there many options and features available on the market that are not dictated by code.

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