Molly Ayala
A transfer switch is a device that transfers electrical loads between power sources, such as utility power and generator power. It is designed to remain connected to power distribution circuits under all foreseeable circumstances, including very large fault and short-circuit currents. Some transfer switches are manual, requiring an operator to effect the transfer by throwing a switch, while others are automatic and trigger when they sense that a source has lost or gained power. An Automatic Transfer Switch (ATS) is often installed where a backup generator is located, so that the generator may provide temporary electrical power if the utility source fails.
| Characteristics | Values |
|---|---|
| Function | Transfers electrical loads between power sources |
| Types | Manual, automatic, closed transition (CTTS), open transition, soft-loading (SLTS), static, non-automatic electrical push-button, delayed transition, make-before-break, break-before-make, service entrance-rated |
| Applications | Residential, commercial, industrial (e.g., data centers, hospitals, factories) |
| Benefits | Ensures continuous power supply, improves safety, enhances compliance with electrical standards, reduces downtime, protects from overloads |
| Considerations | Number of generators, machine spacing, primary and alternate power source frequencies, voltage requirements, maintenance, cost |
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What You'll Learn
Transfer switches transfer electrical loads between power sources
A transfer switch is a device that transfers electrical loads between power sources. It is designed to connect a power source to a circuit, even in the presence of overcurrents up to the device's maximum rating. Transfer switches are available in manual and automatic variants. Manual transfer switches require an operator to effect the transfer by throwing a switch, while automatic transfer switches trigger when they sense that a source has lost or gained power.
Automatic Transfer Switches (ATS) are often installed where a backup generator is located, so that the generator may provide temporary electrical power if the utility source fails. ATSs may also command the backup generator to start, based on the voltage monitored on the primary supply. They are also responsible for disconnecting the generator from the electric utility when it is on and providing temporary power.
Transfer switches can be set up to provide power to critical circuits or entire electrical subpanels. They can also allow for load shedding or the prioritization of optional circuits, such as heating and cooling equipment. Some transfer switches are designed to transfer loads between utility power and generator power without interrupting downstream equipment or introducing a strategic delay. This is known as a closed transition transfer switch (CTTS).
In contrast, an open transition transfer switch breaks contact with one source of power before making contact with another. This type of transfer switch is used in typical emergency systems, where there is a momentary interruption of power to the load when it is transferred from one available source to another.
Transfer switches are essential in providing backup power to homes, businesses, and critical facilities during power outages. They are also necessary to ensure compliance with relevant electrical codes and standards, such as the National Electric Code, which mandates the use of a permanent switching means to connect a contingent power source in establishments with a single generator.
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They can be manual or automatic
A transfer switch is a device designed to transfer electrical loads between power sources, such as utility power and generator power. During a power outage, it connects electrical loads to a backup power source, typically a generator.
Transfer switches can be manual or automatic. Manual transfer switches are operated by a person who effects the transfer by throwing a switch. An example is a manual three-position switch or circuit breaker, with utility power on one side, the generator on the other, and "off" in the middle. The operator must switch through the full disconnect "off" position before making the next connection.
Non-automatic electrical push-button switches are another type of manual switch. They do the same job as a lever, but use an electronically operated mechanical device to trigger the switch. The button can be situated on the transfer switch or located elsewhere, such as in a control room. The decision of when and whether to press the button remains in human hands.
Automatic transfer switches (ATS) trigger when they sense that one of the sources has lost or gained power. They include a smart controller, which decides, independently of human intervention, when to activate the switch. An ATS may, for example, detect a power outage, automatically start up a standby generator, and make the switch to accept power from the generator. When the service is restored, the ATS will switch back and shut down the generator.
ATS are often installed where a backup generator is located, so that the generator may provide temporary electrical power if the utility source fails. ATS may also command the backup generator to start, based on the voltage monitored on the primary supply. The principal purpose of an ATS is to ensure the continuous delivery of electrical power from one of two power sources to a connected load circuit.
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They prevent backfeeding from an emergency generator
A transfer switch is an electrical switch that switches a load between two sources. It is often used to connect a backup generator during a power outage. An Automatic Transfer Switch (ATS) is usually installed where a backup generator is located, so that the generator may provide temporary electrical power if the utility source fails.
Transfer switches are essential for safely connecting a generator to a home's electrical system, preventing backfeeding and enabling power control. Backfeeding occurs when a generated power source is introduced without isolating the power supply, creating a new and unintentional path for power that reverses the typical flow of energy. This can cause power to flow back into the utility line, creating a risk of electric shock to utility workers and damaging electrical equipment.
Transfer switches prevent backfeeding by safely isolating the power source from its primary source to a backup or secondary source. During a power outage, a transfer switch can be used to connect electrical loads to a backup power source, typically a generator. The transfer switch isolates the backup generator from the electric utility when the generator is on and providing temporary power. This ensures that the generator powers only the home's circuits, isolating it from the utility lines for a safe and compliant setup.
There are different types of transfer switches available, including manual and automatic transfer switches. A manual transfer switch requires an operator to effect the transfer by throwing a switch, while an automatic transfer switch triggers when it senses that one of the sources has lost or gained power. Automatic transfer switches have the most sophisticated control mechanisms, including a smart controller that decides when to activate the switch independently.
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They ensure compliance with electrical codes and standards
A transfer switch is an electrical switch that transfers electrical loads between power sources, such as utility power and generator power. During a power outage, it is used to connect electrical loads to a backup power source, typically a generator. Transfer switches are available in different types to suit various needs. They can be manual, automatic, or a combination of both.
Incorporating transfer switches into electrical systems helps ensure compliance with relevant electrical codes and standards. For example, the National Electric Code® requires a way to disconnect the electrical service where it enters a building. This requirement is usually satisfied by the presence of a main circuit breaker in the electrical panel. A 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. This ensures compliance with the National Electric Code's requirement for a means to disconnect the electrical service.
Transfer switches also help to ensure compliance with electrical standards related to safety. For example, the UL1008 standard permits transfer switches to be marked with one or more short-circuit and/or short-time WCRs specific to an overcurrent protection device type. Transfer switches themselves are not designed to protect load circuits and equipment from overcurrents. Instead, they work in conjunction with overcurrent protection devices such as circuit breakers to protect downstream circuits and equipment.
Additionally, transfer switches prevent generators from back-feeding power into the grid, which is an important safety objective. They isolate the backup generator from the electric utility when the generator is on and providing temporary power. This isolation protects utility workers who expect the lines to be dead during an outage.
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They are essential for emergency power systems
A transfer switch is an electrical switch that transfers electrical loads between two power sources. They are essential for emergency power systems as they allow for the transfer of power from a primary source to a backup generator. This is especially important in hospitals, factories, data centres, and other facilities that require continuous uptime.
During a power outage, a transfer switch connects electrical loads to a backup power source, typically a generator. This ensures that equipment remains connected to a power source and can continue operating without interruption. For example, in a home equipped with a backup generator and an ATS, when a power outage occurs, the ATS will signal the backup generator to start. Once the generator is ready, the ATS disconnects the home from the electric utility and connects the generator to the home's main electrical panel.
Transfer switches can be manual or automatic. Manual transfer switches require an operator to physically throw the switch, while automatic transfer switches (ATS) trigger automatically when they sense that one of the sources has lost or gained power. ATSs are often installed with backup generators, so the generator can provide temporary power if the utility source fails. They can also command the backup generator to start based on the voltage of the primary supply.
In addition to providing uninterrupted power, transfer switches also offer safety benefits. They prevent generators from back-feeding power into the grid, which is important for worker safety. They also ensure compliance with relevant electrical codes and standards, such as the National Electric Code, which mandates the use of a permanent switching means to connect a contingent power source in establishments with a single generator.
Overall, transfer switches play a crucial role in emergency power systems by providing a reliable means of transferring power, ensuring continuous operation of equipment, and enhancing safety.
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Frequently asked questions
A transfer switch is an electrical switch that transfers electrical loads between two power sources.
Transfer switches can be manual or automatic. Manual transfer switches require an operator to throw the switch, while automatic transfer switches trigger when they sense that a source has lost or gained power.
Transfer switches are used to provide a reliable means of transferring electrical loads between primary and alternate sources of electrical power. They are especially important in data centres, hospitals, factories and other facilities that require continuous uptime.
A transfer switch remains connected to power distribution circuits under all foreseeable circumstances. When the primary power source fails, the transfer switch shifts the load to the emergency power source. Once the primary source is restored, the transfer switch shifts the load back to the primary source and shuts down the generator.
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