Transfer of Power

The transfer switch allows multiple sources of power to supply a given load. While the transfer from one power source to another can be controlled manually or automatically, all transfer switches are built to certain code requirements for transferring loads. No matter what type of transfer switch is used, it is important that the electrical system designer understand the basics about these devi...


The transfer switch allows multiple sources of power to supply a given load. While the transfer from one power source to another can be controlled manually or automatically, all transfer switches are built to certain code requirements for transferring loads.

No matter what type of transfer switch is used, it is important that the electrical system designer understand the basics about these devices when selecting and applying a transfer switch so as to maintain reliable power and minimize unwanted power outages. Consequently, a comparison of the basic types of transfer switch is in order.

The manual transfer switch is used to provide simple, cost-effective switching for the electrical system where there is no requirement for automatic connection to the alternate power source. Manual transfer switches are also used to isolate portions of the power distribution system for maintenance or modifications. Moreover, manual transfer switches may be controlled locally or via remote control stations. But the designer must be sure to confirm that the manual transfer switch is rated to transfer under load.

The automatic transfer switch, on the other hand, commonly referred to as an ATS, is by far the most common form of transfer switch. Required for emergency or legally required standby power systems, the ATS traditionally consists of switches, but it might also use a collection of motor-operated circuit breakers, or more recently, might use solid-state “static transfer” components.

Although solid-state transfer switches have been around for a long time, only recently have they started being produced in larger ampere sizes to be used as alternatives to mechanical-based ATS. Because of their high-speed electronic switching, one avoids any interruption to even the most sensitive loads upon transfer between sources. This makes them ideal in applications using uninterruptible or continuous power supply systems.

But there is an additional advantage. The lack of mechanical movement eliminates the normal wear and tear associated with the physical operation of a switch, allowing for many more operations. Unfortunately, the costs and complexity of static transfer switches are still very high compared to ATS, so the engineer should examine these uses carefully.

ATS contain voltage and frequency monitors that have settings when the primary power source has dropped out (typically 75% to 95% of normal levels) and when it has returned (typically 85% to 98% of normal levels). When the power has dropped below the preset level, the ATS will transfer to the alternate power source within a normally selectable time of 0 to 6 seconds. Delays to retransfer back to normal power are provided from 0 to 30 minutes. This delay to retransfer is desirable to ensure that the normal power source has fully returned and is stable before retransferring. If the ATS uses a normally off-line power source, such as an emergency generator, the ATS will send a start signal to the generator.

So much for the types of transfer switches to consider. What is also necessary is a review of the types of power sources.

Classifying Sources

ATS equipment is an integral part of most emergency or standby power systems. It is important that the engineer review the code requirements when deciding on the classification of an alternate power source. For example, loads within a facility are normally grouped into three categories: emergency, standby and optional. These classifications affect how transfer switches are applied within the distribution system. Often engineers and facility owners refer to their “emergency” generator when, in fact, it is a “standby” generator. There are notable code differences between these two types of transfer systems.

Several relevant codes and standards have requirements for these classifications:

  • NFPA, 70-2005 National Electrical Code, Articles 700, 701 and 702

  • NFPA, 99-2005 Health Care Facilities

  • NFPA, 101-2006 Life Safety Code

  • NFPA, 110-2005 Standard for Emergency and Standby Power Systems

  • ANSI/IEEE Std 446-1995, Emergency and Standby Power Systems

  • UL 1008 Automatic Transfer Switches.

The NEC article 700 classifies emergency systems as “those systems legally required and classified as emergency by municipal, state, federal or other codes, or by any governmental agency having jurisdiction.” The NEC further states that “these systems ... automatically supply illumination and power… essential for safety to human life.” In practical terms, this would normally include egress lighting, fire detection and protection, elevators, public safety communications or any system where loss of power would cause serious endangerment to life safety or health. The emergency power isrequired to be available within 10 seconds or less. The engineer should carefully review all applicable codes and requirements from the authorities having jurisdiction to categorize the loads for a new project.

When applying ATS to emergency power systems, several requirements should be noted by the engineer. The NEC requires that ATS as part of the emergency system be tested under load on a regular basis, typically every three months. The ATS must have adequate capacity and rating to handle all loads operating simultaneously. This does not allow the engineer to apply diversity or demand factors normally used to determine the capacity and ratings of normal power systems.

It is often also desired to manually bypass or isolate the power around the ATS. In such cases, the bypass should be specified as an internal feature within the ATS. In addition, due to NFPA 70E, which covers arc-flash safety concerns, the manual bypass should be accomplished without it being necessary to open the main compartment door of the ATS. The National Electrical Code also requires that the emergency power system be routed independently of other power systems. Moreover, the NEC also requires one-hour fire resistance or fire protection for the entire emergency distribution system, including the location of the ATS equipment.

And finally, there is a requirement for emergency power sources to have ground fault indication—not interruption—on the system.

It should also be pointed out that the NEC also contains requirements for legally required standby systems. Standby systems may include communications, selected ventilation or smoke removal systems, lighting, or certain types of industrial processes that may create hazards or hamper firefighting operations if power were not available. Standby systems must be available within 60 seconds and may be routed in the same raceway as normal power systems.

Finally, the last type of power supplied by transfer switches is classified by the NEC as optional standby systems. These systems consist of loads that do not affect life safety, but a power event on these loads could result in unacceptable financial or operational losses to a facility. Typically, this type of load would include the following applications: data processing, communication systems, refrigeration, selected HVAC loads and manufacturing or critical industrial processes.

But what happens in the system where a single alternate power source, such as a generator, is used to supply both emergency and non-emergency loads? The system must be carefully designed to provide separate pathways and ATS equipment for the different types of loads and include load shedding, unless the generator source has the capacity to supply all the loads fully. Always check with the local authority having jurisdiction (AHJ) when this is the case.

As one can see, there are many considerations in specifying ATS. What follows are some final criteria that must be taken into account. One of the primary concerns—one that often is not given enough consideration—is the grounding system.

Specification and Application Criteria

Grounding is an important and often overlooked consideration in the application of ATS. In most instances the alternate source of power is a generator. The primary power source is often a solidly grounded utility supply transformer. In addition, the NEC requires ground fault protection for 480/277-volt equipment services above 1,000 amps, but not on required emergency power sources. The engineer has the option of:

1) grounding the generator neutral by solidly connecting to the primary power source ground or

2) grounding the generator to create a separately derived power source.

When using the primary power source ground, a 3-pole ATS with a solidly connected neutral should be specified to maintain the ground path, as shown if Figure 1 (p. 20). When the generator is grounded to create a separately derived source, an ATS with a switched neutral is required, as shown in Figure 2.

The engineer should carefully review the advantages and disadvantages of separately grounding the generator neutral or relying on the primary power source ground. However, it is usually recommended that the generator neutral be separately grounded. This approach will improve the application and accuracy of ground fault protection schemes, which is discussed in more detail in ANSI/IEEE Std. 446-1995. When a switched neutral is involved, the engineer is encouraged to review the potential ground fault issues of using overlapping contacts vs. a true switched fourth pole.

A number of current specifications should be considered for ATS. The main specifications include continuous current capacity, interrupting ability, fault current withstand and ability to close against high inrush currents.

ATS must be able to handle the anticipated full-load current continuously, 24 hours a day, seven days a week, for an anticipated minimum life of at least 20 years. Current ratings range from 30 to 4,000 amps. Typical ratings include 30, 40, 70, 80, 100, 150, 225, 260, 400, 600, 800, 1000, 1200, 1600, 2000, 2500, 3000 and 4000 amps. The engineer should select a transfer switch which is equal to or greater than the calculated continuous current.

Keep in mind that a transfer switch must be capable of withstanding the available fault current at its location in the system until the overcurrent protection device upstream clears the fault.

The engineer should determine the available fault current at the transfer switch location and the ratings of the overcurrent protection devices to be used with the ATS. Methods for calculating short circuits and applying protective devices can be found in ANSI/IEEE Std. 242-2001, “Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems” and will be not be covered here. In addition, Underwriters Laboratories (UL) 1008, “Standard for Safety Automatic Transfer Switches,” lists minimum requirements and testing for ATS including withstand ratings and coordination ratings with overcurrent protection. The engineer should be familiar with the contents of these publications before specifying and applying ATS.

When the ATS transfers from one source to another, arcing currents are drawn from the active source contacts as the poles are switched. Arcing currents may be at their highest level when motor load(s) are starting under locked rotor characteristics, which are up to six times the normal full-load current. The transfer switch must be able to interrupt these high arcing currents.

When the automatic transfer switch closes on an alternate source, its contacts may be required to handle substantial inrush currents. Certain types of loads, such as motors, lighting and electric heating, may draw substantial starting currents. The ATS must be rated to handle these high inrush currents when closing on an alternate source. Underwriter Laboratories (UL) 1008 classifies ATS for various types of loads.

Some Final Considerations

Another consideration for ATS is the means of transferring loads. The two main types of ATS are the “break before make” (open transition) type and the “make before break” (closed transition) type. In the open transition ATS, there is a small open transition period transferring the load from one source to another. Most facility loads will not be affected negatively by the slight open delay, but some sensitive electronic loads may drop out during this time. There are some types of large or industrial facilities where large motor loads may actually benefit from a delayed or programmed transition to avoid potential power quality issues. Conversely, there are some types of facilities, such as hospitals, where the need to test regularly under load, with no delay or interruption to power, may require the closed transition type ATS. Engineers and facility operators are encouraged to consult with the ATS manufacturer to help you review your specific needs.

Lastly, be sure to plan for the space and location of an ATS. Depending on the ampere rating, configuration and bypass features, the ATS may take up considerable floor space. It may require rear or side access for cable terminations and can't be set against a wall or corner.

Understanding the basics of transfer switches is essential in maintaining power in critical facility electrical systems. With increasing needs and options available for alternate power sources, transfer switches are an essential part of today's power systems. It is important that the system designer understand the basics about these devices when selecting and applying a transfer switch to maintain power reliability.

Transfer Switch Design Guidelines

The automatic transfer switch requirements and functionality are typically dictated by several codes, owner requirements and by the type of facility it is serving. The engineer needs to be aware of several critical issues related to the design of electrical distribution systems that involve automatic transfer switches. Some of the engineering criteria and critical issues involved in the proper design of a system utilizing automatic transfer switches would include some of the following items.

For the proper design of an electrical distribution system utilizing automatic transfer switches, the engineer must determine the following:

Total number of automatic transfer switches required

Requirement of a load-shedding scenario in a system with multiple transfer switches

Number of poles (three or four) that are required to be switched for each transfer switch

Requirement for bypass isolation

Any required generator exercising protocols, which can be determined through programming on the transfer switch

Withstand and close rating of the automatic transfer switch based on the fault current available in the electrical distribution

Transfer time of the transfer switch, which can be a normal transition of 6 to 10 cycles, a solid state static transfer switch or a closed transition automatic transfer switch.

Any transfer switch requirements to be service entrance rated

Required communication between the transfer switch and other building systems.

By Keith Lane, P.E., SASCO, Seattle

AS Per NEC 700

According to the National Electric Code, Section 700, the life-safety branch of power will require its own automatic transfer switch. If an electrical distribution is designed to also feed legally required and/or optional standby loads, additional transfer switches will be required.

It is possible to have a single generator feed life-safety and non-life-safety loads. NEC 700.5 B allows an alternate power source to supply emergency, legally required standby and optional standby loads, where automatic selective load pickup and load shedding as needed is implemented to ensure adequate power to life safety, legally required standby and optional loads, in that order. This can be achieved by providing a separate ATS for each of the three branches of loads—life safety, legally required standby and optional. One way to achieve load shedding is to require standby and optional ATS to have a center off position and a load shed relay. When the generator is critically close to an overload condition, it will send a signal to the load shed relays to remove the non-life safety ATS and non-life safety loads and ensure that the life safety loads remain on emergency generator power.

Three of Four Pole?

In a typical three phase electrical distribution system, the engineer has the choice between 3- and 4-pole transfer switches. There are certain circumstances where a 4-pole transfer switch will be required. A 4-pole transfer switch is typically required where the distribution system has two or more levels of ground fault sensing and two or more automatic transfer switches. Per the National Electric Code, Article 230-95, ground fault protection is required on solidly grounded equipment 277/480-volt rated 1,000 amperes or more. Where a single level of ground fault is required by the NEC, hospitals will require a second level of ground fault protection to ensure service continuity during a ground fault on a branch feeder. Although not required by code, other critical facilities may opt for multiple levels of ground fault protection to minimize the effect of a short to ground. In these cases of multiple levels of ground fault and of multiple transfer switches, a 4-pole transfer switch is required. If the neutral is not switched, neutral current can split in the parallel paths and cause nuisance tripping. A 4-pole transfer switch switches the neutral, a 3-pole transfer switch does not.

By Keith Lane, P.E., SASCO, Seattle

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