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Overview

Recent changes to the National Electrical Code® (NEC) require the selective coordination of overcurrent protective devices at hospitals and other mission-critical facilities. Transfer switches with 30-cycle closing and withstand ratings dramatically simplify designing to that requirement.

Selective Coordination Requirements

Selective coordination was first required by the NEC in 1993 for elevator circuits. Amendments to the Code in 2005 and 2008 strengthened the requirements and expanded them to include emergency and legally required standby systems, as well as critical operations power systems. Selective coordination, as defined in the 2008 NEC, is the “localization of an overcurrent condition to restrict outages to the circuit or equipment affected, accomplished by the choice of overcurrent protective devices and their ratings or settings.” It is a complicated process of coordinating the ratings and settings of overcurrent protective devices, such as circuit breakers, fuses, and ground fault protection relays, to limit overcurrent interruption (and the resultant power outages) to the affected circuit or equipment (the smallest possible section of a circuit). In other words, the only overcurrent protective device that should open is the device immediately “upstream” from the circuit/equipment experiencing an overcurrent condition.

UL Standards and Testing

Underwriter Laboratories (UL) Standard 1008 is the industry-accepted standard that establishes the criteria by which automatic transfer switches are listed. The listing process includes passing tests for closing and withstand short-circuit values. Switches may be listed under several different testing protocols, including:

• Testing with a specific overcurrent device so that the listing is dependent on use of that device or another with identical or faster time/overcurrent curves. While this approach makes it easier for the manufacturer to pass testing, it actually complicates the process of selective coordination for the design engineer.
• Testing for a 3-cycle fault duration. A switch passing this test is considered to be coordinated with any molded-case circuit breaker capable of interrupting the test closing and withstand value. This test is more stringent, but in no way simplifies selective coordination.
• Testing for a specific amount of time beyond 3 cycles to establish a short-time rating. To pass this test, a switch has to close and withstand a fault current for the specified test duration. Close and withstand for 30 cycles is considered to be coordinated with any circuit breaker having only short-time overcurrent protection (not instantaneous). A 30-cycle-rated switch therefore eliminates a host of coordination considerations and dramatically simplifies the entire selective coordination process.

If transfer switches are being protected by circuit breakers with short-time overcurrent protection only, and the switches have only 3-cycle closing and withstand ratings, they are not properly coordinated with their protective breakers. Under these circumstances, transfer switches with 30-cycle ratings are needed to properly coordinate.

Error and Trial

Selective coordination is best done on the drawing board, at the beginning of the design process. Although achieving genuine, documented selective coordination as defined by the NEC can be time-consuming and expensive, flawed selective coordination is even more so. To comply with requirements, a selective coordination plan must consider, for every pertinent circuit, the full range of maximum available overcurrents, including overloads, all types of faults, and short circuits.

Many contracts and code enforcement authorities require a study to evaluate the pertinent circuits and confirm that the protective devices have been selectively coordinated. Performed after construction, such studies are a minefield for systems that were not designed carefully in the first place. Once a system has been determined to be non-compliant, redesigning it and replacing various components can be extremely costly and time-consuming. Even if proper protective devices are installed as a corrective measure, the cable, bus, or conduit ratings may not be adequate. Or a higher-rated transfer switch or new panelboards may be needed, requiring extra mounting space. Since a change to one component often affects others, new calculations are necessary to see what else must be replaced. Such retrofitting to obtain a certificate of occupancy is a design engineer’s nightmare.

Numerous modifications of the NEC requirements have been adopted by local and state governments with varying degrees of enforcement, but let designers be forewarned: It is far better to err on the side of too much protection than not enough. The specifier might be called upon to prove that the time-current curves for circuits in his/her selective coordination scheme comply with the NEC by not overlapping at the available fault current. Even in a locality where selective coordination requirements on the books are not enforced, a specifier and his/her engineering firm could be found liable for injuries suffered due to inferior selective coordination — for the life of the building!

Hold That Line

In a selectively coordinated electrical system using circuit breakers, the breaker for every load circuit must have the proper ratings, interrupting capacity, and settings for the point at which it is installed, based on the highest potential overcurrent from either power source (normal or backup). Progressing “upstream” through the circuit paths, from the smallest load branch circuit all the way to the normal and backup power sources, the specifications of a true selective coordination plan must ensure that every circuit breaker has a higher overcurrent rating and a longer time-delay than the one below it, so that every overload/fault will be cleared by the breaker farthest “downstream” (the breaker immediately “upstream” of the problem).

Today, most transfer switch designs have only 3-cycle closing and withstand ratings. The ability to withstand fault current for 10 times that duration (one-half second) necessitates that 30-cycle transfer switches are mechanically stronger by orders of magnitude. Because of its function — switching from normal to backup power and back again — a transfer switch is obviously in a key location, and its ability to withstand a fault condition is vital to supply power to the served load. In the event of a fault, a transfer or bypass/isolation switch that can withstand 30 cycles of overcurrent is like a sturdy defensive lineman in a football game. Holding the line long enough to allow the coordinated overcurrent protection to interrupt the fault, a 30-cycle switch assists in protecting downstream equipment, such as expensive medical devices.

Another major benefit of 30-cycle transfer switches is the extra capacity they provide for later expansions of electrical systems. The design phase of a renovation that upgrades available fault current or replaces overcurrent protective devices will proceed more smoothly if 30-cycle switches are already installed.

Proceed With Caution

Several things should be taken into consideration when selecting a 30-cycle transfer switch. With the right switch, the additional security and system design simplicity offered by a 30-cycle closing and withstand rating can become reality. Asking a few important questions can make a difference.

Does the manufacturer offer a full line of 30-cycle transfer switches? If so, the specification of a switch is simply a matter of its continuous current rating and is not complicated by gaps in the manufacturer’s product line, or by the necessity of specifying a much higher continuous-rated switch than the circuit would normally require.

Has the 30-cycle transfer switch been tested according to UL standards, and is it UL listed and labeled? The switch’s closing and withstand rating must be a performance value based on actual testing to UL Standard 1008. Because the 30-cycle closing and withstand test is optional under UL-1008, specifiers and purchasers of 30-cycle switches should carefully scrutinize the presentation of any manufacturer’s 30-cycle ratings to be certain that they are based on actual testing by UL and that the switches are UL listed and labeled.

Cost is always a consideration in the choice of any piece of equipment. In the final analysis, however, a transfer switch is a key component in an emergency/backup power system designed to protect lives and/or vital assets. Since the switch serves such a critical function in the system — for both normal and emergency loads — and since the potential losses from any malfunction are so great, the cost of the switch should be secondary to its performance. With this in mind, system designers and owners should insist on the best switch they can find. And given the robustness of its design and construction and its proven ability to withstand 30 cycles of punishment, a 30-cycle-rated switch makes perfect sense.

Conclusion

The 30-cycle transfer switch holds tremendous promise as perhaps the single most cost-effective and simple solution to the complex challenges of selective coordination. The right 30-cycle switch can simplify a backup power system’s design and offer more reliable protection. Plus, it provides unmatched flexibility for future system upgrades and expansion.

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By John Meuleman, Russelectric Inc., Hingham, Mass. — Consulting-Specifying Engineer, 6/1/2009

With daily advances in information technology and other processes and services, the world is becoming a more complicated and power-hungry place. Many industries and services have become increasingly more dependent on a continuous, uninterrupted supply of electric power. However, a continually shrinking electric generation margin reduces the reliability of utility-provided power.

Consequently, the use of backup power control systems and generator control switchgear has grown and will continue to grow—in number, capacity, and complexity—in the coming years. A savvy specifier considers many factors to make sure such equipment performs as needed and its service life is not only long, but virtually trouble-free.

Some facilities cannot be without power for extended periods, but can still tolerate a short outage. For such facilities, a backup power system can be fairly simple. In other applications, however, even a momentary interruption of power could be disasterous. To avoid this, systems should include an uninterruptible power supply (UPS) system to prevent even the slightest “blink.” Generator control switchgear also can be specified to allow closed-transition retransfer or test without power interruption when both sources are available.

Generator control switchgear, typically used as part of a multi-generator electric backup power system at a mission critical facility, should have a life expectancy of at least 15 years. However, another important consideration for maximizing the life expectancy of the switchgear is the possibility of future load growth. Don’t just focus on immediate needs.

When extra capacity is specified, future expansions/upgrades can be accommodated more smoothly and at a much lower cost. For example, a hospital’s standby electrical load today can be three times what it used to be—75% or more of its total connected load.

Construction and components

Regardless of the switchgear’s application, the quality of its construction is key. Start with the cabinets, which should be fabricated from heavy-gauge steel with welded reinforcing gussets for strength and rigidity. For many geographic locations, equipment must be built and tested to withstand a seismic event per International Building Code (IBC) requirements. Enclosures should be protected with a corrosion-resistant electrostatic powder coating.

Inside, the construction of the switchgear should be durable. Switchboard wires should be flame-retardant and should have permanent sleeve markers at both ends. The best wire runs are custom-assembled on chassis (not pre-manufactured) and should include extra wires for faster, simpler repairs and future expansion. Cage clamp-type connectors provide sustained, secure control wiring connections.

Busbar should be formed, cut, and punched before being silver-plated, to ensure integrity of the plating. If the manufacturer buys pre-plated bus and forms it later, the process of bending it can crack the plating and expose raw copper. This can reduce conductivity and cause problems with corrosion and overheating, adversely affecting the performance of the whole system. Where insulated bus is used, the insulation also should be applied after the busbar is formed.

Switchgear should be built and tested to the highest Underwriter Laboratories (UL) requirements such as UL Standard 1558 for switchgear 600 V or less, and the UL category for medium-voltage switchgear (“Circuit Breakers and Metal-Clad Switchgear Over 600 V”). The withstand rating should be a performance value based on actual testing. Keep in mind that, for applications in which the utility and generator sources are paralleled, the withstand and interrupting capacity of all breakers must be greater than the sum of the fault capability available from both power sources.

Controls

Programmable logic controller (PLC)-based digital controls should provide automatic starting, synchronizing, and distribution of standby power upon detection of loss of the utility source. A fully redundant PLC will ensure fail-safe operation. Should one PLC fail, the other one will automatically take over systemwide control. Manual start, paralleling, and load controls are also good ideas in case automatic control is lost.

A color touchscreen operator interface will permit system monitoring and parameter selection on-site. Custom supervisory control and data acquisition (SCADA) will allow for remote monitoring, real-time and historical trending, comprehensive reporting, and remote alarm management. In many cases, the SCADA system for the backup power system can be interfaced with a building’s energy management system and other systems via TCP/IP or other Ethernet protocols—to provide a comprehesive overview of power quality and usage and to document energy usage trends and savings.

Long-term considerations

Other concerns are operator safety and ease of maintenance/troubleshooting. Generator control switchgear should have grounded, separately accessible compartments with drawout power circuit breakers located in their own compartments. Controls should be segregated from power bays, and only control voltages should be available in control compartments. For medium-voltage applications, the main bus joints and power connections should be insulated with preformed boots.

A purchaser of generator control switchgear should select a supplier that specializes in the field. When comparing switchgear suppliers, it is best to consult some of their previous customers to learn their track records. A supplier that is experienced in designing complete switchgear systems can contribute important ideas regarding control schemes, sequences of operations, power transfer options, and installation. A top-quality supplier should be able to provide a comprehensive guide specification that deals with most of the considerations discussed here—a specification that can be easily tailored to a customer’s unique requirements.

Negotiate a good warranty, too. A supplier that backs its switchgear with a two-year warranty probably builds it with better components than a supplier offering only a one-year warranty. A longer warranty also may help reduce the lifecycle cost of the equipment.

Needless to say, it also helps when the supplier is a service-oriented company. Factory-direct field service is preferable because the technicians are intimately familiar with the equipment, are aware of the latest updates, and have easy access to spare parts. This facilitates equipment repair and reduces downtime.

Managing cost

In some cases, generator control switchgear can actually produce a revenue stream of its own, thus defraying a portion of its lifecycle cost. In fact, standby power systems now are frequently used for peak shaving, with the system’s generator controls providing automatic operation. Furthermore, many utilities offer an “interruptible power contract” that gives the utility permission to drop the customer’s facility from the electrical grid (with advance notice) during periods of peak demand. In return, for every kilowatt the customer generates while offline, the utility pays the customer a rebate at a rate that, in some cases, is higher than what the utility charges. In certain cases, the facility can get an electricity cost reduction just for being able to provide load-curtailment operation, even if never called upon to do so.

Intitial cost is always a consideration. In the final analysis, however, the lowest first cost solution may not be the lowest total cost solution once installation, commissioning, and maintenance are considered. And, when the switchgear or power control system will be protecting lives (such as at a hospital or airport) or vital electronic records (such as at a data center), the potential losses—in terms of life or money—from an equipment malfunction can be substantial.

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