| MasterSpec - EVALUATIONS | 12/03 (R 06/05) |
The following editing instructions relate directly to specific parts of the Section Text where they are referenced by the applicable Editing Instruction No. in the editor's notes:
Equipment terms used for transfer switches are defined as follows:
Transfer switches of double-throw construction are used primarily for emergency and standby power systems rated 600 V and less. Transfer switches are applied according to NFPA 70. Articles 700 and 701 in NFPA 70, which cover emergency systems and legally required standby systems, require ATSs; Article 702, which covers optional standby systems, permits NATSs. NFPA 110 contains requirements for transfer-switch characteristics and features. Transfer switches are available in ratings from 30 to 5000 A. For reliability, most transfer switches are mechanically held and electrically operated. Control power comes from the power source to which the load is to be transferred.
Transfer-switch requirements are unique and extensive when compared to requirements of other circuit- and feeder-switching devices. Transfer switches must be able to do the following:
A primary consideration in applying transfer switches is providing protection against failure of the utility service. Also consider the possibility of an open circuit within the building on the load side of the incoming service. An overload, fault condition, or electrical or mechanical failure of the electric power-distribution system within the building can also lead to loss of power to vital loads. Therefore, locating transfer switches close to the load and having the operation of the transfer switches independent of overcurrent protection may be desired. Also, it may be better to use multiple transfer switches of lower current rating located near the load rather than one large transfer switch close to the point of incoming service.
Transfer switches are rated in amperes and are often installed to transfer a variety of loads such as electric-discharge lamps, electric-heating loads, and tungsten filament lamps. Lower-rated transfer switches are not limited in the composition of load as long as the total is within the ampere rating marked on the nameplate and the switch is listed for total system loads. However, the listing of larger transfer switches may have limits on the percentages of certain types of loads that may be included in the total. For example, an 800-A transfer switch may not be approved for a load with more than 30% tungsten filament lamp load. Also, UL 1008 permits more restrictive listing of transfer switches for use with specific load types. For example, an ATS can be listed for use with resistance loads. However, transfer switches with restrictive listings are seldom used in specified projects.
Manual Operation: Some manufacturers offer transfer switches with manual-operating handles that function only with the enclosure open. These manual operators are for maintenance purposes and are not to be operated under load. However, regardless of cautionary labeling, operation under load or while energized is possible. For some lines of switches, inadvertent manual operation of the switch could result in injury. Pay special attention to safety labeling and training where such products are installed.
ATSs for Use in Emergency Systems: Nationally recognized testing laboratory (NRTL) listings for these types of switches cover units intended for use in emergency systems as contemplated by Articles 517 and 700 in NFPA 70. These transfer switches are also suitable for use in standby systems according to Articles 701 and 702 in NFPA 70. ATSs that initiate automatic starting of an engine generator need a switching contact to accomplish this.
BP/IS switches can provide a combination of many features and functions, including the following:
Some manufacturers have combined several of these functions and supply the BP/IS switch and the ATS as an integral assembly within a common enclosure. Such combinations are usually limited to ratings of 800 A and less. For higher-rated switch installations, factory-coordinated side-by-side or back-to-back arrangements of BP/IS and ATS enclosures provide equivalent configurations. Such factory combinations ensure proper coordination of functions and minimize the required floor space and installation costs.
Neutral Connections and Switching: Most ATSs for three-phase systems are three-pole devices, except those for which the two sources are separately derived systems as discussed in Editing Instruction No. 5 in these Evaluations. When the load to be transferred is a three-phase, four-wire circuit, the load neutral conductor is usually connected through the transfer switch on a "solid-neutral" terminal, and the neutrals from the two source feeders are terminated on the same neutral bar.
This practice could be problematic if one or both feeders have ground-fault protection. Detecting ground faults often relies on measuring unbalanced neutral current. Providing multiple paths for such current (as is done by the solid-neutral connection) may lead to erroneous indications in the detection circuits. Where ground-fault protection is used, specify transfer switches with neutral-switching poles (a four-pole ATS). The use of a switched neutral requires the primary and alternate sources to be treated as separately derived sources as defined in NFPA 70, Article 250-20d. Independently ground each source according to NFPA 70, Article 250.
When both the normal and alternate power sources for the transfer switch are separately derived sources, each has its own independent system ground. Accordingly, each has its neutral grounded at either the service entrance or source equipment location according to NFPA 70. Connecting these two independently grounded neutrals together on a transfer-switch neutral bus violates the rule that a system should be grounded at a single point.
Neutral switching can be accomplished in two ways: by switching the neutral pole simultaneously with the phase poles (momentary open transition of all four contacts), or by using overlapping neutral contacts (make-before-break neutral contacts). The first method has generated considerable discussion about the possibility of dangerous voltage transients occurring during switching. The second method has the disadvantage of compromising the ground-fault protection system because neutral current can split between the two sources during the period the neutral contacts overlap. This action may necessitate increasing the delay on the ground-fault protection relay to inhibit its functioning until after the period of overlap. Not all manufacturers offer overlapping neutral contacts on their transfer switches.
A paragraph in the Section Text permits specifying an oversize neutral if needed because of a large nonlinear load component in the current being handled by the switch.
Fire-Pump Transfer Switches: NFPA 20 contains specific requirements for transfer switches used with fire pumps. There are specific ratings and interconnection requirements with the fire-pump controller. These functions are necessary to ensure reliability for this specialized application, and transfer switches for fire pumps must be listed for that application.
Fire-pump Sections specify the transfer switch as part of the fire-pump controller. However, NFPA 20 permits arrangements with the ATS separate and upstream from the fire-pump controller. In Project-specific cases, if appropriate, a knowledgeable specifier can revise this Section so the fire-pump transfer switches are specified here along with those for other applications. Otherwise, specify transfer switches for fire pumps as part of the fire-pump controller for proper coordination and require that the transfer switch be specifically listed for fire-pump use.
As indicated in Editing Instruction No. 3, an important part of defining the requirements of each individual transfer switch is stating the closing and withstand ratings for the switch. The closing rating is the fault current in symmetrical amperes at the unit, and the withstand time is the interval, usually expressed in cycles, that the switch must successfully carry the fault current. The length of the interval depends on the clearing time of the overcurrent device protecting the transfer switch from the worst-case fault current. Clearing time, therefore, depends on the type of overcurrent protective device. Among the withstand times stated for the different types of devices is the requirement that molded-case circuit breakers higher than 150-A rating withstand fault current for 3 cycles.
Thus, on a 60-Hz system for a 400-A transfer switch where the maximum available fault current is 42,000 A and the protective device is a molded-case circuit breaker, closing and withstand ratings are 42,000 A at 3 cycles. Indicate the fault-current value on plans, in a riser or single-line diagram, or in a schedule. An alternative method, and a sensible choice if there is only one transfer switch in the Project, is to list in the Specifications the available fault current at each switch.
Although the maximum withstand-duration time recognized by UL 1008 is three cycles, some manufacturers test and rate conventional transfer switches in certain ratings for longer durations. Note that these tests and ratings may apply only to static withstanding of the longer-duration fault and may not include the closing test.
The withstand-duration time given for transfer switches protected by current-limiting fuses is one-half cycle. Due to the nature of the current-limiting fuse operation, withstand durations are inherently short, and it is neither meaningful nor practical to lengthen them for fused units.
Coordinate the type and interrupting capacity of normal and emergency circuit breakers protecting the transfer switches with the switch withstand ratings. Conventional transfer switches may be listed for use with any circuit breaker up to a stated fault current. Some manufacturers list a switch for use with specific circuit breakers or fuses. In the latter case, the stated fault current may be higher for the closing and withstand ratings. A problem may arise when the manufacturer and characteristics of the actual circuit breaker supplied are not known until submittals are received. In this case, clearly indicate the closing and withstand requirements so that overcurrent protective devices can be chosen properly.
Molded-case, switch-type transfer switches have the fault-current closing and withstand ratings inherent in the design of the components used in the switch construction. Contact manufacturers for available ratings.
Static-type ATSs are available with NRTL listings to comply with UL 1008. Much faster and more costly than their electromechanical equivalents, they have a special application in transferring sensitive electronic equipment loads from one utility supply to another where a facility has more than one utility service. They may also be used to transfer motor loads from one supply to another without harmful disturbances. They transfer both load types described above from the energized utility supply to and from the energized peak-shaving generator systems without subjecting the loads to excessive voltage and frequency transients. Static transfer switches are now listed for use only on optional standby systems as defined in NFPA 70, Article 702.
Because of their limited application, static transfer switches are not included in this Section.
If installing transfer switches in areas where significant seismic events may occur, consider the following possibilities:
Equipment installed in areas requiring seismic bracing must meet specific performance requirements. Local codes normally define design forces that must be resisted by electrical systems. Codes also indicate whether equipment must simply remain in place during a seismic event or if it must also be fully functional after the event. The Section Text offers options for both levels of performance.
Functional performance of equipment following a seismic event requires that the manufacturer test one or more prototypical equipment types, and extrapolate the test results to the full line of equipment offered. Some, but not all, equipment manufacturers perform shaker table tests on their equipment and certify that their equipment meets or exceeds requirements of most nationally recognized codes. The performance level certified is for the fully functional operation of the equipment at the end of the seismic event.
The manufacturer's functional performance certification presumes that the equipment is rigidly anchored to the building support system. Flexible support systems can result in equipment/restraint resonant frequencies for which the manufacturer did not test. Design of the support system requires that detailed dimensional data, weights, location of the center of gravity, and mounting provisions be available. Concrete strength, anchor-bolt type and location, and anchor testing and installation are critical components of the seismic-restraint system.
Obtain the services of seismic experts before revising the Section Text or before designing features to contend with seismic conditions.
Refer to Division 26 Section "Vibration and Seismic Controls for Electrical Systems" for information pertinent to bracing and anchoring switchgear.
For additional information on testing and certifying electrical equipment, consult IEEE 344, Recommended Practice for Seismic Qualification of Class 1E Equipment for Nuclear Power Generating Stations. It provides guidelines for analyzing potential seismic problems of equipment. Transfer switches can be modified to comply with seismic conditions and tested according to this standard. For information on equipment anchorage, consult IEEE 693, Recommended Practice for Seismic Design of Substations. Additionally, a detailed development of criteria for functional testing of equipment is available in the pamphlet Earthquake Requirements and Cutler-Hammer Distribution & Control Electrical Equipment Seismic Capabilities. (See the "References" Article in these Evaluations.)
Publication dates represent the editions on which the current Section Text is based. Standards are revised periodically, which may occur before this Section is updated again.
The following publications are useful in specifying transfer switches. Other references may be needed for design purposes.
The list of manufacturers is neither a recommendation for the companies nor an endorsement of their products. Verify manufacturers' capability to comply with indicated requirements each time the Section Text is edited.
AC Data Systems, Inc. Post Falls, ID (800) 890-2569 www.acdatasystems.com Caterpillar Engine Div. Peoria, IL (800) 732-3959; (309) 675-1000 www.cat.com Eaton Electrical Inc. Cutler-Hammer Pittsburgh, PA (800) 354-2070; (412) 937-6547 www.cutler-hammer.com Emerson ASCO Power Technologies, LP Florham Park, NJ (800) 937-ASCO; (973) 966-2625 www.asco.com Generac Power Systems, Inc. Waukesha, WI (262) 544-4811 www.generac.com GE Zenith Controls Chicago, IL (773) 299-6900 www.zenithcontrols.com Hubbell Industrial Controls, Inc. Archdale, NC (800) 634-8660; (336) 434-2800 www.hubbell-icd.com Kohler Power Systems Generator Division Kohler, WI (800) 544-2444; (920) 565-3381 www.kohlergenerators.com Lake Shore Electric Corporation Bedford, OH (440) 232-0200; (866) 620-5982 www.lake-shore-electric.com Onan/Cummins Power Generation Industrial Business Group Minneapolis, MN (800) 797-6468; (763) 574-5000 www.onan.com Russelectric, Inc. Hingham, MA (781) 749-6000 www.russelectric.com Spectrum Detroit Diesel Sheboygan, WI (920) 451-0846 www.spectrumgenerators.com
When only a single unit or unit size is required on a particular project, a schedule is not necessary. However, if a given project requires several units of varying sizes, characteristics, and capacities, a schedule is preferred. It is the editor's option whether this schedule should appear in the Specifications or on the Drawings. Do not duplicate schedule information on both the Drawings and Specifications. If the editor wants to schedule particular units, the example below may be used as a guide.