1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA ICS 10 PART 3-2010Safety Bulletin: NEMA Safety Considerations for Residential Transfer Equipment and Residential Transfer SwitchesA NEMA Industrial Automation Control Products and Systems (01IS) Section Document ICS 10-2010,
2、 Part 3 Safety Bulletin: NEMA Safety Considerations for Residential Transfer Equipment and Residential Transfer Switches Published by: National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, Virginia 22209 www.nema.org Copyright 2010 by the National Electrical Manuf
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13、shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. Copyright 2010 by the National Electrical Manufacturers Association. ICS 10-2010, Part 3 Page 1 Safety Bulletin: NEMA Safety Considerations for Residential Transfer Equipment and Residentia
14、l Transfer Switches For the sake of simplicity, this guide assumes that the structure being powered is a residence, the power in question is 4-wire 125/250 VAC (two line conductors, one neutral, one ground), the normal source of supply is a power utility, the alternate source of supply is an on-site
15、 generator used for optional standby power, and the house loads are connected to a circuit breaker panel. This is certainly not the only configuration; however, it is a very common configuration. Any device that switches two independent sources to one load or group of loads falls into the general ca
16、tegory of transfer equipment. Transfer equipment and residential transfer switches are safety devices that are required by the NFPA 70, the National Electrical Code (NEC). Article 702.6 states, “Transfer equipment shall be required for all standby systems subject to the provisions of this article an
17、d for which an electric-utility supply is either the normal or standby source.” The use of transfer equipment and transfer switches provides protection from inadvertent interconnection of the normal and alternate supplies. Inadvertent interconnection is the condition that occurs when a generator or
18、other power source is connected to residence wiring supplies power through the de-energized utility wiring to the neighbors homes or to the utility transformer. Obviously, inadvertently electrifying circuits that other people believe to be “dead” poses significant safety hazards. Utility workers and
19、 neighbors have been electrocuted by making contact with these backward fed circuits. Residential transfer equipment and residential transfer switches fall into three general categoriesautomatic, non-automatic (for example pushbutton operated), and manual. An automatic transfer switch is one that tr
20、ansfers from the normal to the backup source of power upon detection of loss of power and without human intervention. Non-automatic and manual transfer equipment require a human operator to do something to make the transfer occur. There are three primary kinds of manual transfer equipment for reside
21、ntial applications. Whole house transfer equipment consists of a transfer switching system installed between the utility meter and the main loadcenter, (an example would be a double-pole double-throw safety switch) or between the main breaker and the branch circuit breakers (an example would be a tr
22、ansfer panelboard). Subpanel transfer equipment uses a small loadcenter that contains only the branch circuits to be powered by the generator or alternate power source. Transfer equipment, or a transfer switch, powers this subpanel alternately from the main panel or the standby source, in accordance
23、 with the provisions of the National Electrical Code. Load-side transfer equipment uses multiple transfer devices connected after the branch circuit breaker, one for each circuit to be powered from the generator. SELECTION There is a wide variety of selection criteria to be considered when choosing
24、a residential transfer switch. Safety. Although there are ways to connect backup power to a residence without transfer equipment, they are dangerous and violate the National Electrical Code and most local building codes. Always use transfer equipment to connect power to the circuits of a residence.
25、Local codes and permits. The local codes and permits at the site of installation may restrict some types of installations. Always determine local code and permit restrictions before choosing a system. Copyright 2010 by the National Electrical Manufacturers Association. ICS 10-2010, Part 3 Page 2 Age
26、ncy certification. Transfer equipment will be marked for the intended use and certified to the appropriate product standard(s). Homeowners should verify that the transfer switch is suitable for the intended use by checking the product for a mark from a Nationally Recognized Testing Laboratory (UL, C
27、SA, ETL, MET Labs, Wyle, etc.) and the words “suitable for use under Article 702 of the National Electrical Code.” “Do-it-yourself” versus professionally installed. It is recommended that transfer equipment be professionally installed. Transfer equipment can be simple enough to be installed by a cap
28、able homeowner; when in doubt, a professional should be employed to do the work. Before buying transfer equipment, decide whether you can or want to install it yourself. Fully installed automatic/non-automatic versus portable/manual. There is no question that fully installed automatic transfer equip
29、ment is the ultimate in convenience and ease of operation, requiring virtually no operator intervention. However, there is a cost for all this convenience. Automatic and permanently installed non-automatic systems are generally more expensive than manual systems, often require professional installat
30、ion, and sometimes use a larger generator that may require its own foundation and/or shelter. Manual transfer equipment requires significant work by the homeowner to start and connect the generator and engage the desired circuits, often during bad weather conditions. These systems are generally smal
31、ler, simpler, less expensive, and easier to install. The selection of automatic versus manual should take into consideration factors such as: o Is the homeowner willing and/or capable of doing the tasks required by a manual system? o Are there frequent power outages, wherein the convenience of an au
32、tomatic would offset the increased cost, or are outages so rare that an automatic would be a waste of money? o Are there features to the home such that every power outage must be handled immediately, whether the homeowner is present or not? Whole house versus partial. A whole house transfer system h
33、as the distinct advantage of being able to run all loads in the home. Partial transfer systems, whether automatic, non-automatic, or manual, power only those critical circuits predefined by the homeowner. Thus, they require less attention, but are more restrictive in that they cannot power any loads
34、 other than the predefined critical circuits. Whole house systems usually require professional installation, as their installation generally affects the incoming utility power. Where a whole house transfer system is installed ahead of the existing service disconnect, the transfer equipment must be v
35、erified as marked “suitable for use as service equipment.” Partial transfer systems can sometimes be “do-it-yourself” installations. Subpanel versus load side. Subpanel style transfer equipment generally uses a small loadcenter that is powered by either the main loadcenter or the generator through a
36、 single transfer device. It is a simpler system, but requires that the circuit conductors be moved from the main loadcenter into the subpanel. This can be a daunting challenge in some installations. Load-side transfer equipment is more complicated, involving a transfer switch for each individual loa
37、d. However, they install after the branch circuit breaker, making installation much easier and less expensive. Remember to maintain AFCI and GFCI protection as required by code. Separately derived versus non-separately derived. A separately derived system requires that all current-carrying conductor
38、s, including the neutral, be switched. This requires the use of a three-pole (line 1, line 2, neutral) transfer equipment solution. (Ground conductors are not switched.) A non-separately derived system requires that both neutrals be solidly connected, not Copyright 2010 by the National Electrical Ma
39、nufacturers Association. ICS 10-2010, Part 3 Page 3 switched. This allows for the use of two-pole (line 1, line 2) transfer equipment. The NEC allows either configuration. For additional information, see NEMA ICS 10, Part 1, Annex B. Generally speaking, if the generator is a “portable” generator and
40、 has an integrally mounted GFCI (ground-fault circuit interrupter) protecting the outlet supplying the transfer equipment, it should be treated as a separately derived system. Copyright 2010 by the National Electrical Manufacturers Association. NATIONAL ELECTRICAL MANUFACTURERS ASSOCIATION 1300 NORTH 17TH STREET, SUITE 900 ROSSLYN. VA 22209www.NEMA.orgTO ORDER ADDITIONAL NEMA STANDARDS VISITWWW.GLOBAL.IHS.COM OR CALL 1-800-854-7179/1-303-397-79565612_0514TB
