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1、 N NE EMMA A DC C 20 0 RESIDENTIAL CONTROLS CLASS 2 TRANSFORMERS NEMA Standards Publication DC 20-1992 (R2003, R2009) Residential ControlsClass 2 Transformers Published by National Electrical Manufacturers Association 1300 North 17th Street, Suite 1752 Rosslyn, Virginia 22209 www.nema.org Copyright
2、2009 by the National Electrical Manufacturers Association. All rights including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Literary and Artistic Works, and the International and Pan American Copyright Conventions. ?
3、 NOTICE AND DISCLAIMER The information in this publication was considered technically sound by the consensus of persons engaged in the development and approval of the document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person parti
4、cipating in the development of this document. The National Electrical Manufacturers Association (NEMA) standards and guideline publications, of which the document contained herein is one, are developed through a voluntary consensus standards development process. This process brings together voluntee
5、rs and/or seeks out the views of persons who have an interest in the topic covered by this publication. While NEMA administers the process and establishes rules to promote fairness in the development of consensus, it does not write the document and it does not independently test, evaluate, or verify
6、 the accuracy or completeness of any information or the soundness of any judgments contained in its standards and guideline publications. NEMA disclaims liability for any personal injury, property, or other damages of any nature whatsoever, whether special, indirect, consequential, or compensatory,
7、directly or indirectly resulting from the publication, use of, application, or reliance on this document. NEMA disclaims and makes no guaranty or warranty, expressed or implied, as to the accuracy or completeness of any information published herein, and disclaims and makes no warranty that the infor
8、mation in this document will fulfill any of your particular purposes or needs. NEMA does not undertake to guarantee the performance of any individual manufacturer or sellers products or services by virtue of this standard or guide. In publishing and making this document available, NEMA is not undert
9、aking to render professional or other services for or on behalf of any person or entity, nor is NEMA undertaking to perform any duty owed by any person or entity to someone else. Anyone using this document should rely on his or her own independent judgment or, as appropriate, seek the advice of a co
10、mpetent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publication may be available from other sources, which the user may wish to consult for additional views or information not covered by this pub
11、lication. NEMA has no power, nor does it undertake to police or enforce compliance with the contents of this document. NEMA does not certify, test, or inspect products, designs, or installations for safety or health purposes. Any certification or other statement of compliance with any health or safe
12、ty related information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement. ? DC 20-1992 (R2003, R2009) Page i CONTENTS Page Foreword ii History of Standardsiii Section 1 GENERAL 1.1 Scope.1 1.2 References 1 1.3 Definitions1
13、Section 2 RATINGS AND CLASSIFICATIONS 2.1 Electrical Ratings.3 2.1.1 Primary Voltage Ratings.3 2.1.2 Secondary Voltage Ratings3 2.2 Classification .3 Section 3 MOUNTING 3.1 General4 3.2 Plane-Surface Mounting4 3.3 Outlet-Box Mounting4 Figure 3-1 Dimensions (in inches mm) for Mounting .4 Section 4 PE
14、RFORMANCE AND TEST 4.1 General5 4.2 Regulation .5 4.2.1 Test Equipment 5 4.2.2 Test Procedures.7 Figure 4-1 Test Circuit7 4.2.3 Curves 7 4.2.4 Data Sheet8 Table 4-1 Test Points .8 Table 4-2 Example Data Sheet 8 Figure 4-2 Example Data Sheet.9 Section 5 MARKING 5.1 General10 5.2 Additional Marking.10
15、 Copyright 2009 by the National Electrical Manufacturers Association. ? DC 20-1992 (R2003, R2009) Page ii Foreword This Standards Publication represents suggested standards for the construction and performance of Class 2 transformers designed primarily for use with residential controls. It has been
16、developed in the public interest and is designed to promote a better understanding between the manufacturer and the user and to assist the user in selecting and obtaining the proper transformer for a particular application. If a value for measurement as given in this Standards Publication is followe
17、d by an equivalent value in other units, the first stated value is to be regarded as the requirement. The given equivalent value may be only approximate. This standard is periodically reviewed by the Residential Controls Section of NEMA for any revisions necessary to keep it up-to-date with advancin
18、g technology. Proposed or recommended revisions should be submitted to: Vice President, Technical Services National Electrical Manufacturers Association 1300 North 17th Street Suite 1752 Rosslyn, Virginia 22209 This Standards Publication was developed by the Residential Controls Section. Section app
19、roval of the standard does not necessarily imply that all section members voted for its approval or participated in its development. At the time it was approved, the group was composed of the following members: Apcom Inc.Franklin, TN Emerson Electric Company, White-Rodgers DivisionSt. Louis, MO GE I
20、ndustrial SystemsMorrison, IL Honeywell, Inc.Golden Valley, MN Therm-O-DiscMansfield, OH Copyright 2009 by the National Electrical Manufacturers Association. ? DC 20-1992 (R2003, R2009) Page iii History of Standards In March 1958, the Residential Controls Section of NEMA adopted a standard regulatio
21、n curve for Class 2 transformers. This standard was issued under the title “Voltage Ratings and Load Characteristics for Primary Control Equipment of 30 Volts and Less,“ and was a part of Publication No. DC 1, General Standards for All Automatic Temperature Controls. At that time the transformer in
22、general use by the industry was one having a 40-volt-ampere rating. It was, therefore, around this rating that the standard was developed. Subsequently, transformers having other ratings were introduced. These transformers, although they might have had the same electrical ratings, in many cases vari
23、ed quite radically in performance. This presented problems from an interchangeability standpoint. Recognizing these problems, the Residential Controls Section in 1964 activated a program to write standards for all transformer sizes. Publication No. ST 2-1973 was the result of the development and rev
24、ision of several drafts developed by the Residential Controls Section of NEMA and approved by the Dry-Type and Specialty Transformer Section whose scope at that time included the products covered in this Standards Publication. It was reaffirmed and editorially revised in 1979. DC 20-1992 (R2003, R20
25、09) revises and supersedes ST 2-1973 (R 1979). This current revision of the standard has deleted all requirements which are contained in ANSI/UL 1585, and references that document as appropriate. Other superfluous information has been deleted. Copyright 2009 by the National Electrical Manufacturers
26、Association. ? DC 20-1992 (R2003, R2009) Page iv Copyright 2009 by the National Electrical Manufacturers Association. ? DC 20-1992 (R2003, R2009) Page 1 Section 1 GENERAL 1.1 SCOPE This Standards Publication covers Class 2 transformers intended primarily for use in 30 volts rms maximum low-voltage r
27、esidential control circuits. It includes definitions, electrical ratings, performance standards, mounting dimensions, and marking requirements. 1.2 REFERENCES The following publications are adopted, in whole or in part as indicated, by reference in this Standards Publication and are available from t
28、he organizations below. National Fire Protection Association Batterymarch Park Quincy, Massachusetts 02269 American National Standards Institute 11 West 42nd Avenue New York, NY 10036 ANSI/NFPA 70 National Electrical Code Underwriters Laboratories, Inc. 333 Pfingsten Road Northbrook, IL 60062 Americ
29、an National Standards Institute 11 West 42nd Avenue New York, NY 10036 ANSI/UL 514 A Metallic Outlet Boxes ANSI/UL 514 B Fittings for Cable and Conduit ANSI/UL 514 C Nonmetallic Outlet Boxes, Flush-Device Boxes and Covers ANSI/UL 1585 Safety Standard for Class 2 and 3 Transformers 1.3 DEFINITIONS cl
30、ass 2 transformer: A Class 2 transformer is a step-down transformer of the low-secondary-voltage type, suitable for use in Class 2 remote-control low-energy circuits in accordance with ANSI/NFPA 70. It shall be of the energy-limiting type or of non-energy-limiting type equipped with over-current pro
31、tection. energy-limiting transformer: An energy-limiting transformer is a transformer that is intended for use on an approximately constant-voltage supply circuit and that has sufficient inherent impedance to limit the output current to a thermally safe maximum value. non-energy-limiting transformer
32、: A non-energy-limiting transformer is a constant-potential transformer which does not have sufficient inherent impedance to limit the output current to a thermally safe maximum value. primary winding: The primary winding is on the input side. Copyright 2009 by the National Electrical Manufacturers
33、Association. ? DC 20-1992 (R2003, R2009) Page 2 secondary winding: The secondary winding is on the output side. rating of transformer: The rating of a transformer consists of a voltampere output, together with any other characteristics, such as voltage or current; other characteristics such as frequ
34、ency and power factor may be included. voltampere rating of a transformer: The voltampere rating of a transformer is the output which can be delivered continuously at rated secondary voltage and rated frequency without exceeding the specified temperature rise limitations. load: The load of a transfo
35、rmer is expressed in voltamperes and is the product of the secondary voltage and the secondary current, expressed as voltamperes. Copyright 2009 by the National Electrical Manufacturers Association. ? DC 20-1992 (R2003, R2009) Page 3 Section 2 RATINGS AND CLASSIFICATION 2.1 ELECTRICAL RATINGS 2.1.1
36、Primary Voltage Ratings Primary voltage ratings shall be as follows: 120 volts at 60 hertz 208 volts at 60 hertz 240 volts at 60 hertz 277 volts at 60 hertz 480 volts at 60 hertz Transformers with multiple rated primaries shall be tested at each primary voltage ratings. 2.1.2 Secondary Voltage Ratin
37、gs Secondary voltage ratings shall be as follows: No load: 26.5 0.5 volts 100 percent rated current: 24.0 1.0 volts 200 percent rated current: 21.5 1.5 volts Secondary voltage ratings shall be applicable to all loads between 0.5 power factor and unity power factor with the primary connected to a sou
38、rce of rated voltage and frequency in accordance with 2.1.1. 2.2 CLASSIFICATION Class 2 transformers shall be one of the following types. NEMA Type Rated Secondary Current (in Amperes) at Nominal 24 volts A0.4 B0.8 C1.2 D1.6 E2.0 . Copyright 2009 by the National Electrical Manufacturers Association.
39、 ? DC 20-1992 (R2003, R2009) Page 4 Section 3 MOUNTING 3.1 GENERAL Class 2 transformers shall conform to ANSI/UL 1585. The enclosure of a Class 2 transformer shall be designed for mounting either on a plane surface or on an outlet box. 3.2 PLANE-SURFACE MOUNTING For mounting on a plane surface, the
40、enclosure shall be so designed that it will make contact only at the points of support. An air space of not less that inch (6.4 mm) shall be maintained between the enclosure and the supporting surface. 3.3 OUTLET-BOX MOUNTING If a plate is provided for mounting on standard junction boxes, the dimens
41、ions and provisions for mounting shall be in accordance with Figure 3-1. Note I Holes A and B shall be optional. Note II 3/16 inch (4.7) diameter holes shall be permitted to be any of the slotted shapes shown. Note III Openings into the box when the transformer is installed shall meet the requiremen
42、ts under “Closure of Openings” in ANSI/UL 514A. Note IV See 3.0 and ANSI/UL 1585, Section 8. Figure 3-1 DIMEMSIONS (IN INCHES (MM) FOR MOUNTING Copyright 2009 by the National Electrical Manufacturers Association. ? DC 20-1992 (R2003, R2009) Page 5 Section 4 PERFORMANCE AND TEST 4.1 GENERAL All trans
43、formers shall meet the performance requirements for Class 2 transformers as described in ANSI/UL 1585. 4.2 REGULATION Transformers classified according to 2.2 shall meet the appropriate requirements of this standard when tested at no load, 100 percent rated load, and 200 percent rated load at load f
44、actors between 0.5 and unity. Load tests shall be performed at an ambient temperature between 68F (20C) and 80F (26.6C). 4.2.1 Test Equipment The test equipment listed in this subclause should be of good quality instrumentation; the accuracy of the meters being within 1 percent and periodically veri
45、fied by the user. To ensure accuracy of test results, it is important to verify that measuring and recording instruments provide true rms readings. The test equipment should be connected in accordance with Figure 4-1. All resistance measurements should be measured at the load switch with an accurate
46、 resistance bridge. a. Power Supply 1. Should be of the nonregulating type. 2. Should be capable of producing the rms test voltages of 120, 208, 240, 277, or 480 volts. 3. Should have a minimum capacity of 500 volt-amperes. b. Rms Voltmeter (for primary) 1. Should be of a proper range to provide acc
47、urate readings. c. Rms Voltmeter (for secondary) 1. Should be of a proper range to provide accurate readings. d. Rms Current Meter 1. Should be of a proper range to provide accurate readings. 2. There should be a circuit impedance to meter resistance ratio of at least 50:1 e. Resistance Bridge f. Sw
48、itches 1. Should have a maximum contact resistance of 0.02 ohms at 5 amperes rms. g. There should be four resistive loads, as follows: 1. 60 ohms + 1 percent, 25 watts (R1) 2. 30 ohms + 1 percent, 50 watts (R2) 3. 15 ohms + 1 percent, 75 watts (R3) 4. 7.5 ohms + 1 percent, 100 watts (R4) Copyright 2
49、009 by the National Electrical Manufacturers Association. ? DC 20-1992 (R2003, R2009) Page 6 The resistors should be arranged so that they can be switched into the circuit, singly or in parallel, to produce the rated current or 200 percent rated current, as follows: 5. 60 ohms for 0.4 ampere (R1) 6. 30 ohms for 0.8 ampere (R2) 7. 60 plus 30 ohms for 1.2 amperes (R1 + R2) 8. 15 ohms for 1.6 amperes (R3) 9.