NEMA PB 2 2-2014 Application Guide for Ground Fault Protective Devices for Equipment.pdf

1、NEMA Standards PublicationNational Electrical Manufacturers AssociationNEMA PB 2.2-2014Application Guide for Ground Fault ProtectiveDevices for EquipmentNEMA Standards Publication PB 2.2-2014 Application Guide for Ground Fault Protective Devices for Equipment Published by: National Electrical Manufa

2、cturers Association 1300 North 17th Street, Suite 900 Rosslyn, Virginia 22209 www.nema.org 2014 National Electrical Manufacturers Association. All rights, including translation into other languages, reserved under the Universal Copyright Convention, the Berne Convention for the Protection of Literar

3、y and Artistic Works, and the International and Pan American copyright conventions. 2014 National Electrical Manufacturers Association 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

4、 document at the time it was developed. Consensus does not necessarily mean that there is unanimous agreement among every person participating in the development of this document. The National Electrical Manufacturers Association (NEMA) standards and guideline publications, of which the document con

5、tained herein is one, are developed through a voluntary consensus standards development process. This process brings together volunteers 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 pr

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10、omeone else. Anyone using this document should rely on his or her own independent judgment or, as appropriate, seek the advice of a competent professional in determining the exercise of reasonable care in any given circumstances. Information and other standards on the topic covered by this publicati

11、on may be available from other sources, which the user may wish to consult for additional views or information not covered by this publication. 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 product

12、s, designs, or installations for safety or health purposes. Any certification or other statement of compliance with any health or safetyrelated information in this document shall not be attributable to NEMA and is solely the responsibility of the certifier or maker of the statement.NEMA PB 2.2-2014

13、Page i 2014 National Electrical Manufacturers Association Foreword This Standards Publication is intended to provide a basis of common understanding within the electrical community. User needs have been considered throughout the development of this publication. Proposed or recommended revisions shou

14、ld be submitted to: Vice President, Technical Services National Electrical Manufacturers Association 1300 N 17th Street, Suite 900 Rosslyn, VA 22209 This Standards Publication was developed by the Panelboard and Distribution Board Section. Section approval of the standard does not necessarily imply

15、that all section members voted for its approval or participated in its development. At the time it was approved, the Section was composed of the following members: ABB Inc.New Berlin, WI Eaton Cooper BussmannEllisville, MO Eaton Electrical Inc.Pittsburgh, PA GEPlainville, CT Hubbell, Inc.Bridgeport,

16、 CT Milbank Manufacturing CompanyKansas City, MO Penn Panel they can limit only the length of time that a ground fault above the selected pickup current setting is allowed to exist. Relay setting tolerances and operating times for the disconnects or protective devices to open and clear the fault mus

17、t be included in any analysis of coordination or clearing speeds. It may be advisable to consider different settings for the construction and operational phases of a project. Minimum settings should be selected during the construction phase while equipment is being installed and the probability of c

18、onstruction accidents is high. For the operational phase, in addition to protection, settings should allow for proper system coordination and provide optimum service continuity to avoid possible safety problems resulting from loss of lights and operating power required for elevators, exhaust fans, a

19、nd other safety-oriented electrical loads. Loss of power resulting from too sensitive GFP device settings could cause more serious consequences than the equipment damage incurred as a result of a minor ground fault. 2.5 METHODS OF GFP DEVICE SELECTIVITY There are two basic methods of achieving selec

20、tivity between GFP devices in a coordinated distribution system: (1) time-current band and (2) zone-selective. Each of these methods is described in Section 6. 2.7 DAMAGE LEVELS Tolerable damage levels should be established for each system as a basis for selecting equipment and determining time and

21、current settings of all devices. Annex A illustrates the methods for calculating tolerable damage levels used in this guide. NEMA PB 2.2-2014 Page 6 2014 National Electrical Manufacturers Association Section 3POWER CIRCUIT 3.1 VOLTAGE GFP devices are designed for use on power systems of 600 volts or

22、 less. They may be used on higher-voltage power circuits if sufficient insulation and spacing is provided. 3.2 FREQUENCY GFP devices should be used only on power circuits of the frequency for which they are marked, or as recommended by the manufacturer. 3.3 CURRENT RATING GFP devices are designed to

23、 withstand continuously a fault current at least equal to the maximum pick-up setting available on the relay. They should not be applied to a power circuit in such a way that a continuous unbalanced current in excess of the maximum continuous current rating can flow through the sensing device withou

24、t being interrupted. When GFP devices are applied to a system, their maximum permissible fault current withstand rating must not be exceeded. This involves the calculation of the maximum ground fault current to which the devices may be subjected. 3.4 LOCATION OF GFS DEVICES The location of a GFS dev

25、ice defines the point beyond which downstream (in direction of power flow) ground faults can be detected. Consequently, it should be located as close to its associated disconnecting means as possible and is usually mounted immediately downstream from it. There are two basic methods of using GFS devi

26、ces to detect ground fault current. 3.4.1 GROUND RETURN METHOD The GFS device detects the total ground current flowing in the grounding electrode conductor and the main bonding jumper. This method can be used only for the main disconnect of services or separately derived systems. 3.4.2 OUTGOING CIRC

27、UIT METHOD (RESIDUAL OR ZERO-SEQUENCE) The GFS device detects the ground current by making a vectorial summation of the phase and neutral (if any) currents. It is located downstream from the point at which the wiring system is grounded. Only this method can be used for feeders. It is also often used

28、 on the incoming main disconnect, multiple mains and ties. 3.5 GROUND CONNECTIONS If any grounding connection to the power system exists downstream of the main disconnect, neither the ground return nor outgoing circuit method is adequate and the manufacturer should be consulted for recommendations.

29、Power supply systems exist in which there are multiple grounding connections, e.g., networks, multiple source systems, and so forth, and care must be exercised when applying ground fault protection to NEMA PB 2.2-2014 Page 7 2014 National Electrical Manufacturers Association distribution systems fed

30、 from them. In general, the ground return method should not be used because multiple paths exist for the return of ground fault current to the source. Thus, if the ground return method is used in a switchboard fed by a network system, not only is there the possibility that the ground fault sensor wi

31、ll detect only a part of the ground fault current originating in its own system (the remainder returning through the grounding conductors of other systems fed from the same source and via the source grounding connection), but it may also detect some part of the ground fault current originating in an

32、y other system fed from the same source. Since the maximum setting permitted by Article 230 of the NEC is 1200 amperes and the available ground fault current from such a source is very high, it is very possible that a ground fault in one of the connected systems will trip any GFP equipment of other

33、systems fed from that source and using the ground return method. 3.6 MOUNTING AND CONNECTION OF GFS DEVICES Commonly used GFS devices are window or through-type magnetic devices. They should be mounted without undue mechanical stresses, twisting or misalignment being exerted by clamps, supports, bus

34、 bars, or cables. Split sensing devices should have clean gaps or lamination stacks and be properly assembled and clamped. Power conductor bus bars or cable should be symmetrically located in the ground fault sensor window. Clearances between conductors and the ground fault sensor body should be not

35、 less than those recommended by the manufacturer. The manufacturers instructions on the polarity of GFS secondary windings should be followed. Generally, the maintenance of polarity is not necessary unless two or more are connected in parallel, but for differential or summation methods, winding pola

36、rity and mounting instructions must be observed and the manufacturers wiring diagram followed. 3.7 DELTA-CONNECTED SYSTEMS Although when the NEC requires GFP it refers specifically and only to wye-connected solidly grounded systems, many delta-connected systems are solidly grounded at one corner or

37、at the center point of one leg. They are also vulnerable to destruction by arcing ground faults, and protection by GFP devices can reduce damage levels. Usually GFP can be applied in the same way as for wye-connected systems. 3.8 NEUTRALS AND GROUNDING Systems in which multiple individually grounded

38、 power sources are used, such as utility and emergency generator systems, need careful consideration. It is helpful if no tie exists between neutrals of multiple sources. Four-pole transfer switches are one way of breaking the neutral tie when sources are not connected in parallel. Relaying schemes

39、are available that operate properly with continuous neutrals. Care must be exercised when terminating equipment grounding conductors in a switchboard. They must never be connected to the neutral bus. All equipment grounding conductors must be terminated at the switchboard ground bus. In a system wit

40、h a single power source, the switchboard ground bus should be connected to the neutral bus only within the main disconnect section and by means of a single conductor, i.e., the neutral grounding conductor or main bonding jumper. Multiple-source systems require similar control of the connection(s). T

41、his is essential when the ground return method of sensing is used to ensure that all of the ground fault current returns to the source via the conductor(s) encircled by the GFP sensing device. It is also essential, when the vectorial summation method is used, to avoid any ground connection to the ne

42、utral on the load side of any sensor in the switchboard. 3.9 NEED FOR COMPLETE SYSTEM INFORMATION It is common for an engineer to be asked to design a switchboard, with no detailed knowledge of the supply other than the voltage, frequency, number of phases and possibly the fact that it is grounded.

43、If NEMA PB 2.2-2014 Page 8 2014 National Electrical Manufacturers Association ground fault protection is to be designed into the system, it is essential that complete information, including power sources and all grounding connections, be obtained. 3.10 SYSTEMS CONTAINING TRANSFORMERS A GFP system in

44、stalled on the supply side (primary) of a transformer protects only the zone downstream as far as the transformer primary when that winding is electrically isolated from all other windings. If the primary winding is electrically connected (such as an autotransformer or buckboost type), then protecti

45、on is also provided on the connected secondary side. If GFP is required on an electrically isolated secondary side of a transformer, it must be treated as an entirely separate system. It follows from the foregoing that the phase overcurrent protection on the primary side of any transformer in the di

46、stribution system must be properly coordinated with the GFP device nearest to the transformer secondary on the secondary side if nuisance tripping of the primary disconnecting means is to be avoided. 3.11 SYSTEM DESIGN See the checklist in Annex B.NEMA PB 2.2-2014 Page 9 2014 National Electrical Man

47、ufacturers Association Section 4CONTROL CIRCUIT 4.1 CONTROL POWER Control power, if required, may be ac or dc depending upon the requirements of the GFP devices and the disconnects. The control power source should be in accordance with the nameplate and the manufacturers instructions. 4.2 AC CONTROL

48、 POWER The primary of a control transformer, if provided and supplied from the power circuit, must be connected line to line. If a control power transformer were connected line to neutral, its primary voltage could drop too low for satisfactory operation if a ground fault occurred on the phase to wh

49、ich the transformer was connected. All ac shunt trip devices suitable for use with GFP devices should operate down to 55% of rated voltage, and all ac auxiliary relays essential for the tripping function should also operate down to 55% of rated voltage unless supplied from a reliable separate control power source. (If supplied from a reliable separate control power source, conventional relays may be used. A receptacle or lighting circuit is not a reliable source.) 4.3 DC CONTROL POWER Where freedom from line voltage fluctuations is required, control power can be suppl