Part 1 a Simple Approach to Short Circuit Calculations
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www.stevenengineering.com 9200 - 8 System Distribution Electrical For An Protection Dependable Engineering Outside Local Area: (800) 25 Bussmann 9200 - 8 Part 1 A SimpleApproach Calculations Main Office: (650) 58 6370 - 0 Short Circuit To 230 Ryan Way, South San Francisco, CA, 9408 Bulletin EDP-1 (2004-1) Courtesy of Steven Engineering, Inc. Electrical Distribution System Basic Considerations of Short-Circuit Calculations Why Short-Circuit Calculations Sources of short circuit current that are normally taken Several sections of the National Electrical Code relate under consideration include: to proper overcurrent protection. Safe and reliable - Utility Generation application of overcurrent protective devices based on - Local Generation these sections mandate that a short circuit study and a - Synchronous Motors and www.stevenengineering.com selective coordination study be conducted. - Induction Motors 9200 These sections include, among others: Capacitor discharge currents can normally be - 8 110-9 Interrupting Rating neglected due to their short time duration. Certain IEEE 110-10 Component Protection (Institute of Electrical and Electronic Engineers) publications 230-65 Service Entrance Equipment detail how to calculate these currents if they are substantial. 240-1 Conductor Protection 250-95 Equipment Grounding Conductor Protection Asymmetrical Components 517-17 Health Care Facilities - Selective Coordination Short circuit current normally takes on an asymmetrical characteristic during the first few cycles of duration. That is, Compliance with these code sections can best be it is offset about the zero axis, as indicated in Figure 1. accomplished by conducting a short circuit study and a Outside Local Area: (800) 25 selective coordination study. The protection for an electrical system should not only be safe under all service conditions but, to insure continuity 9200 - 8 of service, it should be selectively coordinated as well. A coordinated system is one where only the faulted circuit is C U isolated without disturbing any other part of the system. R Overcurrent protection devices should also provide short- R TIME circuit as well as overload protection for system E components, such as bus, wire, motor controllers, etc. N T To obtain reliable, coordinated operation and assure Main Office: (650) 58 that system components are protected from damage, it is necessary to first calculate the available fault current at Figure 1 6370 various critical points in the electrical system. - 0 Once the short-circuit levels are determined, the In Figure 2, note that the total short circuit current Ia is engineer can specify proper interrupting rating require- the summation of two components - the symmetrical RMS ments, selectively coordinate the system and provide current IS, and the DC component, IDC. The DC component component protection. is a function of the stored energy within the system at the initiation of the short circuit. It decays to zero after a few General Comments on Short-Circuit Calculations cycles due to I2R losses in the system, at which point the Short Circuit Calculations should be done at all critical short circuit current is symmetrical about the zero axis. The points in the system. RMS value of the symmetrical component may be deter- These would include: mined using Ohm`s Law. To determine the asymmetrical - Service Entrance component, it is necessary to know the X/R ratio of the - Panel Boards system. To obtain the X/R ratio, the total resistance and total - Motor Control Centers reactance of the circuit to the point of fault must be - Motor Starters determined. Maximum thermal and mechanical stress on - Transfer Switches the equipment occurs during these first few cycles. It is - Load Centers important to concentrate on what happens during the first half cycle after the initiation of the fault. 230 Ryan Way, South San Francisco, CA, 9408 Normally, short circuit studies involve calculating a bolted 3-phase fault condition. This can be characterized as all three phases “bolted” together to create a zero impedance connection. This establishes a “worst case” condition, that results in maximum thermal and mechanical stress in the system. From this calculation, other types of fault conditions can be obtained. Courtesy of Steven Engineering, Inc. 3 Electrical Distribution System Basic Considerations of Short-Circuit Calculations To accomplish this, study Figure 2, and refer to Table 8. Interrupting Rating, Interrupting Capacity and Short-Circuit Currents Interrupting Rating can be defined as “the maximum IP = 115,450A Ia short-circuit current that a protective device can safely clear, under specified test conditions.” I = a Interrupting Capacity can be defined as “the actual 66,500A IDC www.stevenengineering.com short circuit current that a protective device has been Is = tested to interrupt.” 50,000A 9200 The National Electrical Code requires adequate - C 8 U interrupting ratings in Sections 110-9 and 230-65. R TIME R Is E Section 110-9 Interrupting Rating. Equipment intended to N break current at fault levels shall have an interrupting rating T sufficient for the system voltage and the current which is Ia - Asymmetrical RMS Current available at the line terminals of the equipment. IDC - DC Component Is - Symmetrical RMS Component Section 230-65. Available Short-Circuit Current. Service IP - Instantaneous Peak Current Equipment shall be suitable for the short circuit current Outside Local Area: (800) 25 available at its supply terminals. Figure 2 Low voltage fuses have their interrupting rating 9200 - 8 Figure 2 illustrates a worst case waveform that 1 phase expressed in terms of the symmetrical component of short- of the 3 phase system will assume during the first few circuit current, IS. They are given an RMS symmetrical cycles after the fault initiation. interrupting rating at a specific power factor. This means For this example, assume an RMS symmetrical short that the fuse can interrupt any asymmetrical current circuit value of 50,000 amperes, at a 15% short circuit associated with this rating. Thus only the symmetrical power factor. Locate the 15% P.F. in Table 8. Said another component of short-circuit current need be considered to way, the X/R short circuit ratio of this circuit is 6.5912. determine the necessary interrupting rating of a low voltage Main Office: (650) 58 fuse. For U.L. listed low voltage fuses, interrupting rating The key portions are: equals its interrupting capacity. 6370 - Symmetrical RMS Short Circuit Current = Is Low voltage molded case circuit breakers also have - 0 - Instantaneous Peak Current = Ip their interrupting rating expressed in terms of RMS - Asymmetrical RMS Short Circuit Current symmetrical amperes at a specific power factor. However, (worst case single phase) = Ia it is necessary to determine a molded case circuit breaker’s interrupting capacity in order to safely apply it. The reader From Table 8, note the following relationships. is directed to Buss bulletin PMCB II for an understanding of this concept. Is = Symmetrical RMS Current Ip = Is x Mp (Column 3) Ia = Is x Mm (Column 4) For this example, Figure 2, Is = 50,000 Amperes RMS Symmetrical Ip = 50,000 x 2.309 ( Column 3) = 115,450 Amperes Ia = 50,000 x 1.330 (Column 4) 230 Ryan Way, South San Francisco, CA, 9408 = 66,500 Amperes RMS Asymmetrical With this basic understanding, proceed in the systems analysis. Courtesy of Steven Engineering, Inc. 4 3ø Short-Circuit Current Calculations – Procedures and Methods 3Ø Short-Circuit Current Calculations, To begin the analysis, consider the following system, Procedures and Methods supplied by a 1500 KVA, three phase transformer having a To determine the fault current at any point in the full load current of 1804 amperes at 480 volts. (See System system, first draw a one-line diagram showing all of the A, below) Also, System B, for a double transformation, will sources of short-circuit current feeding into the fault, as well be studied. as the impedances of the circuit components. www.stevenengineering.com To start, obtain the available short-circuit KVA, MVA, or To begin the study, the system components, including SCA from the local utility company. those of the utility system, are represented as impedances 9200 The utility estimates that System A can deliver a short- - 8 in the diagram. circuit of 100,000 MVA at the primary of the transformer. The impedance tables given in the Data Section System B can deliver a short-circuit of 500,000 KVA at the include three phase and single phase transformers, current primary of the first transformer. Since the X/R ratio of the transformers, safety switches, circuit breakers, cable, and utility system is usually quite high, only the reactance need busway. These tables can be used if information from the be considered. manufacturers is not readily available. With this available short-circuit information, begin to It must be understood that short circuit calculations are make the necessary calculations to determine the fault performed without current limiting devices in the system. current at any point in the electrical system. Calculations are done as though these devices are Four basic methods will be presented in this text to replaced with copper bars, to determine the maximum Outside Local Area: (800) 25 instruct the reader on short circuit calculations. “available” short circuit current. This is necessary to These include : project how the system and the current limiting devices will - the ohmic method 9200 - perform. 8 - the per unit method Also, current limiting devices do not operate in series - the TRON® Computer Software method to produce a “compounding” current limiting effect. The - the point to point method downstream, or load side, fuse will operate alone under a short circuit condition if properly coordinated. System A System B 3Ø Single Transformer System Main Office: (650) 58 3Ø Double Transformer System Available Utility Available Utility S.C. MVA 100,000 6370 S.C. KVA 500,000 - 0 1500 KVA Transformer 1000 KVA Transformer, 480Y/277V, 25’ - 500kcmil 480/277 Volts 3Ø 3.5%Z, 3.45%X, .56%R 30’ - 500 kcmil 6 Per Phase 3.45%X, .60%R If.l.