Component Protection - Cooper Industries

1 2005 Cooper Bussmann67 This issue analyzes the Protection of electrical system components from faultcurrents. It gives the specifier the necessary information regarding the short- circuit current or withstand rating of electrical circuit components, such aswire, bus, motor starters, etc. Proper Protection of circuits will improve reliability and reduce the possibility of injury. Electrical systems can bedestroyed if the overcurrent devices do not limit the short- circuit current towithin the withstand rating of the system s components. Merely matching theamp rating of a Component with the amp rating of a protective device will notassure Component Protection under short circuit National Electrical Code covers Component Protection in several sections.

2 The first section to note is Protection and The National Electrical Code circuit Impedance and Other Characteristics:The overcurrentprotective devices, the total impedance, the Component short- circuit currentratings, and other characteristics of the circuit to be protected shall beselected and coordinated to permit the circuit -protective devices used toclear a fault to do so without extensive damage to the electrical components of the circuit . This fault shall be assumed to be either betweentwo or more of the circuit conductors or between any circuit conductor andthe grounding conductor or enclosing metal raceway. Listed productsapplied in accordance with their listing shall be considered to meet therequirements of this requires that overcurrent protective devices, such as fuses and circuitbreakers be selected in such a manner that the short- circuit current (withstand) ratings of the system components will not be exceeded should ashort circuit short- circuit withstand rating is the maximum short- circuit current that acomponent can safely withstand.

3 Failure to provide adequate Protection mayresult in Component destruction under short circuit calculating the fault levels throughout the electrical system, the next stepis to check the withstand rating of wire and cable, circuit breakers, transferswitches, starters, etc., under short circuit :The let-through energy of the protective device must be equal to or lessthan the short- circuit withstand rating of the Component being :Choosing overcurrent protective devices strictly on the basis ofvoltage, current , and interrupting rating alone will not assure componentprotection from short- circuit currents. High interrupting capacity electro-mechanical overcurrent protective devices ( circuit breakers), especiallythose that are not current -limiting, may not be capable of protecting wire,cable or other components within high short circuit ranges.

4 The interruptingrating of a protective device pertains only to that device and has absolutelyno bearing on its ability to protect connected downstream often, an improperly protected Component is completely destroyedunder short circuit conditions while the protective device is opening thefaulted proceeding with the study of Component withstandability, the technology concerning current -limitation will be DefinedToday, most electrical distribution systems are capable of delivering very highshort- circuit currents, some in excess of 200,000A. Many circuit componentshave relatively low short circuit withstandability of a few thousand amps. If thecomponents are not capable of handling these short- circuit currents, theycould easily be damaged or destroyed.

5 The current -limiting ability of today smodern fuses allows components with low short- circuit withstand ratings to bespecified in spite of high available fault offers the following definition of a current -limiting device: current -Limiting Overcurrent Protective Device: A device that, when interrupting currents in its current -limiting range, reduces the current flowingin the faulted circuit to a magnitude substantially less than that obtainable inthe same circuit if the device were replaced with a solid conductor havingcomparable concept of current -limitation is pointed out in the following graph, wherethe prospective available fault current is shown in conjunction with the limitedcurrent resulting when a current -limiting fuse clears. The area under the current curve is representative of the amount of short circuit energy being dissipated in the circuit .

6 Since both magnetic forces and thermal energy aredirectly proportional to the square of the current , it is important to limit theshort- circuit current to as small a value as possible. The maximum magneticforces vary as the square of the PEAK current and thermal energy varies asthe square of the RMS Effect of FusesComponent ProtectionIntroduction and current -LimitationProspective available short-circuitcurrent that would flow when a fuse is not ,000 Peak Let-ThroughCurrent of Fusetc10,000 Time Total Clearing Time of Fuse0 current (100,000)2= 10010,000*Thus, the current -limiting fuse in this example (above waveform) would limitthe let-through energy to a fraction of the value which is available from thesystem. In the first major loop of fault current , standard non- current -limiting,electro-mechanical protective devices would let-through approximately 100times* as much destructive energy as the fuse would let-through.

7 2005 Cooper Bussmann68 Analysis of current -Limiting fuse Let-Through ChartsThe degree of current -limitation of a given size and type of fuse depends, ingeneral, upon the available short- circuit current that can be delivered by theelectrical system. current -limitation of fuses is best described in the form of alet-through chart that, when applied from a practical point of view, is useful todetermine the let-through currents when a fuse let-through charts are plotted from actual test data. The test circuit thatestablishes line A-B corresponds to a short circuit power factor of 15%, that isassociated with an X/R ratio of The fuse curves represent the cutoff valueof the prospective available short- circuit current under the given circuit conditions.

8 Each type or class of fuse has its own family of let-through let-through data has been generated by actual short- circuit tests of current -limiting fuses. It is important to understand how the curves are generated, and what circuit parameters affect the let-through curve , there are three circuit parameters that can affect fuse let-throughperformance for a given available short- circuit current . These are:1. Short- circuit power factor2. Short- circuit closing angle3. Applied voltageCurrent-limiting fuse let-through curves are generated under worst case conditions, based on these three variable parameters. The benefit to the useris a conservative resultant let-through current (both Ipand IRMS). Under actual field conditions, changing any one or a combination of these will resultin lower let-through currents.

9 This provides for an additional degree of reliability when applying fuses for equipment Let-Through Charts for Cooper Bussmann fuses are near theback of this of a current -Limiting FusePrior to using the fuse Let-Through Charts, it must be determined what let-through data is pertinent to equipment withstand withstand ratings can be described as: How Much Fault Currentcan the equipment handle, and for How Long? Based on standards presentlyavailable, the most important data that can be obtained from the fuse Let-Through Charts and their physical effects are the following:A. Peak let-through current : mechanical forcesB. Apparent prospective RMS symmetrical let-through current : heating effectC. Clearing time: less than 1 2cycle when fuse is in it s current -limiting range (beyondwhere fuse curve intersects A-B line).

10 This is a typical example showing the short- circuit current available to an 800 Acircuit, an 800A Low-Peak current -limiting time-delay fuse , and the let-throughdata of Amp Low-Peak current -Limiting Time-DelayFuse and Associated Let-Through DataComponent ProtectionHow To Use current -Limitation ChartsAMPRATING800 APROSPECTIVE SHORT- circuit current SYMMETRICAL RMS AMPSINSTANTANEOUS PEAK LET-THROUGH current IN AMPS 10002000300040006000800010,00020,00030,0 0040,00060,00080,000100,000200,000I400,0 00300,000200,000100,00080,00060,00030,00 020,00010,000800060004000300020001000 ABAvailable Peak Short- circuit current = 198,000 AAvailable RMS Short- circuit current = 86,000 APeak Let-Through Currentof fuse = 49,000 ARMS Let-Through current of fuse = 21,000 Atm = fuse Melt Timeta = fuse Arc Timetc = fuse Clearing TimetctmtaTIMEHow to Use the Let-Through ChartsUsing the example given, one can determine the pertinent let-through data forthe KRP-C-800SP amp Low-Peak fuse .