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Component protection.pdf


<p>Component ProtectionIntroduction and Current-LimitationThis issue analyzes the protection of electrical system components from fault currents. It gives the specifier the necessary information regarding the shortcircuit current or withstand rating of electrical circuit components, such as wire, bus, motor starters, etc. Proper protection of circuits will improve reliability and reduce the possibility of injury. Electrical systems can be destroyed if the overcurrent devices do not limit the short-circuit current to within the withstand rating of the systems components. Merely matching the amp rating of a component with the amp rating of a protective device will not assure component protection under short circuit conditions. The National Electrical Code covers Component Protection in several sections. The first section to note is 110.10.</p> <p>Current-Limitation DefinedToday, most electrical distribution systems are capable of delivering very high short-circuit currents, some in excess of 200,000A. Many circuit components have relatively low short circuit withstandability of a few thousand amps. If the components are not capable of handling these short-circuit currents, they could easily be damaged or destroyed. The current-limiting ability of todays modern fuses allows components with low short-circuit withstand ratings to be specified in spite of high available fault currents. NEC 240.2 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 flowing in the faulted circuit to a magnitude substantially less than that obtainable in the same circuit if the device were replaced with a solid conductor having comparable impedance. The concept of current-limitation is pointed out in the following graph, where the prospective available fault current is shown in conjunction with the limited current 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. Since both magnetic forces and thermal energy are directly proportional to the square of the current, it is important to limit the short-circuit current to as small a value as possible. The maximum magnetic forces vary as the square of the PEAK current and thermal energy varies as the square of the RMS current.</p> <p>Component Protection and The National Electrical Code110.10 Circuit Impedance and Other Characteristics: The overcurrent protective devices, the total impedance, the component short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit-protective devices used to clear a fault to do so without extensive damage to the electrical components of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or between any circuit conductor and the grounding conductor or enclosing metal raceway. Listed products applied in accordance with their listing shall be considered to meet the requirements of this section. This requires that overcurrent protective devices, such as fuses and circuit breakers be selected in such a manner that the short-circuit current (withstand) ratings of the system components will not be exceeded should a short circuit occur. The short-circuit withstand rating is the maximum short- circuit current that a component can safely withstand. Failure to provide adequate protection may result in component destruction under short circuit conditions. After calculating the fault levels throughout the electrical system, the next step is to check the withstand rating of wire and cable, circuit breakers, transfer switches, starters, etc., under short circuit conditions. Note: The let-through energy of the protective device must be equal to or less than the short-circuit withstand rating of the component being protected. CAUTION: Choosing overcurrent protective devices strictly on the basis of voltage, current, and interrupting rating alone will not assure component protection from short-circuit currents. High interrupting capacity electromechanical overcurrent protective devices (circuit breakers), especially those that are not current-limiting, may not be capable of protecting wire, cable or other components within high short circuit ranges. The interrupting rating of a protective device pertains only to that device and has absolutely no bearing on its ability to protect connected downstream components. Quite often, an improperly protected component is completely destroyed under short circuit conditions while the protective device is opening the faulted circuit. Before proceeding with the study of component withstandability, the technology concerning current-limitation will be reviewed.</p> <p>Current-Limiting Effect of Fuses100,000</p> <p>Prospective available short-circuit current that would flow when a fuse is not used.</p> <p>Current</p> <p>10,000 0</p> <p>Peak Let-Through Current of Fuse</p> <p>tc Total Clearing Time of Fuse</p> <p>Time</p> <p>Thus, the current-limiting fuse in this example (above waveform) would limit the let-through energy to a fraction of the value which is available from the system. In the first major loop of fault current, standard non-current-limiting, electro-mechanical protective devices would let-through approximately 100 times* as much destructive energy as the fuse would let-through.2</p> <p>*</p> <p>(100,000) = 100 10,000</p> <p>2005 Cooper Bussmann</p> <p>67</p> <p>Component ProtectionHow To Use Current-Limitation ChartsAnalysis of Current-Limiting Fuse Let-Through ChartsThe degree of current-limitation of a given size and type of fuse depends, in general, upon the available short-circuit current that can be delivered by the electrical system. Current-limitation of fuses is best described in the form of a let-through chart that, when applied from a practical point of view, is useful to determine the let-through currents when a fuse opens. Fuse let-through charts are plotted from actual test data. The test circuit that establishes line A-B corresponds to a short circuit power factor of 15%, that is associated with an X/R ratio of 6.6. The fuse curves represent the cutoff value of the prospective available short-circuit current under the given circuit conditions. Each type or class of fuse has its own family of let-through curves. The 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 data. Typically, there are three circuit parameters that can affect fuse let-through performance for a given available short-circuit current. These are:1. Short-circuit power factor 2. Short-circuit closing angle 3. Applied voltage</p> <p>Prior to using the Fuse Let-Through Charts, it must be determined what letthrough data is pertinent to equipment withstand ratings. Equipment withstand ratings can be described as: How Much Fault Current can the equipment handle, and for How Long? Based on standards presently available, the most important data that can be obtained from the Fuse LetThrough Charts and their physical effects are the following:A. Peak let-through current: mechanical forces B. Apparent prospective RMS symmetrical let-through current: heating effect C. Clearing time: less than 12 cycle when fuse is in its current-limiting range (beyond where fuse curve intersects A-B line).</p> <p>This is a typical example showing the short-circuit current available to an 800A circuit, an 800A Low-Peak current-limiting time-delay fuse, and the let-through data of interest.</p> <p>800 Amp Low-Peak Current-Limiting Time-Delay Fuse and Associated Let-Through Data</p> <p>Current-limiting fuse let-through curves are generated under worst case conditions, based on these three variable parameters. The benefit to the user is a conservative resultant let-through current (both Ip and IRMS). Under actual field conditions, changing any one or a combination of these will result in lower let-through currents. This provides for an additional degree of reliability when applying fuses for equipment protection. Current-Limiting Let-Through Charts for Cooper Bussmann fuses are near the back of this book.</p> <p>Analysis of a Current-Limiting FuseB I</p> <p>INSTANTANEOUS PEAK LET-THROUGH CURRENT IN AMPS</p> <p>400,000 300,000 200,000</p> <p>Available Peak ShortCircuit Current = 198,000A Available RMS ShortCircuit Current = 86,000A</p> <p>100,000 80,000 60,000</p> <p>800A</p> <p>Peak Let-Through Current of Fuse= 49,000A RMS Let-Through Current of Fuse = 21,000A tm ta tc tm = Fuse Melt Time ta = Fuse Arc Time tc = Fuse Clearing Time TIME</p> <p>30,000 20,000 10,000 8000 6000</p> <p>A2000 1000 1000 2000 3000 4000 6000</p> <p>8000 10,000</p> <p>20,000</p> <p>30,000 40,000</p> <p>60,000 80,000</p> <p>100,000</p> <p>PROSPECTIVE SHORT-CIRCUIT CURRENT SYMMETRICAL RMS AMPS</p> <p>68</p> <p>200,000</p> <p>AMP RATING</p> <p>4000 3000</p> <p>2005 Cooper Bussmann</p> <p>Component ProtectionHow To Use Current-Limitation ChartsHow to Use the Let-Through ChartsUsing the example given, one can determine the pertinent let-through data for the KRP-C-800SP amp Low-Peak fuse. The Let-Through Chart pertaining to the 800A Low-Peak fuse is illustrated. A.Step 1.</p> <p>Determine the PEAK let-through CURRENT.Enter the chart on the Prospective Short-Circuit current scale at 86,000 amps and proceed vertically until the 800A fuse curve is intersected. Follow horizontally until the Instantaneous Peak Let-Through Current scale is intersected. Read the PEAK let-through CURRENT as 49,000A. (If a fuse had not been used, the peak current would have been 198,000A.)</p> <p>Step 2. Step 3.</p> <p>Most electrical equipment has a withstand rating that is defined in terms of an RMS symmetrical-short-circuit current, and in some cases, peak let-through current. These values have been established through short circuit testing of that equipment according to an accepted industry standard. Or, as is the case with conductors, the withstand rating is based on a mathematical calculation and is also expressed in an RMS short-circuit current. If both the let-through currents (IRMS and Ip) of the current-limiting fuse and the time it takes to clear the fault are less than the withstand rating of the electrical component, then that component will be protected from short circuit damage. The following Table shows typical assumed short-circuit current ratings for various unmarked components.</p> <p>B.Step 1.</p> <p>Determine the APPARENT PROSPECTIVE RMS SYMMETRICAL let-through CURRENT.Enter the chart on the Prospective Short-Circuit current scale at 86,000A and proceed vertically until the 800A fuse curve is intersected. Follow horizontally until line A-B is intersected. Proceed vertically down to the Prospective Short-Circuit Current. Read the APPARENT PROSPECTIVE RMS SYMMETRICAL let-through CURRENT as 21,000A. (The RMS SYMMETRICAL let-through CURRENT would be 86,000A if there were no fuse in the circuit.)</p> <p>Typical Short-Circuit Current Ratings For Unmarked Components*Component Industrial Control Equipment:a. Auxiliary Devices b. Switches (other than Mercury Tube Type) c. Mercury Tube Switches Rated over 60 amperes or over 250 volts Rated 250 volts or less, 60 amperes or less, and over 2kVA Rated 250 volts or less and 2kVA or less Meter Socket Base Photoelectric Switches Receptacle (GFCI Type) Receptacle (other than GFCI Type) Snap Switch Terminal Block Thermostat 5 5 5 3.5 1 10 5 10 2 5 10 5</p> <p>Short- Circuit Rating, kA</p> <p>Step 2. Step 3. Step 4.</p> <p>Current-Limitation Curves Cooper Bussmann Low-Peak Time-Delay Fuse KRP-C-800SP</p> <p>*Based upon information in UL 891 (Dead-Front Switchboards) The following components will be analyzed by establishing the short-circuit withstand data of each component and then selecting the proper currentlimiting fuses for protection: Wire and Cable Bus (Busway, Switchboards, Motor Control Centers and Panelboards) Transfer Switches HVAC Equipment Ballasts Circuit Breakers A detailed analysis of motor circuit component protection is provided later in the section on motor circuits.</p> <p>C. Clearing timeIf the RMS Symmetrical available is greater than the point where the fuse characteristic curve intersects with the diagonal A-B line, then the fuse clearing time is 12 cycle or less. In this example, the intersection is approximately 9500A; so for short-circuit currents above approximately 9500A, this KRP-C-800SP fuse is current-limiting. The current-limiting charts and tables for Cooper Bussmann fuses are in the rear of this book under Current-Limiting Let-Through Charts. Refer to these tables when analyzing component protection in the following sections.</p> <p>2005 Cooper Bussmann</p> <p>69</p> <p>Component ProtectionWire &amp; CableThe circuit shown originates at a distribution panel where 40,000 amps RMS symmetrical is available. To determine the proper fuse, first establish the shortcircuit withstand data for the 10 AWG THW copper cable shown in the diagram.</p> <p>Short-Circuit Current Withstand Chart for Copper Cables with Thermoplastic Insulation</p> <p>Short-Circuit Protection of Wire and Cable</p> <p>The following table shows the short-circuit withstand of copper cable with 75C thermoplastic insulation based on Insulated Cable Engineers Association (ICEA) formulae. The short-circuit withstand of the 10 AWG THW copper conductor is 4300A for one cycle (0.0167 seconds). Short-circuit protection of this conductor requires the selection of an overcurrent device which will limit the 40,000A RMS symmetrical available to a value less than 4300A, and clear the fault in one cycle or less. The Low-Peak dual-element fuse let-through chart shows that the LPS-RK30SP Low-Peak dual-element fuse will let-through an apparent prospective RMS current of less than 1800A, when 40,000A is available (and would clear the fault in less than 12 cycle).</p> <p>Short-Circuit Currents for Insulated CablesThe increase in kVA capacity of power distribution systems has resulted in possible short-circuit currents of extremely high magnitude. Conductor insulation may be seriously damaged by fault induced, high conductor temperatures. As a guide in preventing such serious damage, maximum allowable short circuit temperatures, which damage the insulation to a slight extent only, have been established for various insulation as follows: Paper, rubber and varnished cloth 200C Thermoplastic 150C The chart at the top of next column shows the currents which, after flowing for the times indicated, will produce these maximum temperatures for each conductor size. The system short circuit capacity, the conductor crosssectional area and the overcurrent protective device opening time should be such that these maximum allowable short-circuit currents are not exceeded. Using the formula shown on the ICEA protection table will allow calculating withstand ratings of conductors. It may be advantageous to...</p>