component short-circuit protection

8/9/2019 Component Short-Circuit Protection

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9Component Short-Circuit Protection

9.1.0 Introduction9.1.1 Short-Circuit Current Withstand Chart for Copper Cables with Paper,

Rubber, or Varnished-Cloth Insulation9.1.2 Short-Circuit Current Withstand Chart for Copper Cables with

Thermoplastic Insulation9.1.3 Short-Circuit Current Withstand Chart for Copper Cables withCross-Linked Polyethylene and Ethylene-Propylene-Rubber Insulation

9.1.4 Short-Circuit Current Withstand Chart for Aluminum Cables with Paper,Rubber, or Varnished-Cloth Insulation

9.1.5 Short-Circuit Current Withstand Chart for Aluminum Cables withThermoplastic Insulation

9.1.6 Short-Circuit Current Withstand Chart for Aluminum Cables withCross-Linked Polyethylene and Ethylene-Propylene-Rubber Insulation

9.1.7 Comparison of Equipment Grounding Conductor Short-Circuit WithstandRatings

9.1.8 NEMA (Standard Short-Circuit Ratings of Busway)9.1.9 U.L. No. 508 Motor Controller Short-Circuit Test Ratings9.1.10 Molded-Case Circuit Breaker Interrupting Capacities9.1.11 NEC Table 450.3(A), Maximum Rating or Setting of Overcurrent

Protection for Transformers over 600 V (as a Percentage of Transformer-Rated Current)

9.1.12 NEC Table 450.3(B), Maximum Rating or Setting of OvercurrentProtection for Transformers 600 V and Less (as a Percentage of Transformer-Rated Current)

9.1.13 U.L. 1008 Minimum Withstand Test Requirement (for AutomaticTransfer Switches)

9.1.14 HVAC Equipment Short-Circuit Test Currents, Table 55.1 of U.L.Standard 1995

9.2.1 Protection through Current Limitation9.2.2 Current-Limiting Effect of Fuses9.2.3 Analysis of a Current-Limiting Fuse9.2.4 Let-Thru Data Pertinent to Equipment Withstand9.2.5 How to Use the Let-Thru Charts9.2.6 Current-Limitation Curves: Bussmann Low-Peak Time-Delay Fuse

KRP-C800SP

Section

9.1


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9.1.0 IntroductionMost electrical equipment has a withstand rating that is defined in terms of anrms symmetrical short-circuit current and, in some cases, peak let-thru current.These values have been established through short-circuit testing of that equipmentaccording to an accepted industry standard. Or, as is the case with conductors, thewithstand rating is based on a mathematical calculation and is also expressed as anrms symmetrical short-circuit current.

The following provides the short-circuit withstand data for each system component.Please note that where industry standards are given (e.g., NEMA), individual manu-facturers of equipment often have withstand ratings that exceed industry standards.

9.1.1 Short-Circuit Current Withstand Chartfor Copper Cables with Paper, Rubber, orVarnished-Cloth Insulation ( see page 9.3 )

9.2 Section Nine


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Component Short-Circuit Protection 9.3

9.1.1


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9.1.2 Short-Circuit Current Withstand Chart forCopper Cables with Thermoplastic Insulation

9.4 Section Nine

9.1.2


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9.1.3 Short-Circuit Current Withstand Chart forCopper Cables with Cross-Linked Polyethyleneand Ethylene-Propylene-Rubber Insulation


9.1.3


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9.1.4 Short-Circuit Current Withstand Chartfor Aluminum Cables with Paper, Rubber, orVarnished-Cloth Insulation

9.6 Section Nine

9.1.4


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9.1.5 Short-Circuit Current Withstand Chart forAluminum Cables with Thermoplastic Insulation


9.1.5


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9.1.6 Short-Circuit Current Withstand Chart forAluminum Cables with Cross-Linked Polyethyleneand Ethylene-Propylene-Rubber Insulation

9.8 Section Nine

9.1.6


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9.1.7 Comparison of Equipment GroundingConductor Short-Circuit Withstand Ratings


TABLE 9.1.7


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9.1.9 U.L. No. 508 Motor ControllerShort-Circuit Test Ratings

9.10 Section Nine

TABLE 9.1.8

TABLE 9.1.9

9.1.8 NEMA (Standard Short-CircuitRatings of Busway)


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9.12 Section Nine

TABLE 9.1.10 (Continued )

(continued )


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9.14 Section Nine


(continued )


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9.16 Section Nine


(continued )


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(Courtesy of Siemens Corpora


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9.1.11 NEC Table 450.3(A), Maximum Rating orSetting of Overcurrent Protection for Transformersover 600 V (as a Percentage of Transformer-RatedCurrent)

9.18 Section Nine

TABLE 9.1.11

( 2001, NFPA)


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9.1.13 U.L. 1008 Minimum Withstand TestRequirement (for Automatic Transfer Switches)


TABLE 9.1.12

TABLE 9.1.13

9.1.12 NEC Table 450.3(B), Maximum Rating orSetting of Overcurrent Protection for Transformers600 V and Less (as a Percentage ofTransformer-Rated Current)

( 2001, NFPA)


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9.1.14 HVAC Equipment Short-Circuit TestCurrents, Table 55.1 of U.L. Standard 1995

9.20 Section Nine

TABLE 9.1.14

9.2.1 Protection through Current LimitationToday, most electrical distribution systems are capable of delivering very highshort-circuit currents, some in excess of 200,000 A. If the components are not capa-ble of handling these short-circuit currents, they could be easily damaged ordestroyed. The current-limiting ability of todays modern fuses and current-limitingbreakers (with current-limiting fuses) allows components with low short-circuitwithstand ratings to be specified despite high available fault currents.

The concept of current limitation is pointed out and analyzed in Figures 9.2.2 and9.2.3, respectively, where the prospective available fault current is shown in conjunc-tion with the limited current resulting when a current-limiting fuse clears. The areaunder the current curve indicates the amount of short-circuit energy being dissipatedin the circuit. Since both magnetic forces and thermal energy are directly propor-tional to the square of the current, it is important to limit the short-circuit currentto as small a value as possible. Magnetic forces vary as the square of the peak cur-rent, and thermal energy varies as the square of the rms current.

Thus the current-limiting fuse in this example would limit the let-thru energy toa fraction of the value that is available from the system. In the first major loop of the fault current, standard non-current-limiting electromechanical devices would

let through approximately 100 times as much destructive energy as the fusewould let through.


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9.2.2 Current-Limiting Effect of Fuses


9.2.2

9.2.3

9.2.3 Analysis of a Current-Limiting Fuse


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9.2.4 Let-Thru Data Pertinentto Equipment Withstand

Prior to using the fuse let-thru charts, it must be determined what let-thru dataare pertinent to equipment withstand ratings.

Equipment withstand ratings can be described as, How much fault current can theequipment handle, and for how long? Based on standards currently available, themost important data that can be obtained from the fuse let-thru charts and theirphysical effects are the following:

I Peak let-thru currentmechanical forcesI Apparent prospective rms symmetrical let-thru currentheating effect

Figure 9.2.4 is a typical example showing the short-circuit current available to an800-A circuit, an 800-A Bussmann Low-Peak current-limiting time-delay fuse, andthe let-thru data of interest.

9.22 Section Nine

9.2.4

9.2.5 How to Use the Let-Thru ChartsUsing the example given in Figure 9.2.4, one can determine the pertinent let-thrudata for the Bussmann KRP-C800SP ampere Low-Peak fuse. The let-thru chartpertaining to the 800-A Low-Peak fuse is illustrated in Figure 9.2.6.

Determine the peak let-thru current

Step 1. Enter the chart on the prospective short-circuit current scale at 86,000 A, and proceedvertically until the 800-A fuse curve is intersected.

Step 2. Follow horizontally until the instantaneous peak let-thru current scale is intersected.

Step 3. Read the peak let-thru current as 49,000 A. (If a fuse had not been used, the peak

current would have been 198,000 A.)

Determine the apparent prospective rms symmetrical let-thru current

Step 1. Enter the chart on the prospective short-circuit current scale at 86,000 A, and proceedvertically until the 800-A fuse curve is intersected.

Step 2. Follow horizontally until line AB is intersected.

Step 3. Proceed vertically down to the prospective short-circuit current.

Step 4. Read the apparent prospective rms symmetrical let-thru current as 21,000 A. (Therms symmetrical let-thru current would be 86,000 A if there were no fuse in the circuit.)

Refer to different fuse manufacturers current-limitation characteristics for appli-cations of different fuse types and sizes under various circuit conditions.


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9.2.6 Current-Limitation Curves: BussmannLow-Peak Time-Delay Fuse KRP-C800SP


9.2.6

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