IEEE C37.5-1979 pdf free.IEEE Guide for Calculation of Fault Currents for Application of AC High-Voltage Circuit Breakers Rated on a Total Current Basis.

This standard is a guide for the calculation offault currents for the application of ac high-voltage circuit breakers rated on a total currentbasis.This guide is intended for general use in theapplication of circuit breakers. Familiarity withother American National Standards applying tocircuit breakers is assumed，and provisions ofthese standards are indicated herein only whennecessary for clarity in describing applicationrequirements.

General. Thisguide gives a simplifiedmethod of approximating the magnitude offault currents: however，other more rigorousmethods should be used where required.

For determination of the system X/R ratio,it should be noted that there is no completely accurate way of combining parallel circuitswith different values of X/R into a single cir-cuit with one value of x/Rsince the currentfrom the several circuits will be the sum ofseveral exponentially decaying terms，usuallywith different exponents,while that from asingle circuit contains just one such term.Investigation has shown that for practical purposes the procedure of reducing the reactanceto a single value with the resistance assumed tobe zero, and reducing the resistance to a singlevalue with the reactance assumed to be zero,gives,in general，better accuracy than anyother reasonably simple procedure (includingthephasor representation used at systemfrequency). In addition, the error for practicalcases is on the conservative side.

In cases where an E/Z calculation is made, itis acceptable to substitute Z/R for x/Rpro-vided that the is obtained from a separatereduction of resistance with the reactanceassumed to be zero.

Rotating Machine Reactances.Basically,initial shortcircuit current of rotating machinesis determined by the machine subtransient re-actances.For the shortcircuit and momentarycurrent calculation,the reactances in Table 1are used.

Resistance of System and Typical X/RRatio. For the purpose of determining theequivalent X/R ratio,it is recommended thatthe manufacturer’s advice be obtained concern-ing the resistance value to be used for importantelectrical devices. In the absence of such recom-mendations,the approximate resistance is suggested in Table 2. In both cases,measuredvalues on rotating machines should be convertedto normal operating temperature.IEEE C37.5 pdf download.

# IEEE C37.5-1979 pdf free

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