CURRENT-LIMITING REACTORS 469 



circuit currents which are liable to be set up due to faults in the 

 various parts of the system. When a short-circuit occurs, the 

 maximum short-circuit current is limited by the total effective 

 impedance at that instant in the generators, transformers, and 

 transmission lines to the fault in question. This value is, how- 

 ever, not constant, but decreases rapidly until a value limited by 

 the synchronous impedance of the generators is reached (see 

 " Synchronous Generators," page 292). A sharp distinction must, 

 therefore, be made between an instantaneous and a sustained 

 short-circuit, the former being dependent upon the instantaneous 

 effective impedance of the system and the latter on the sustained 

 effective impedance. Except for long transmission and distribution 

 lines, the resistance is, as a rule, of such small value compared to the 

 reactance, that for all practical purposes it may be neglected and 

 the calculations based on reactance only instead of impedance. 



As previously stated, a severe short-circuit may result in a 

 mechanical destruction of the apparatus or an overheating of the 

 same. The former is, of course, chiefly due to the instantaneous 

 current rush, while the sustained short-circuit current ordinarily 

 determines the thermal effect. 



The instantaneous short-circuit current is readily calculated, 

 being equal to the normal current multiplied by 100 and divided 

 by the total reactance to the fault, expressed in per cent. For 

 modern water-wheel-driven generators the inherent reactance 

 varies from 15 to 25 per cent and for transformers from 6 to 10 

 per cent. As expressed in per cent it may be obtained from the 

 formula: 



where p- reactance in per cent; 



X = single-phase reactance in ohms; 

 E = voltage between phases in kilovolts. 



The reactance in ohms per mile of one wire of a symmetrical 

 three-phase circuit is 



X = 27T/L = 2wf\ (.74 logic |+ .0805^ 10~ 3 1 , 



in which s = spacing between centers of conductors in inches; 

 r = radius of conductors in inches. 



In considering the amount of current that will feed into a short 



