522 



ELECTRICAL EQUIPMENT 



diagram (Fig. 325A), illustrates the connections for one conduc- 

 tor. It involves a standard overload relay but a special current 

 transformer. This has three windings; two primary to which the 

 two halves of the split conductor are connected, and one secondary 

 connected to the relay which controls the circuit breaker trip coil. 

 Under normal operation the current divides equally between the 

 two parallel paths and in each transformer the magnetizing effect 

 of the two primary coils are equal and opposite. The transformer, 

 therefore, offers no impedance to the current flow and the sec- 

 ondary windings and relays are unaffected. If a fault develops 

 in one of the two parallel conductors, however, it is evident that 

 the balance between the two primary transformer windings will 

 be upset, thus producing a magnetizing effect on the secondary 

 windings, exciting the relays and tripping the circuit breakers. 



Differential Relays. These are intended for the protection of 

 generators, transformers, etc., from internal short circuits and 



A.C. Generator 



r To Trip Coils 



Current 

 Transformers 



FIG. 326. Differential Relay Con- 

 nection for Generator Protection. 



FIG. 327. Differential Relay Con- 

 nection for Transformer Protection. 



operate always instantaneously. They are of the ordinary plunger 

 type and may be provided with one or two coils, one generally being 

 used for generator protection and two for protecting transformers, 

 as shown in Figs. 326 and 327. 



When one current coil is used, the secondaries of the current 



