396 



ELECTRICAL EQUIPMENT 



lines 1 and 4, 1 and 3, 2 and 3, or 2 and 4 or their connection, 

 causes insulation stresses \/2 times this value or 1414 volts. On 

 the low-voltage winding the corresponding stress would be 141 

 volts. 



With the two low-voltage windings connected for a three-wire 

 distribution, as in Fig. 236, the maximum stress when one of the 

 outside wires becomes grounded is 141 volts, while, if the junc- 

 tion or neutral point is grounded it is limited to 100 volts. 



Some systems are supplied with two-phase generators in which 

 the neutral points of each winding are connected together. In 

 this case simultaneous grounding or connection of any two lines 



FIG. 240. 



from the generator cause a short-circuit on one-half the generator 

 winding. 



For grounding two-phase systems several methods are em- 

 ployed. With a four-wire distribution the mid-point of each 

 transformer winding should be independently grounded unless 

 the motor windings served are interconnected so as to prevent it. 

 In that event the neutral of one transformer only should be 

 grounded. With the three-wire system the neutral point should 

 be grounded and the same applies to the systems shown in Figs. 

 239 and 240. 



Three-phase. The following are the most common methods 

 in which transformers may be connected for a three-phase system: 



Delta-Delta. 



Delta- Y, or vice versa. 



Y-Y. 



Open-delta. 



T-connection. 



