The Diseases Of Electrical Machinery

78 THE DISEASES OF enters the low-voltage Circuit, and may do great damage with great danger to property and to life. (VMVWVWM / Fig. 42. Such occurrencesare fortunately rare—how they may occur can be seen from Fig. 42. Here one lead, carrying high-voltage cur- rent, may have a short-circuit to earth while at the same time the low-voltage side is also earthed. If the lightning then travels in the direction of the arrow, and a short-circuit between the primary and secondary winding is caused by the action of the arc, the high-voltage current enters directly into the low- voltage circuits, and destroys lamps, motors and everything taking current, and may, possibly, end with fatal results. Transformation of Three-Phase Current by Two Single-Phase Transformers.—If one of two transformers connected together for transforming two-phase current should break down, the plant can only be driven single-phase until the break-down is repaired. On the other hånd, if one transformer of three connected in mesh should break down, the plant can still be worked three-phase. The two remaining transformers will naturally be very heavily loaded.