288 ELECTRICAL MACHINERY 



a fatal shock by connecting a high-voltage wire to ground 

 through his body. 



Danger of a " Broken-down " Transformer. This pos- 

 sibility is illustrated in Fig. 180; it is supposed that the 

 high-voltage insulation has broken down so that the two 

 coils are connected together through the point of breakdown 

 at E. Then the low-voltage line (either C or D) becomes 

 charged with a high voltage of nearly the same potential 

 with respect to the ground as the line A . If a partial ground 

 exists on line B (as it generally will) and a person standing 

 on the ground touches either of the low-voltage wires, 

 C and Z), he will receive a shock the same as though he 

 grounded the high-voltage line, A. And even if B has no 



Partial 

 ground 



FIG. 180. Sketch to Show the Danger of a Transformer Broker 

 Down between its High-tension and Low-tension Windings. 



partial ground, a fatal shock might still be received due 

 to the charging current (the earth serving as a condenser 

 plate) his body will have to carry. 



Construction of Coils. To obtain good insulation the 

 coils of a transformer are wound with insulated wire, 

 covered with a thick layer of tape. After they are wound 

 they are impregnated with some insulating compound, 

 and oiled cambric and special insulating papers are freely 

 used between the coils and between the coils and the core. 

 After being completed, the transformer insulation is tested 

 for break-down by using another high-voltage transformer 



