442 ELECTRICAL EQUIPMENT 



terminates in a threaded stud mounted in a circular fiber disc with 

 arrangement for interconnection by short links. To prevent the 

 possibility of short-circuiting sections of the winding, all threaded 

 studs, between which short circuits could be made, have the same 

 thread and dimensions, while the studs to be connected differ in 

 size. The connecting link is also fitted with couplings which differ 

 in size from one another so that unlike studs on the circular disc 

 may be coupled; this arrangement rendering harmful connections 

 impossible. 



When the above-described multi-conductor arrangement does 

 not prove practical on account of very high voltages or too many 

 taps, a terminal connection board, generally made of oil-treated 

 maple, can be used, to which all leads are brought, separately 

 bushed and provided with suitable terminals for interconnections. 

 This board is normally submerged in the oil. 



The main leads in self and water-cooled power transformers 

 are brought out through insulating bushings in the cover. Usually 

 only two high-tension terminals are brought out for single-phase 

 units, while for three-phase units three or four bushings may be 

 provided, depending on whether the neutral is to be brought out. 

 The same also applies to the low-tension leads. 



The design of the leads for moderate voltages involves no 

 difficulties. For indoor transformers they usually consist of a 

 metal rod heavily insulated with several wrappings of black var- 

 nished cambric, fiber collars being added for the high-voltage 

 ranges to increase the creeping surface (Fig. 272). For outdoor 

 service these leads are covered with a petticoated porcelain 

 bushing. The conductor may also consist of a flexible cable 

 passing up through a tube making connection between the line 

 and the transformer winding. The lead proper (flexible cable) 

 may, therefore, be disconnected from the line at the top of the 

 bushing and slid down through it, in case it is desirable to remove 

 the cover from the tank without disturbing the cone. 



For higher voltages, above 70,000, the bushing design involves 

 greater difficulties, it being necessary to carefully equalize the 

 potential and keep the gradient at or below the amount which is 

 safe for the weakest point. The latest type of compound-filled 

 bushings with one-piece porcelain shells is undoubtedly the most 

 satisfactory design brought out to date (Fig. 280). They consist 

 of a single top and a single bottom porcelain with a central section 



