The Diseases Of Electrical Machinery

76 THE DISEASES OF to single out any special advantages of one method over the other. It is, however, an advantage that the arrangement with sandwiched coils lends itself to a quicker repair than the other arrangement. With regard to voltage-drop there is nothing to choose, both arrangements being equal with correctly dimen- sioned gap between the primary and secondary windings, and a sufficient number of separate coils. Faults. Bridge-over in the Winding.—Faults occur relatively much more rarely with transformers than with dynamos. The most usual fault is a short- circuit in the high-voltage winding, which causes it to carbonize in layers, resulting in a more or less serious bridging over of part of the high-voltage winding. It is at once seen that such a carbonisation will extend less with transformers having sandwiched coils than with the other type. The windings short-circuited by this carbonisation are then subject to a heavy short- circuit current. Beyond this, they have further no magnetic action upon the low-voltage winding. The result will usually be that the secondary voltage will increase, since the ratio of transformation (i.e. the number of primary turns divided by the number of secondary turns) is altered. In addition, the heating of part of the transformer and the high no-load current will point out this fault. The conditions will be much the same if the fault appears in the other winding, as will usually happen if the high-voltage winding is not the primary but the