The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 77 secondary winding, as is the case in transformers for stepping-up. In this case the secondary voltage will be lowered, not increased. It can easily be seen, from what has gone before, that the same effect will be caused by a double short- circuit to iron. Lightning.—Lightning is considerably feared in transformer stations, and many faults are laid to the credit of lightning which can be otherwise explained in the most simple manner. If lightning strike the primary or secondary transformer leads, owing to the high self-induction of the windings, it is not to be ex- pected that it will pass through either coil; it will, more likely, seek the quickest way to earth, and, owing to the high voltage of such a discharge, it will cer- tainly find one. It thereby destroys the insulation of the particular winding, and damages, by the heat evolved, both a part of that winding and also usually a part of the other winding as well. There are, how- ever, many possibilities for lightning to find its way to earth through a transformer without in any way affecting its action, so long as there is no second short-circuit in the transformer. In many cases an earthed sheet or wire is provided between primary and secondary to prevent disaster. Great danger can be produced with badly insu- lated transformers, by the high-voltage winding Corn- ing into contact with low-voltage winding in two places, since the high-voltage alternating current then