The Diseases Of Electrical Machinery

54 THE DIS EAS ES OF demagnetises the magnet system. A temporary repair can be made without immediately inserting a new coil (that is, of course, if the point of breakdown cannot be made good again by inserting mica as insulation), by cutting out the defective coil and connecting the two neighbouring coils (of the same phase) together. The circuit in the defective coil must, of course, be opened. Since single-phase generators also usually possess a fairly large number of coils, the absence of a single coil can be made up for without unduly load- ing the armature iron and the magnet coils too much by increasing the magnetising current. With modern types of generators, the winding is so arranged that it is impossible for two coils separated from one another any distance to come into contact with one another. This cause of breakdown need therefore not be considered. Short-Cireuit between the Stator Winding and Iron.—A short-circuit to iron, in the case of an alter- nator, may be not only dangerous for the winding, but also for human beings. Modern alternators are built for very high pressures. A voltage of only 200 volts can, under certain circumstances, cause a fatal shock. Such generators have, therefore, earthed frames. The Earthing of Generators.—This earthing has the following effect. In an alternating-current system of distribution with many branches, a fault in one cable is by no means an unusual occurrence. If such occur, then everybody who is not insulated from earth is