The Diseases Of Electrical Machinery

ELECTRICAL MACHINERY. 5$ under the influence of this short-circuit to the earth. If it were now possible for the alternating current to flow through the fault, and then through anybody back to the other pole, it would usually result in a fatal shock. Consider now in what manner most accidents have arisen. It is owing to someone having touched the frame of the alternators. This is a sufficient reason for earthing the frame. If the alternating current were to come into contact with. the iron, then, if the machine were not earthed, anyone touching the frame would be the connecting link between the machine and earth, If, therefbre, there occur at one and the same time a short-circuit in the cable and one to the iron of the machine, a pressure would act upon the man, the exact value of which would depend upon the ratio of the resistance of the connection to earth and the resistance of the man. This voltage could, under certain circumstances, be fatal. If, however, the ma- chine is not insulated from earth, but is connected by a low resistance path, the man will be protected, since he has a much higher resistance. With an insulated machine it is possible for the simultaneous appearance of a fault in the cable and the winding to pass unnoticed. With an earthed frame, however, a current would flow through earth between the two faults, and, accordmg to the extent of the faults, the coils of the armature will be more or less over-loaded. If two short-circuits to iron occur in the winding