Flashing of D-C machines caused by short circuits

STUDIES MADE ON THE flashover characteristics of two d-c machines revealed in part the mechanism of flashover as a result of short circuits. Experimental data obtained from oscillographic records and from high-speed motion pictures substantiated the concepts of arcing currents and flashing currents as follows: 1. Arcing current is related to commutation and produces ions over the surface of the commutator. 2. Flashing current is in the opposite direction to arcing current. 3. Flashing results from the ionized film of gas that envelopes the commutator and is initiated where the brush-to-commutator voltage is high enough to sustain an arc with the ionized gases present.