. The Street railway journal . tatic field becomesmore intense the air fails and brush discharge is seenaround the different shells near the conductors and at thecemented portions. As the potential is raised stresses form,further increasing the zone of ruptured air until the airnear the shells becomes sufficiently conducting. Then an arcforms from shell to shell and finally from the pin to thewire, the arc straightening out to nearly the shortest dis-tance between them, but around the edges of the , of course, assumes that the insulator is sufficientlystrong to withstand the flashin
. The Street railway journal . tatic field becomesmore intense the air fails and brush discharge is seenaround the different shells near the conductors and at thecemented portions. As the potential is raised stresses form,further increasing the zone of ruptured air until the airnear the shells becomes sufficiently conducting. Then an arcforms from shell to shell and finally from the pin to thewire, the arc straightening out to nearly the shortest dis-tance between them, but around the edges of the , of course, assumes that the insulator is sufficientlystrong to withstand the flashing-over voltage without punc-turing. The entire top surface of the same insulator under heavyrainstorm conditions at once becomes wet and the potentialof the line is immediately carried to the outer rim of thispiece. Some of the rain beats against the lower shells,wetting them, not only where it strikes, but on the undersurface of the upper shells, due to spattering. The amountof surface so acted upon depends upon the force and. FIG. 1.—ORDIN.\RY INSU- FIG. 2. — IDEAL MULTI- LATOR UNDER VARYING PART INSUL \TOR SERVICE CONDITIONS angularity with which the rain is driven and upon thedesign of the shells. Under a severe storm it is quitepossible for practically all of the insulator surfaces tobecome wet and conducting except possibly the under sur-face of the center or bottom shell. If the insulator is prop-erly mounted with respect to the cross arm and the centershell is correctly formed, this inner surface will remaindry. In the meantime the line potential has followed thewet surfaces of the insulator until it has reached the bot-tom edge E of the center shell, so that the full line potentialis now being carried by the center shell alone. The insulator can now fail in either of two ways: (a) Byrupture of the air film adjacent to the inner surface, be-cause of the intense electrostatic field, resulting in an arcfrom the shell to the pin, or (b) by puncture of the shellitself.
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Keywords: ., bookcentury1800, bookdecade1880, booksubjectstreetr, bookyear1884
