Publication: Western Electrician
Chicago, IL, United States
BY ALBERT SCHEIBLE.
While it has been customary to judge insulators for high-voltage circuits chiefly by the voltage at which they maintain a continuous arc, or at which they break down, this in itself may not always form a fair basis of comparison between different styles and makes. Conditions will sometimes arise in practice which will impose an abnormal voltage on an insulator for a brief period of time, and while this abnormal voltage may momentarily annul the insulation, the latter need not necessarily be broken down. Under such conditions preference should certainly be given to the design and make of insulator which will most readily resume its high insulation after the abnormal strain is removed, either by a cessation of the momentarily excessive voltage, or by an increase in effective insulation, due to some change in the insulator itself.
The latter fact was only recently, mentioned by Mr. C. J. Greene in his report on me tests of various designs and makes of European and American high-voltage insulators. (London Electrical Review of April 14, 1908.) Among these tests were some on an insulator of the mushroom type, which, in a clean and dry condition, sparked at 80,000 volts and broke down at 90,000 volts. After being wet all over, the same insulator sparked at 45,000 volts, but the heat of the arc soon dried the insulator, thereby causing the arc to cease. A higher voltage restored the arc, but it was again extinguished by the further drying of the insulator. This restoring and ceasing of the arc was repeated with each increase of voltage until a permanent arc was established at 90,000 volts, the insulator then being presumably quite dry. Continuing this line of experimentation, the same insulator was then tested under conditions imitating those in proximity to railways or collieries, by first coating the entire surface (excepting the interior of the inner sleeve) with a mixture of coal dust and water. When thus coated the first spark started at about 50,000 volts, but the insulator soon began to dry, and the discharge from it carried off enough of the coal dust so as partially to clean the surface and thus stop the arc. Gradual increases of voltage showed a repetition of this drying and cleaning process, until the arc became permanent at 85,000 volts (or about 5,000 volts less than when the same insulator was entirely free from the coal dust). It will be noted that this self-cleaning of the insulator, like the partial drying in the previous instances, occurred at voltages which are still rare even in this country, it being doubtful whether at lower voltages the action would be rapid enough, to be effective. However, this interesting phenomenon may partly account for the unexpectedly long life which such insulators have shown under trying conditions, and it may also form a new basis of comparison for judging insulators of various designs and materials.
Some years ago the writer had occasion to note another phase of the self-restoring of insulators, which has since found practical application, though unknown to a large share of the, users. In designing some arc-lamp insulators for use on high-voltage circuits, the requirements aside from high insulation were mechanical strength and an interlocking of parts to prevent a possible dropping of the lamp, in case the body of the insulator was damaged by a blow or by lightning. Careful study led to a form of insulator having a petticoated porcelain bell for its body and specially designed metal fittings connected thereto. With this arrangement of parts, a space between some of the metal fittings in the interior of the device had to be filled with some mixture which could be introduced in a plastic state, and which would not puncture at too low a voltage. Considerable experimenting as to suitable mixtures for this purpose led to the adoption of one having so high an insulation that the arc would form over the petticoats on the outer porcelajn bell before it would puncture the inner compound. Then a further increase of the voltage while the discharge was passing over the outer surface would puncture the inner plastic insulation, thus practically breaking down the insulator. After allowing the device to cool. Professor Woodworth of Chicago, who was conducting the tests at the Lewis Institute, was surprised to find that the puncture break had automatically healed itself, so that a new breakdown test on the same insulator would require approximately the same, and, in some instances, even a higher voltage than that at which the original puncture had occurred.
The importance of such a self-restoring feature in arc-lamp insulators-will readily be apparent, as it implies that no permanent damage would be done by the average static discharged during an electric storm. The fact that such a feature has already been introduced in arc-lamp insulators and insulating cross-arms, which have been widely used in this country, may account for some of the fine service obtained from them under severe conditions. Taken in connection with Mr. Greene's experiment, it also suggests that insulators for high-voltage work should no longer be judged merely by their intimate breakdown voltages, but at least partly by the voltages from which they will readily recover, and the readiness with which they will recuperate if subjected to strains considerably below their puncture voltages.
|Date completed:||October 5, 2009 by: Bob Stahr;|