. American hydroelectric practice; a compilation of useful data and information on the design, construction and operation of hydroelectric systems, from the penstocks to distribution lines . cture Voltage , . , ,for 230 Insulators Taken from the Baltimore Transmission aamiraDiy a punc-Line of Pennsylvania Water and Power Company ture test or a flash- over test dry, theUmitation being the size and design of the petticoats. The value of a flash-over test when the insulator is wet depends upon the nature of the ordinary spray test approximates a heavy but rather quiet shower ofrain. S
. American hydroelectric practice; a compilation of useful data and information on the design, construction and operation of hydroelectric systems, from the penstocks to distribution lines . cture Voltage , . , ,for 230 Insulators Taken from the Baltimore Transmission aamiraDiy a punc-Line of Pennsylvania Water and Power Company ture test or a flash- over test dry, theUmitation being the size and design of the petticoats. The value of a flash-over test when the insulator is wet depends upon the nature of the ordinary spray test approximates a heavy but rather quiet shower ofrain. Service conditions would be more nearly approximated by a veryfine spray directed obliquely upon an insulator which had previously beencoated with fine dust. Testing Insulators with Megger.—In comparing the resistance of sus-pension types of insulators, as determined by a megger, with the puncturevoltage, the Pennsylvania Water and Power Company has found that thelatter increases in a general way with the resistance. In fact, the ratio ofpuncture voltage to resistance followed very closely the curve of Fig. having a resistance of about 50 megohms had a puncture value about. 20,000 10,000 350 TRANSMISSION LINE CONSTRUCTION AND OPERATION 207 Flashover —T Flasliover 60,000 volts, while those having a resistance of 350 megohms had a puncturevoltage close to the flash-over value. It seems advisable, therefore, not toallow insulators to remain in service on a 70,000 volt line unless each unithas a resistance of at least 500 megohms. Flash-over of Suspension Insulators in String.—Interesting observationshave also been made by this company on two 70,000 volt lines bothequipped with the same design of suspension insulators, the one line withfive units to the string in suspension and six units on strain towers, the otherline with seven units to the string in suspension and eight in strain. Rec-ords for one year for both hnes showed the larger number of Hghtning strokesfor
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