. 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 . rs by heat, as it did not pass alongside them but away from insulator record for 1913 showed this clearly. Thus it was necessaryto remove 210 insulators from circuit No. 2, (Fig. 115) due to lightningdamage, and only three from No. 1 circuit. The other advantage gainedwas that it proved to be easier to extinguish the arcs on No. 1 circuit andthat it could be done with much less loss of the synchro
. 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 . rs by heat, as it did not pass alongside them but away from insulator record for 1913 showed this clearly. Thus it was necessaryto remove 210 insulators from circuit No. 2, (Fig. 115) due to lightningdamage, and only three from No. 1 circuit. The other advantage gainedwas that it proved to be easier to extinguish the arcs on No. 1 circuit andthat it could be done with much less loss of the synchronous load connectedto the system, on account of the preponderance of two-wire short circuitscompared with three-wire short circuits. The following year one moreunit was added to No. 2 circuit suspension towers and two more to the straintowers, and this is the way it is at present. The number of insulators re-moved from the line due to lightning damage is small, not more than aboutone hundred per year, on the average, and is not on the increase. Puncture and Flash-over Ratio for Insulators.—There has been somedifference of opinion among operating engineers and insulator designers as. —J Flashovet A1 irrm-mTm Cable300,000 Strands Fig. 115.—Different Paths Takenby Lightning Flashovers ■ 208 HYDROELECTRIC PRACTICE to the proper relationship between puncture voltage and flash-over is usually considered that a proper ratio of puncture voltage to flash-over is about 2 to 1. Some engineers hold, however, that an insulator ofvery large striking distance may have a comparatively low flash-over volt-age and may easily have a dielectric strength as based on oil tests of 2 to 1,yet as far as operating conditions go, such an insulator may not be nearly asgood as one of some other ratio, because of the fact that the latter may havea shorter striking distance. In the National Electrical Safety Code issued as Circular No. 54 by theBureau of Standards, November 15, 1916
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