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R R E E E

leakage and insulation

R: R (E E') E' .

and assuming that the reading of the voltmeter with the insulation resistance connected is 5, we have: R (30-5) a 100,000 ohms. 5 If the test shows an excessive amount of leakage, or a ground or short circuit, the location of the trouble may be determined by the process of elimination—that is, by cutting out the various feeders until the ground or leakage disappears, and, when the feeder on which the trouble exists has been located, by following the same process with the branch circuits.

Of course, the larger the installation and the longer and more numerous the circuits, the greater the leakage will be; and the lower will be the insulation resistance, as there is a greater surface exposed for leakage. The Rules of the National Electric Code give a sliding scale for the requirements as to insulation resistance, depending upon the amount of current carried by the various feeders, branch circuits, etc. The rule of the National Electric Code (No. 66) covering this

point, is as follows: "The wiring in any building must test free from grounds; i. e., the com plete installation must have an insulation between conductors and between all conductors and the ground (not including attachments, sockets, recepta cles, etc.) not less than that given in the following table: Up to 5 amperes 4,000,000 ohms „ 2,000,000 ,.

25 800,000 " If50 400,000 " 100 200,000 " " 200 " 100,000 " " 400 " 50,000 " " 800 " 25,000 " " 1,600 el12,500 " "The test must be made with all cut-outs and safety devices in place. If the lamp sockets, receptacles, electroliers, etc., are also connected, only one half of the resistances specified in the table will be required."