Cyclopedia of heating, plumbing and sanitation; a complete reference work . Fig. 33. Connections of Voltmeter and Battery for Testing Insulation Resistance. 328 ELECTRIC YVIRTXG 39 It is a fact which the reader undoubtedly knows, that the E. M. F. asindicated by the voltmeter in Fig. 34 is inversely proportional to theresistance: that is, the greater the resistance, the lower will be thereading on the voltmeter, as this reading indicates the leakage or cur-rent passing through the resistance. Putting this in the shape of aformula, we have from the theory of proportion: or, Transposing, and E :
Cyclopedia of heating, plumbing and sanitation; a complete reference work . Fig. 33. Connections of Voltmeter and Battery for Testing Insulation Resistance. 328 ELECTRIC YVIRTXG 39 It is a fact which the reader undoubtedly knows, that the E. M. F. asindicated by the voltmeter in Fig. 34 is inversely proportional to theresistance: that is, the greater the resistance, the lower will be thereading on the voltmeter, as this reading indicates the leakage or cur-rent passing through the resistance. Putting this in the shape of aformula, we have from the theory of proportion: or, Transposing, and E :E :: R + Rx : R ;E R + E Rx = E Rx = E R- E 7? = R (E-E\R(E - E) Rx = E. Or, expressed in words, the insulation resistance is equal to the resist-ance of the volt-meter multiplied bythe difference be-tween the first read-ing (or the voltagein the cells) andthe second reading(or the reading ofthe voltmeter withthe insulation re-sistance in series with the voltmeter), divided by this last reading ofthe voltmeter. Example. Assume a resistance of a voltmeter (R) of 20,000 ohms,and a voltage of the cells (E) of 30 volts; and suppose that the insula-tion resistance test of a wiring installation, including switchboard,feeders, branch circuits, panel-boards, etc., is to be made, the insula-tion resistance being represented by the letter Rx. By substitutingin the formula R (E - E) >+Bus ^-Bus Fig. 34. Insulation Resistance Placed in Circuit, Ready forTesting. R3 E and assuming that the reading of the voltmeter with the insulationresistance connected is 5, we have: 20,000 X (30-5) R = 100,000 ohms. If the test shows an excessive amount of leakage, or a ground or
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