The advantage of the voltmeter method is that it requires merely a direct-current voltmeter, which can be used for many other purposes, and which all engineers or contractors should possess, together with a box of cells having a potential of preferably over 30 volts. The volt-. meter should have a scale of not over 150 volts, for the reason that if the scale on which the battery is used covers too wide a range (say 1,000 volts) the readings might be so small as to make the test inac curate. A good arrangement would be to have a voltmeter having two scales—say, one of 60 and one of 600—which would make the voltmeter available for all practical potentials that are likely to be used inside of a building. If desired, a voltmeter could be obtained with three connections having three scales, the lowest scale of which would be used for testing insulation resistances.
Before starting a test, all of the fuses should be inserted and switches turned on, so that the complete test of the entire installation can be made. When this has been done, the voltmeter and battery should be connected, so as to obtain on the lowest scale of the volt meter the electromotive force of the entire group of cells. This connection is shown in Fig. 33. Immediately after this has been done, the insulation resistance to be tested is placed in circuit, whether the insulation to be tested is a switch board, slate panel-board, or the entire wiring installation; and the connections are made as shown in Fig. 34. A reading should then again be taken of the voltmeter; and the leakage is in proportion to the difference between the first and second readings of the volt meter. The explanation given below will show how this resistance may be calculated: It is evident that the resistance in the first case was merely the resistance of the volt meter and the internal resistance of the battery. As a ruts), the internal resistance of the battery is so small in companson with the resistance of the voltmeter and the external resistance, that it may be entirely neglected, and this will be done in the following calculation. In the
second case, however, the total resistance in circuits is the resistance of the voltmeter and the battery, plus the entire insulation resistance on all the wives, etc., connected in circuit.
To put this in mathematical form, the voltage of the cells may be indicated by the letter E; and the reading of the voltmeter when the insulation resistance is connected by the circuit, by the letter E'. Let R represent the resistance of the voltmeter and represent the insulation resistance of the installation which we wish to measure.
It is a fact which the reader undoubtedly knows, that the E. M. F. as indicated by the voltmeter in Fig. 34 is inversely proportional to the resistance: that is, the greater the resistance, the lower will be the reading on the voltmeter, as this reading indicates the leakage or cur rent passing through the resistance. Putting this in the shape of a formula, we have from the theory of proportion: Or, expressed in words, the insulation resistance is equal to the resist ance of the volt meter multiplied by the difference be tween the first read ing (or the voltage in the cells) and the second reading (or the reading of the voltmeter with Fig. 84. Insulation Resistance Placed in Circuit, Ready Iur the insulation re- Testing.
sistance in series with the voltmeter), divided by this last reading of the 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 . By substituting in the formula