MEGOHMMETER

1-38 MEGOHMMETER An ordinary ohmmeter cannot be used for measuring resistance of multimillions of ohms, such as in conductor insulation. To adequately test for insulation break down, it is necessary to use a much higher potential than is furnished by the battery of an ohmmeter. This potential is placed between the conductor and the outside surface of the insulation. An instrument called a MEGOHMMETER (MEGGER) is used for these tests. The megger (fig. 1-36) is a portable instrument consisting of two primary elements: (1) a hand-driven dc generator, G, which supplies the high voltage for making the measurement, and (2) the instrument portion, which indicates the value of the resistance being measured. The instrument portion is of the opposed-coil type, as shown in figure 1-36(A). Coils a and b are mounted on the movable member c with a fixed relationship to each other, and are free to turn as a unit in a magnetic field. Coil b tends to move the pointer counterclockwise, and coil a tends to move the pointer clockwise. Figure 1-36.—A megger internal circuit. Coil a is connected in series with R₃ and the unknown resistance, R_x, to be measured. The combination of coil, R₃, and R_x forms a direct series path between the positive (+) and negative (-) brushes of the dc generator. Coil b is connected in series with R₂ and this combination is also connected across the generator. There are no restraining springs on the movable member of the instrument portion of the megger. Therefore, when the generator is not operated, the pointer floats freely and may come to rest at any position on the scale.