Test Instruments & Measuring Devices
The multimeter can be used as an ammeter, an ohm meter, or a voltmeter.
Meggers are used to measure insulation resistance.
EXPLAIN how the following electrical test equipment
and measuring devices are connected to a circuit:
STATE the electrical parameters measured by each of
the following test instruments:
The multimeter is a portable single instrument capable of measuring various electrical values
including voltage, resistance, and current. The volt-ohm-milliammeter (VOM) is the most
commonly used multimeter. The typical VOM has a meter movement with a full scale current
of 50 µA, or a sensitivity of 20 KW/V, when used as a DC voltmeter. A single meter movement
is used to measure current, AC and DC voltage, and resistance. Range switches are usually
provided for scale selection (e.g., 0-1V, 0-10V, etc).
The megger is a portable instrument used to measure insulation resistance. The megger consists
of a hand-driven DC generator and a direct reading ohm meter. A simplified circuit diagram of
the instrument is shown in Figure 17.
The moving element of the ohm meter consists of two coils, A and B, which are rigidly mounted
to a pivoted central shaft and are free to rotate over a C-shaped core (C on Figure 17). These
coils are connected by means of flexible leads. The moving element may point in any meter
position when the generator is not in operation.
As current provided by the hand-driven generator flows through Coil B, the coil will tend to set
itself at right angles to the field of the permanent magnet. With the test terminals open, giving
an infinite resistance, no current flows in Coil A. Thereby, Coil B will govern the motion of the
rotating element, causing it to move to the extreme counter-clockwise position, which is marked
as infinite resistance.