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ELECTROSTATIC METER MOVEMENT | Up
Neets Module 03-Introduction to Circuit Protection, Control, and Measurement | Next
USING THE OHMMETER |
1-32
move. The movement of the plates is opposed by a spring attached to the plates. A pointer that indicates
the value of the voltage is attached to these movable plates. As the voltage increases, the plates develop
more torque. To develop sufficient torque, the plates must be large and closely spaced. A very high
voltage is necessary to provide movement, therefore, electrostatic voltmeters are used only for HIGH
VOLTAGE measurement.
VOLTMETER SAFETY PRECAUTIONS
Just as with ammeters, voltmeters require safety precautions to prevent injury to personnel and
damage to the voltmeter or equipment. The following is a list of the MINIMUM safety precautions for
using a voltmeter.
·
Always connect voltmeters in parallel.
·
Always start with the highest range of a voltmeter.
·
Deenergize and discharge the circuit completely before connecting or disconnecting the
voltmeter.
·
In dc voltmeters, observe the proper circuit polarity to prevent damage to the meter.
·
Never use a dc voltmeter to measure ac voltage.
·
Observe the general safety precautions of electrical and electronic devices.
Q38. What type of meter movement reacts to voltage rather than current?
Q39. What is the only use for the voltage sensitive meter movement?
Q40. List the six safety precautions for the use of voltmeters.
OHMMETERS
The two instruments most commonly used to check the continuity (a complete circuit), or to measure
the resistance of a circuit or circuit element, are the OHMMETER and the MEGGER (megohm meter).
The ohmmeter is widely used to measure resistance and check the continuity of electrical circuits and
devices. Its range usually extends to only a few megohms. The megger is widely used for measuring
insulation resistance, such as between a wire and the outer surface of the insulation, and insulation
resistance of cables and insulators. The range of a megger may extend to more than 1,000 megohms.
The ohmmeter consists of a dc ammeter, with a few added features. The added features are:
1. A dc source of potential (usually a 3-volt battery)
2. One or more resistors (one of which is variable)
3. A simple ohmmeter circuit is shown in figure 1-31.
The ohmmeter’s pointer deflection is controlled by the amount of battery current passing through the
moving coil. Before measuring the resistance of an unknown resistor or electrical circuit, the test leads of
the ohmmeter are first shorted together, as shown in figure 1-31. With the leads shorted, the meter is
calibrated for proper operation on the selected range. While the leads are shorted, meter current is
maximum and the pointer deflects a maximum amount, somewhere near the zero position on the ohms
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