Tests and Measurements
The two instruments most often used to check continuity, or to measure the resistance of a circuit or circuit component, are the ohmmeter (or multimeter) and the megger (megohmmeter). The ohmmeter is widely used to measure resistance and to 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 that between a wire and the outer surface of its insulation, and the insulation resistance of cables and insulators. The range of a megger can be extended to more than 1,000 megohms.
Multimeter (Ohmmeter) Method
Multimeter applications include resistance measurements; continuity checks; and inductor, capacitor, and transformer checks. A transformer, for example, may be tested by checking whether there is an open or short, low-insulation resistance to ground, or improper continuity between transformer windings. A capacitor may be tested to determine whether it is open or shorted. Ensure that capacitors are properly discharged before you test them; otherwise, damage to the multimeter may occur.
When measuring small values of resistances, remember to consider the resistance of your test leads. Most digital multimeters cannot be zeroed in the way old analog multimeters can. With digital multimeters, you have to short the leads, read the lead resistance displayed, and then subtract the reading from subsequent component measurements that you make.
Meggers produce the large voltages that are required to measure resistances as high as 10,000 megohms — only high resistance values can be measured. The unknown resistance is connected between the megger terminals, and the hand generator part of the meter is cranked. Some meggers are capable of producing in excess of 1,000 volts, so use caution when you operate them. Typical applications for a megger are testing unterminated transmission lines and AC power cords for insulation breakdown.
Resistance can also be measured by RCL bridges. These bridges operate on the principle of the Wheatstone bridge; that is, an unknown resistance is balanced against known resistances and, after the bridge has been balanced, the unknown resistance is calculated in terms of the known resistance. Resistance is usually measured with direct current for maximum accuracy. The RCL bridges can be used to measure resistance with alternating current, but external reactance compensation is usually required.