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“A Stitch in Time...”

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The generator can be hand-cranked or l<strong>in</strong>e-operated to develop a high<br />

DC voltage which causes a small current through and over surfaces of the<br />

<strong>in</strong>sulation be<strong>in</strong>g tested (Fig. 2). This current (usually at an applied voltage<br />

of 500 volts or more) is measured by the ohmmeter, which has an <strong>in</strong>dicat<strong>in</strong>g<br />

scale. Fig. 3 shows a typical scale, which reads <strong>in</strong>creas<strong>in</strong>g resistance values<br />

from left up to <strong>in</strong>f<strong>in</strong>ity, or a resistance too high to be measured.<br />

Figure 3–Typical scale on the Megger <strong>in</strong>sulation tester.<br />

How to Interpret Resistance Read<strong>in</strong>gs<br />

As previously mentioned, <strong>in</strong>sulation resistance read<strong>in</strong>gs should be considered<br />

relative. They can be quite different for one motor or mach<strong>in</strong>e tested three<br />

days <strong>in</strong> a row, yet not mean bad <strong>in</strong>sulation. What really matters is the trend<br />

<strong>in</strong> read<strong>in</strong>gs over a time period, show<strong>in</strong>g lessen<strong>in</strong>g resistance and warn<strong>in</strong>g<br />

of com<strong>in</strong>g problems. Periodic test<strong>in</strong>g is, therefore, your best approach to<br />

preventive ma<strong>in</strong>tenance of electrical equipment, us<strong>in</strong>g record cards as shown<br />

<strong>in</strong> Fig. 4.<br />

Figure 4–Typical record of <strong>in</strong>sulation resistance of a mill motor. Curve A shows test values as<br />

measured; Curve B shows same values corrected to 20°C (see page 22), giv<strong>in</strong>g a def<strong>in</strong>ite downward<br />

trend toward an unsafe condition. Reverse side of card (at right) is used to record the test data.<br />

<br />

A STITCH IN TIME

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