Using insulation testing to diagnose electrical problems

By Jeff Jowett, Senior Application Engineer, AVO International, Valley Forge, Pennsylvania

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If a critical piece of capital equipment "dies", the obvious response is to attempt to bring it back online through troubleshooting and repair. Is there power? Connect a voltmeter. Check the fuses. Is it drawing too much current? Take a reading with a clamp-on ammeter. Hook up an insulation tester and look for shorts to ground. These commonplace diagnostic procedures are the most familiar applications of electrical test instrumentation to the maintenance of capital equipment.

The sophistication of electronic test instruments, however, has broadened and refined the capabilities of the devices, adding a new dimension to electrical maintenance. Comparatively crude "go/no-go" tests are still as useful as ever. But they can now be supplemented with analytical procedures that furnish the astute maintenance technician with a reliable picture of the equipment's condition with respect to its normal life cycle.

A fortunate property of insulation is that it can be used as a barometer of the overall condition of the electrical system it supports. Insulation deteriorates steadily and gradually over time. Its electrical resistance can be measured and used as the relevant indicator. Circuitry exhibits no such predictive deterioration, while components tend to fail catastrophically and can be difficult or inconvenient to test. Insulation, in addition to its primary function, provides the ideal indicator of the equipment's condition.

New technology = new opportunity

The challenge of electrical instrumentation, then, is to facilitate maximum use of this opportunity. The enhanced performance of newer technology lets established procedures yield greater insights and make new maintenance methods available. Traditional testers commonly generated only 1,000 volts, and the measurement range was often limited to hundreds of, or a few thousand, megohms. The quantum leap to a 5-kV tester was often made on a "take it or leave it" basis, purely to accommodate the demands of high-voltage equipment.

Modern instruments deliver stable voltage, above minimum load requirement, over the full resistance range of the test specimen and feature microprocessor sensitivity in the measuring circuit. The combination of higher voltage and enhanced sensitivity enables the tester to both draw and measure the miniscule current that quality insulation in new, capital equipment passes. Accordingly, a plant can develop and implement sophisticated procedures that rely on precise measurements. The insulation tester isn't limited to values associated with faulty or aged equipment, but can be used to pinpoint the test item's position anywhere along its aging curve. The "infinity" (that is to say, over-range) indication, which is a delight to the repair technician, represents a void to the diagnostician. Improved 5-kV testing fills that void with valuable analytical data.

Standard tests gain increased capability

Familiar standardized test procedures that have been used for years benefit from the improved capabilities of enhanced 5-kV testing. Most basic of these is the time-resistance method. A valuable property of insulation, but one that must be understood, is that it "charges" during the course of a test. The polar DC field the tester applies causes realignment of the insulating material on the molecular level, as dipoles orient themselves with the electric field. This movement of charge, of course, constitutes a current. Its value as a diagnostic indicator is based on two opposing factors: the current dies away as the structure reaches its final orientation, while "leakage" promoted by deterioration passes a comparatively large, constant current. The net result is that in "good" insulation, leakage current is relatively small, and resistance rises dramatically as charging goes to completion. This changing resistance bedevils the unschooled, but is exactly what the diagnostician wants to see. Deteriorated insulation passes relatively large amounts of leakage current at a constant rate for a given applied voltage. This leakage current "floods out" the charging effect.

Polarization index

Time-resistance methods, as they are known, take advantage of this effect. Graphing the resistance reading as a function of time from initiation of the test yields a smoothly rising curve for "good" insulation, but a "flat" curve if the insulation shows signs of deterioration (see Figure 1). The ultimate simplification of this technique is represented by the popular polarization index test that requires only two readings and simple division. The 10-minute reading is divided by the one-minute reading to give a ratio. Obviously, a low ratio indicates little change, hence poor insulation, while a high ratio indicates the opposite. References to typical polarization index values are common in the literature, which makes this test very easy and readily used. Note that resistance readings alone are almost meaningless since they may range from enormous values in new equipment down to a few megohms just before removal from service. A test like the polarization index is particularly useful because it can be performed on even the largest equipment and yields a self-contained evaluation based on relative readings rather than absolute values. No polarization index can be calculated with a tester of limited range, because "infinity" is not a meaningful number that one can use in a calculation. Advanced 5-kV testers readily reach the teraohm range-1012, or thousands of thousands of "megs"-and so do not run off the graph. The largest and newest capital equipment can be readily tested to yield repeatable data for recording and subsequent trend evaluation. Figure 2 shows typical polarization index test results.

Step voltage

The step voltage method is another familiar technique that enjoys expanded applicability when a 5-kV tester is available. Quality insulation can be expected to withstand over-voltage stress. Deteriorated insulation, however, exhibits an increase in current passing to ground as higher voltages exploit more structural flaws. Hence, resistance decreases noticeably with voltage. A recognized standard procedure is to increase voltage in five one-minute increments. If the readings deviate by more than 25 percent, the test item is judged to be unfit and in need of service. Obviously, a typical 500- and 1,000-V tester only approximates this test. Advanced 5-kV models perform a rigorous test, typically in increments from 1 to 5 kV or from 0.5 to 2.5 kV, and are fully automated (see Figure 3). Like the polarization index, the step voltage test is a repeatable, self-evaluating test free of extraneous influences such as temperature effect.

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