When a battery-operated megohmmeter (insulation tester) is used to perform a dielectric absorption test, the resistance is measured and recorded from each conductor to ground each minute for 10 consecutive minutes. What condition accounts for a gradual rise in resistance each successive minute?
• Behavior of good insulation vs. deteriorated insulation under a constant DC test voltage • Meaning of a steadily increasing megohm reading in a dielectric absorption test • How moisture or a ground fault typically affect resistance readings over time
• Ask yourself what should happen to the resistance reading over several minutes if the insulation is dry, clean, and solid when a steady DC voltage is applied. • Consider how moisture or contamination would likely change the resistance value: would it go up, down, or stay low and unstable? • Think about what a direct continuity to ground would look like on a megohmmeter: would the readings be high and rising, or very low and possibly near zero?
• Trend of readings: Are they steadily increasing, steadily decreasing, or staying low? What does a rising trend usually indicate about insulation polarization and dryness? • Effect of defects: Cracks, moisture, or a solid ground fault usually make resistance readings do what—higher or lower? • Test principle: In a dielectric absorption test, good insulation tends to show what kind of change in resistance as the insulation charges and polarizes over time?
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