Before testing insulation with a megohmmeter, the windings of large machines should be grounded for about 15 minutes just prior to the test. Why is this procedure recommended?
• Megohmmeter (megger) testing measures insulation resistance by applying a high DC voltage • Large rotating machines can retain electrical charge after operation • Purpose of grounding (earthing) windings before high-voltage tests
• Think about what might remain on the windings of a large machine after it has been running at high voltage and then switched off • Ask yourself: what problem could occur if you test with a megger while that leftover condition is still present? • Which choice explains why grounding first makes the test safer and more accurate, not just what could be wrong with the insulation itself?
• Which option talks about a condition that builds up during operation and can be removed by grounding? • Which options describe problems with the insulation that grounding for 15 minutes would NOT really fix (like damage or moisture)? • Focus on the answer that explains both safety for personnel/equipment and reliability of the insulation resistance reading.
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