Large machines undergoing a resistance insulation testing using a megohmmeter should be discharged to remove any accumulated electrostatic/capacitive/dielectric-absorption charge stored. When should this discharge be performed?
• Capacitive charging of large motor and generator windings during a megohmmeter (megger) test • How stored charge can remain on windings before the test begins and after voltage is applied • Manufacturer and safety guidance on discharging windings to protect personnel and equipment
• Think about what happens to the windings when a high DC test voltage is applied, and what happens to that energy when the test is finished • Consider at which moments a person is most at risk of touching a charged winding: only during the test, or also before and/or after? • Ask yourself whether a machine that has been sitting idle could already be charged, and whether turning off the megger instantly removes all stored charge
• Verify whether a megger test applies DC voltage that can leave the winding acting like a capacitor • Confirm whether stored charge can remain on the winding after the test is complete if it is not purposefully discharged • Check standard testing practice: do technicians usually connect a discharge path only once, or do they ensure safe discharge at all potentially hazardous stages of the procedure?
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