Engineers request that 5 meg ohm is the bottom limit. I use 1 meg as a bottom limit when megging conductors before they are energized. I have run motors as low as .5 meg, when the moisture is driven out by the heat of the motors operation the motors usually climbs up to about 2 meg ohms.
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Briefly, a megger test is testing an electric circuit at load. The circuit might prove good at normal resistance measuring but fails when applying high voltage. Read the megger result correctly, every electric circuit will fail if stressed high enough. Disconnect any servo drive or frequency controller before testing.
A good number on a megger test should be around 2 megohms.
The value of insulation resistance varies according to the type of CT and manufacturer. It is best to check with the manufacturer for acceptable values. The word 'Megger' is a registered trademark of Megger Limited. Megger Limited's product range includes insulation testers, relay testers, battery testers and many other electrical test and measurement products. Tony Wills, Applications Engineer, Megger Ltd.
Megger tests are important because the test makes sure that your electrical insulation is safe and operating as intended. Megger tests are performed using a Meg-ohmmeter.
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