I am assuming you are talking about a megger test on a motor. This is a widely debated subject as there are many different factors involved.
The short answer is this: the IEEE 43 standard for the IR testing of electric motors gives a minimum acceptable value of 1 megohm plus 1 megohm per kilovolt of motor operating voltage. For a 460 volt motor, the pass/fail threshold value would be 1.46 megohms and 1.23 for a 230 volt motor.
I personally have a different opinion. A megohm test is best used with a begining baseline test and subsequent daily, weekly, monthly or yearly tests, (depending on application) to determine actual breakdown of insulation over time. It is important to conduct these comparitive tests under the same or very similar conditions of the motor. I usually allow the motor to get to operating temp under normal environmental conditions, then LOTO the motor from its power source, disconnect the motor at its terminal box and meg the motor at its leads. I do this mainly because the motor contactor can give a false meg reading due to excessive carbon tracing on the contacts.
This is what I have been taught by master tradesmen and personal experience:
5 megohm or less: I condemn the motor
5 to 10 megohm: the motor is in danger and I will still run it but follow up with weekly megger testing.
10 to 20 meg: I have cause for concern and will follow up with monthly megger testing.
Above 20 meg: the motor is ok and will follow up with yearly megger testing.
Bear in mind these are quidelines and many other factors can affect motor megger tests. A perfect example is a motor that is partially or fully encased in oil such as an oil filled sump pump motor or a hermetic compressor motor. Contaminants in the oil such as moisture or high levels of metal will give a false low megger reading so oil testing or replacement may be necessary.
I hope this has answered your question.
Regards.
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