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Thread: Acceptable Megger Readings

  1. #1
    Join Date
    Jun 2004
    Posts
    21

    Acceptable Megger Readings

    The onsite refinery electricians at our facility were given a "NCCER Skills Validation" test recently. One of the questions had everyone stumped, including myself. The question was focused on a formula used to calucate the acceptable Megger reading (per the NEC) on a cable. We normally refer to the manufacturer's recommendations for new cable. But this was not one of the muliple choice answers.
    I realize specific information is needed to calculate this properly (i.e. AWG/kcmil, resistance & length).
    Can anyone give me direction of where to find this Megger formula?
    COP QC

  2. #2
    Join Date
    Feb 2005
    Location
    Charlotte, NC
    Posts
    6,327

    Re: Acceptable Megger Readings

    Never heard of a "formula", we go by NETA specs for min IR readings.

    I have heard of a "Thumbrule" of (voltage rating x 2) + 1000 ohms but this gives values much lower than I (or NETA) would consider acceptable.

  3. #3
    Join Date
    Feb 2003
    Location
    Illinois
    Posts
    18,977

    Re: Acceptable Megger Readings

    There is nothing in the NEC that covers this as far as I know. The most common rule of thumb that I hear is 1 meg/1000 volts.
    Don
    Don, Illinois
    "It is the first responsibility of every citizen to question authority." B Franklin

  4. #4
    Join Date
    Jul 2004
    Posts
    328

    Re: Acceptable Megger Readings

    There is nothing with a set minimum for megger readings. Merely suggested minimum's. You may think I'm crazy but I have started equipment with .1 Meg to ground on 1000V megger. NEC especially does not address this.

    Megger reading is another one of those things that you need historical data and know what environmental conditions you are in. The only time I have even worried about it is if I get below 1 Meg.

    Now I work mainly on older used equipment (kind of like myself ) and I am very used to low megger readings. When you turn motors of especially in a humid or cold environment they have a tendency to draw in a lot of moisture and the megg reading drops like a rock. If you wait for it to come back up you will be a long time, and you just may have to build a fire under it.

    JMHO

    [ July 19, 2005, 04:07 PM: Message edited by: 69boss302 ]

  5. #5
    Join Date
    Oct 2003
    Location
    Pennsville, N.J.
    Posts
    222

    Re: Acceptable Megger Readings

    I have given this information out on previous threads. There have been several people reply back to me via Private Mail, and I gave them the contact for the two manuals that the AVO company, formerly the Biddle Megger Company have, and I gave them the engineers name and phone number.

    I am going to quote out of one of AVO manuals on testing: "The One-Megohm Rule"
    For many years maintenance professionals have used the one-megohm rule to establish the allowable lower limit for insulation resistance. The rule may be stated:

    Insulation resistance should be approximately one megohm for each 1000 volts of operating voltage, with a minimum value of one megohm.

    For example, a motor rated at 2400 volts should have a minimum insulation resistance of 2.4 megohms. In practice, megohm readings normally are considerably above this minimum value in new equipment or when insulation is in good condition. On a lower voltage, the minimum reading should never be lower than one megohm.

    With all of this said, if you wish to send me a private message, I will put you in contact with the engineer at the AVO company and maybe if you talk nice, Jeff Jowett may send you the two manuals on testing insulation, and also testing Earth Resistance Testing.

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