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Thread: Insulation Resistance Test Standards

  1. #1
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    Insulation Resistance Test Standards

    We are doing a 480V insulation resistance test at 1000Vdc. Does anyone know what are acceptable values for resistance? Also what is the difference between a megger test and an IR test? thank-you

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    Quote Originally Posted by tlona1 View Post
    We are doing a 480V insulation resistance test at 1000Vdc. Does anyone know what are acceptable values for resistance? Also what is the difference between a megger test and an IR test? thank-you
    The spec depends on what you are testing.

    A "megger" test and IR test are the same thing, megger is a brand name that is used instead of the correct term "IR test", like Kleenex.

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    oh, we are simply testing the cables for a three phase 480v, 30A circuit that feeds light pole

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    Quote Originally Posted by tlona1 View Post
    Does anyone know what are acceptable values for resistance?
    In general, you are looking for a really big number.

    More specifically, you can calculate the expected value using the following equation:

    IR = k * log(D/d)

    where k is an insulation resistance constant that depends on the type of insulation you have, D is the diameter of the cable over the insulation, and d is the diameter of the cable under the insulation.

    HOWEVER, the value you get from this equation will only be valid for 1000 feet of cable at 60 degrees F. If your situation is different from that (and it most likely is), then you will need to apply some scaling factors to get the right number. Even then, if the cable is dirty or the humidity where you are is high or you do something wrong, you'll get the wrong answer.

    Perhaps you should consider having a qualified professional testing agency/individual do these tests for you...?
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

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    Quote Originally Posted by tlona1 View Post
    oh, we are simply testing the cables for a three phase 480v, 30A circuit that feeds light pole
    600V cable should be >100M per ANSI/NETA standards. Values need to be tempature corrected to 20 degrees C.
    Attached Files Attached Files

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    Quote Originally Posted by drbond24 View Post
    In general, you are looking for a really big number.

    More specifically, you can calculate the expected value using the following equation:

    IR = k * log(D/d)

    where k is an insulation resistance constant that depends on the type of insulation you have, D is the diameter of the cable over the insulation, and d is the diameter of the cable under the insulation.

    HOWEVER, the value you get from this equation will only be valid for 1000 feet of cable at 60 degrees F. If your situation is different from that (and it most likely is), then you will need to apply some scaling factors to get the right number. Even then, if the cable is dirty or the humidity where you are is high or you do something wrong, you'll get the wrong answer.
    Excellent points. (I thought that equation was based on 68 Degrees F??)

    Quote Originally Posted by drbond24 View Post
    Perhaps you should consider having a qualified professional testing agency/individual do these tests for you...?
    Good advice

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    Quote Originally Posted by zog View Post
    Excellent points. (I thought that equation was based on 68 Degrees F??)
    Thank you.

    I was using the 2006 'A Stitch in Time' as my reference for the temperature baseline. They say that rotating equipment and transformers are 68 degrees F but cable is 60 degrees F. That is on page 22, in the paragraph above the table and in the fine print below the table.

    As for the equation, I don't have that textbook with me at the moment and I can't remember what it said about temperature, so I'll have to look it up later. Thanks for pointing it out either way. I don't want to be spouting stuff that I'm not sure is correct. :cool:
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

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    Quote Originally Posted by drbond24 View Post
    Thank you.

    I was using the 2006 'A Stitch in Time' as my reference for the temperature baseline. They say that rotating equipment and transformers are 68 degrees F but cable is 60 degrees F. That is on page 22, in the paragraph above the table and in the fine print below the table.

    As for the equation, I don't have that textbook with me at the moment and I can't remember what it said about temperature, so I'll have to look it up later. Thanks for pointing it out either way. I don't want to be spouting stuff that I'm not sure is correct. :cool:
    Now I thought rotating equipment and transformes were corrected to 40 degrees C. "A stich in time" seems to contridict it self (Look at page 20), I think that correction table on page 21 is just an example. ANSI?NETA specs say to correct inductive equipment to 40C and everything else to 20C. Now you have me thinking.

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    Quote Originally Posted by zog View Post
    "A stich in time" seems to contridict it self (Look at page 20), I think that correction table on page 21 is just an example. ANSI?NETA specs say to correct inductive equipment to 40C and everything else to 20C.
    Hmmmmm...interesting. I'd go with the ANSI/NETA specs. I've never actually seen them, but I would think they would be a better standard to follow than the company propoganda that Megger puts out. I use 'A Stitch in Time' as my reference quite a bit, but that doesn't mean it is a good idea. I need to get my hands on those ANSI/NETA standards. Those have come up before in my conversations at work and I don't know anything about them.
    Engineers are always honest in matters of technology and human relationships. That's why it's a good idea to keep engineers away from customers, romantic interests, and anyone else who can't handle the truth.

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    Stich in Time is a good reference, I have one in my desk. You Power Plant guys usually dont follow NETA anyways, usually EPRI standards will apply to you right?

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