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Megger testing technique questions

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    Megger testing technique questions

    When performing insulation resistance on equipment, does it matter which lead on a megger goes where? If Iím testing insulation resistance from A to B, does it matter if the Red lead goes on A or B?

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    Quote Originally Posted by Urangtt View Post
    When performing insulation resistance on equipment, does it matter which lead on a megger goes where? If Iím testing insulation resistance from A to B, does it matter if the Red lead goes on A or B?
    According to my insulation tester instruction book, "modern insulating materials give little difference in the reading, if any, regardless of which way the leads are connected." Each megohmmeter terminal outputs equal voltage at opposite polarity.

    With older insulation, however, a phenomenon known as electroendosmosis causes a lower reading to be obtained with the positive terminal connected to the grounded side of the insulation under test.

    One of the technicians I work with shared a story with me about a time he encountered this phenomenon in the field while testing a low-voltage busway that was damaged by a flood.

    He told me that the bus duct was first tested with the positive lead on the conductor and the negative lead on ground, a reading of 2 megohms was measured. When the test was run a second time, with the leads reversed, his tester couldn't bring up voltage (dead short).

    In an electric field, water molecules align themselves so that their positive ends face one way and their negative ends face the other. Once aligned, the water molecules favor negative charges. For this reason, I tend to always put the negative lead on the device being measured.

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    Megger guard terminal potential

    A senior tech once told me the Megger guard terminal is the same potential as the negative terminal. I'm not sure if this holds true with modern test equipment, but I think this should be taken into consideration when guarding equipment that could be damaged by a high test voltage, such as a low voltage transformer winding.

    As an example, letís say you were testing a 13.2-0.48kV transformer. If the guard terminal was at the same potential as the negative terminal, it would not be a good idea to guard the 480V winding while doing a 5kV test with the negative lead on the primary winding. You would effectively be putting 5kV on the low voltage winding. In a special case like this, it would be advisable to use your positive lead on the primary side to protect guarded equipment from high voltages.

    A 1kV test, for example, will apply +500VDC on positive terminal and -500 VDC on negative terminal, when one of the leads becomes grounded, the other will see +/- 1000 VDC. If the negative lead is not connected to ground the guard would also be at +/- 1000 VDC.

    For this reason, when testing transformers, I will use the positive terminal for the windings under test.

    I would recommend you take a look at "A Guide to Diagnostic Testing Above 1kV" by Megger for some really good technical info on insulation resistance testing. http://www.biddlemegger.com/biddle/5...ticTesting.pdf

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    Megger Usage

    Quote Originally Posted by Urangtt View Post
    When performing insulation resistance on equipment, does it matter which lead on a megger goes where? If Iím testing insulation resistance from A to B, does it matter if the Red lead goes on A or B?
    A megger should be used the same way that a hipot or a power factor set would be used with the red positive lead being the hot and the black negative lead being the return. The return lead senses the current that is being carried by the device under test and compares it to the voltage being applied to calculate a resistance. If you are testing across two ungrounded points such as breaker or switch poles then it really doesn't matter which lead is on one side or the other. If you have a test form or instructions that say test from here to there, consider the here to be the red lead and the there to be the black lead.

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    Quote Originally Posted by SecondGen View Post
    A senior tech once told me the Megger guard terminal is the same potential as the negative terminal. I'm not sure if this holds true with modern test equipment, but I think this should be taken into consideration when guarding equipment that could be damaged by a high test voltage, such as a low voltage transformer winding.

    As an example, letís say you were testing a 13.2-0.48kV transformer. If the guard terminal was at the same potential as the negative terminal, it would not be a good idea to guard the 480V winding while doing a 5kV test with the negative lead on the primary winding. You would effectively be putting 5kV on the low voltage winding. In a special case like this, it would be advisable to use your positive lead on the primary side to protect guarded equipment from high voltages.

    A 1kV test, for example, will apply +500VDC on positive terminal and -500 VDC on negative terminal, when one of the leads becomes grounded, the other will see +/- 1000 VDC. If the negative lead is not connected to ground the guard would also be at +/- 1000 VDC.

    For this reason, when testing transformers, I will use the positive terminal for the windings under test.

    I would recommend you take a look at "A Guide to Diagnostic Testing Above 1kV" by Megger for some really good technical info on insulation resistance testing. http://www.biddlemegger.com/biddle/5...ticTesting.pdf
    I believe you are right about the guard being at the same potential as the negative. If you reference a power factor test set this would make sense. I have to call hocus pocus on your water theory though I have never heard of someone overthinking a megger test so much. Let's be honest anything meggered at 2 Meg ohms or hard down is getting investigated.

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    Quote Originally Posted by Ore905 View Post
    I have to call hocus pocus on your water theory though I have never heard of someone overthinking a megger test so much.
    Not my theory, it comes from a senior tech with over 25 years experience in the field. Take a look at the Megger guide I linked to.

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    I was told to put the black lead on whats under test since DC goes negative to positive? Not sure of the validity behind that.

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