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Thread: Insualtion Resistance Tester, aka Megger

  1. #1
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    Insualtion Resistance Tester, aka Megger

    We have had lots of underground lines of customers go bad this year. Some have been easy to troubleshoot, some not. Im curious if a Megger will work to check for underground line
    s, whether good or bad. One example, we read continuity on one, yet only could get 85 volts at receiving end. Obviously we know we have a problem, but I figure with a Megger, it will push more voltage through the line, and give me a more accurate Ohm reading, telling me if its good. Ive been looking at a Fluke 1587, but Have also been looking at Megger....they have a nice facility in Dallas Tx, so im partial to them to. Any help or advice is appreciated.
    GK

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    Quote Originally Posted by gk351 View Post
    We have had lots of underground lines of customers go bad this year. Some have been easy to troubleshoot, some not. Im curious if a Megger will work to check for underground line
    s, whether good or bad. One example, we read continuity on one, yet only could get 85 volts at receiving end. Obviously we know we have a problem, but I figure with a Megger, it will push more voltage through the line, and give me a more accurate Ohm reading, telling me if its good. Ive been looking at a Fluke 1587, but Have also been looking at Megger....they have a nice facility in Dallas Tx, so im partial to them to. Any help or advice is appreciated.
    What are you checking continuity with? A fairly simple digital meter will still tell you you have fairly high resistance, as in go/no go type of evaluation.

    A megger is better for finding high resistance ground faults more so than just checking resistance of a conductor.

    Once you know you have a break in the line, your next issue is finding it, that is what fault locators are for.

  3. #3
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    Quote Originally Posted by kwired View Post
    What are you checking continuity with? A fairly simple digital meter will still tell you you have fairly high resistance, as in go/no go type of evaluation.

    A megger is better for finding high resistance ground faults more so than just checking resistance of a conductor.

    Once you know you have a break in the line, your next issue is finding it, that is what fault locators are for.
    Technically, a 'Megger' is an insulation tester. It is not designed to test the integrity of the conductors.
    Cheers and Stay Safe,

    Marky the Sparky

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    Quote Originally Posted by K8MHZ View Post
    Technically, a 'Megger' is an insulation tester. It is not designed to test the integrity of the conductors.
    Thank you, better words to use than the ones I found.

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    I agree with everyone's statements. The most recent I can think of was using my Fluke T5-600, a tester but not a multimeter. I was just thinking if you used a Megger, say at 250VDc, you would get a very accurate ohm reading.
    GK

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    Quote Originally Posted by gk351 View Post
    I agree with everyone's statements. The most recent I can think of was using my Fluke T5-600, a tester but not a multimeter. I was just thinking if you used a Megger, say at 250VDc, you would get a very accurate ohm reading.
    As other said, a megger at any voltage Will not give you an accurate low ohm reading, some do have a low ohm scale as my megger brand does but unless it has this feature it will not tell you how much ohms the resistance is when below a few hundreds of thousands of ohms , they are design and calibrated to read in the millions of ohms range, to test a conductor for continuity you need a ohm meter that will read in the very low ohm range which many multi-meters can do very accurately, if you suspect a break in the conductor then you can use a megger to see if there is leakage to ground or to another conductor in the same run which will tell you if there is a fault to ground or the other conductor this is helpful when you have an underground circuit that randomly faults a fuse or breaker but a regular ohm meter wont show it because it is only is using a 9 volt battery.

    Like I said above you should be using your megger to check between the subject conductor and Earth, if you have leakage then you know you have a bad conductor, not just from one end but both ends, I would bet if this is a 240 volt circuit then the 85 volts your getting is an open conductor that you seeing the voltage return through Earth at the load end which the megger will prove when you check it to earth.

    The Fluke T5-600 is only a 1k ohm meter, it doesn't place a load on the circuit so it will read phantom voltages so be careful as this can through you off, its not a good meter to trouble shoot ohm readings with as it is a basic meter, it will though tell you if your wire is good if you can read from one end to the other as it should be less then 5 ohms depending upon how long a run it is, but for under ground wiring problems a megger is your best friend if you know how to use one?
    Last edited by hurk27; 08-06-13 at 10:08 PM.
    Wayne A. From: N.W.Indiana
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    Just don't be fairly safe

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    Quote Originally Posted by gk351 View Post
    I agree with everyone's statements. The most recent I can think of was using my Fluke T5-600, a tester but not a multimeter. I was just thinking if you used a Megger, say at 250VDc, you would get a very accurate ohm reading.
    Maybe so, but you need to put the test leads on each end of the conductor being tested, or use another conductor and then the test will include the resistance of that conductor also, but you will also read any test current that finds a leak in the insulation and your reading will include alternate paths taken because of this leakage.

    You really have just two tests of interest here. Insulation testing - which you will have better results with the megger, and low impedance continuity checking - if your conductor reads more than a couple ohms you have a resistance there that will likely give undesired results. Some simple continuity testers will read continuity at pretty high resistances, they are not what you want to use for this kind of test. I have seen some that you can hold the leads in each hand and they give you a continuity beep, that is too high of a resistance for verifying whether or not a conductor is in good condition.

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    Megger and fluke 1587 both have their own importance

    See megger and fluke 1587 both have their own importance according to the requirement. And there is no problem in using megger for under ground work. It has special instruments with features for underground work.

  9. #9
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    Quote Originally Posted by kwired View Post
    Maybe so, but you need to put the test leads on each end of the conductor being tested, or use another conductor and then the test will include the resistance of that conductor also, but you will also read any test current that finds a leak in the insulation and your reading will include alternate paths taken because of this leakage.

    You really have just two tests of interest here. Insulation testing - which you will have better results with the megger, and low impedance continuity checking - if your conductor reads more than a couple ohms you have a resistance there that will likely give undesired results. Some simple continuity testers will read continuity at pretty high resistances, they are not what you want to use for this kind of test. I have seen some that you can hold the leads in each hand and they give you a continuity beep, that is too high of a resistance for verifying whether or not a conductor is in good condition.
    Some Megger (TM) models combine an insulation tester with a low range ohmmeter. They generate a relatively high current compared to the ohmmeter function of a regular multimeter. But for really low resistance measurements, you want a meter than uses a four terminal connection to the wiring under test (one set of leads to drive the current and another set of leads to measure the voltage), often combined with a pulsed test current to reduce the power requirement.

    And if the resistance is actually high (as it would have to be for a drop to 85 volts) a Time Domain Reflectometry (TDR) instrument will tell you exactly how far down the wire the fault is located.

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