Megger testing technique questions

[Urangtt]

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

[SecondGen]

When performing insulation resistance on equipment, does it matter which lead on a megger goes where? If I’m 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.

[SecondGen]

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, let’s 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

[Kerry4407]

When performing insulation resistance on equipment, does it matter which lead on a megger goes where? If I’m 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.

[Ore905]

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, let’s 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.

[SecondGen]

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.

[chils15]

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.

[suchagrrl]

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.

This is what I am told by a senior tech (NETA 2 with more field experience than I) as well, however other techs have told me I am doing it wrong. I'd love for someone to clear that up.

[suchagrrl]

In case anyone else is looking for this, I contacted Megger. This was the reply I got.

There is more flexibility in the manner of lead hookup than is commonly supposed. In the absence of any other considerations, industry standard is minus (-) to circuitry, plus (+) to ground. (Various labelling conventions apply to different models of tester; some are designated “L”, for “line”, and “E”, for “earth”.) This configuration baffles some operators, depending on familiarity with conventions employed in other types of testing. In most cases, it doesn’t matter; the same resistance reading will prevail if the leads are exchanged. However, it has been observed that certain types of exotic insulating materials (e.g., some ceramics) yield different readings depending on the test lead configuration. In such instances, it has been observed that the aforementioned configuration yields the lower of the two readings. This is the desired of the two readings because insulation testing is generally concerned with safety, maintenance, and troubleshooting, and therefore the worst case reading would be the one which yields the most relevant information. Adopting a standard procedure for lead hookup relieves the operator from having to establish a specific knowledge of every type of material to be encountered as to whether it exhibits this effect, and prevents the less-informative higher reading from being inadvertently accepted as the final test result.
Additionally, some authorities assert that the reverse hookup can cause small amounts of contaminants to be carried into the insulation with the leakage current, whereas the accepted configuration would have the opposite effect.
Stated more specifically, if testing wire or cable, the minus lead would go to conductor(s), the positive to ground, shield, armor, or conduit. In extreme cases like direct-buried single conductor, a ground rod can be driven into the soil in the vicinity of the test, and the positive lead connected to it. The additional resistance of the soil as the leakage current travels to the rod is irrelevant when compared to the resistance of the insulation. With motors, generators, and transformers, the negative lead is to windings, the positive to case. With electrical tools and other equipment, negative is to circuitry, positive to frame.
However, the operator has additional freedom to employ other hookup configurations. Just be careful to avoid inadvertent continuity tests when elements thought to be isolated are in fact connected. Be familiar with the basic wiring diagram of the test item. Remember, there is supposed to be an insulation barrier between the two elements to which the leads are connected. As an example, wire and cable can be tested hot-to-neutral or phase-to-phase, but don’t forget to disconnect at the other end of the circuit. Otherwise, it’s only a high-voltage continuity test, and the resultant “zero” reading will be misinterpreted as indicating faulty cable. Worse, if there is equipment left connected, you could end up sending a high voltage through its circuitry.
The operator is free to make a judicious choice whether to test the entire piece of equipment as a single test, or to sectionalize. As an example, hot and neutral conductors can be clipped together and tested to ground; similarly, with three phases. Or, each conductor can be tested separately, either to ground or between each other. The choice is largely the operator’s, but standard procedure is to do a complete test first, then proceed with sectionalized tests only if the first test resulted in an unsatisfactory reading. Remember, testing the entire piece of equipment at once yields a worst case result, because electrically, the insulation is only as good as its weakest point. If the entire piece tests “good”, its individual elements will read even higher.
Finally, many models have a third terminal. This is a guard, not a ground, as operators sometimes misinterpret from the “G” designation. Connecting it to ground will only serve to short-circuit the test and give an invalid reading. Its actual purpose is to act as a shunt circuit to remove parallel leakage paths from the measurement. If the test item has more than one leakage path in parallel, one can be shunted around the measurement circuit by connecting it to the guard, leaving a more specific measurement of the other path. Therefore, the guard acts as an extra diagnostic tool to permit more depth and scope to analytical testing and troubleshooting. A reasonably thorough knowledge of the test item is required, but when employed, the guard can yield invaluable detail.


Please let us know if you have any additional questions.

Regards,
Brian

Brian Hammerschmidt
Applications Specialist
Megger
Valley Forge Corporate Center
2621 Van Buren Ave.
Norristown, Pennsylvania 19403-1007 USA.

[jts1976]

In case anyone else is looking for this, I contacted Megger. This was the reply I got.

There is more flexibility in the manner of lead hookup than is commonly supposed. In the absence of any other considerations, industry standard is minus (-) to circuitry, plus (+) to ground. (Various labelling conventions apply to different models of tester; some are designated “L”, for “line”, and “E”, for “earth”.) This configuration baffles some operators, depending on familiarity with conventions employed in other types of testing. In most cases, it doesn’t matter; the same resistance reading will prevail if the leads are exchanged. However, it has been observed that certain types of exotic insulating materials (e.g., some ceramics) yield different readings depending on the test lead configuration. In such instances, it has been observed that the aforementioned configuration yields the lower of the two readings. This is the desired of the two readings because insulation testing is generally concerned with safety, maintenance, and troubleshooting, and therefore the worst case reading would be the one which yields the most relevant information. Adopting a standard procedure for lead hookup relieves the operator from having to establish a specific knowledge of every type of material to be encountered as to whether it exhibits this effect, and prevents the less-informative higher reading from being inadvertently accepted as the final test result.
Additionally, some authorities assert that the reverse hookup can cause small amounts of contaminants to be carried into the insulation with the leakage current, whereas the accepted configuration would have the opposite effect.
Stated more specifically, if testing wire or cable, the minus lead would go to conductor(s), the positive to ground, shield, armor, or conduit. In extreme cases like direct-buried single conductor, a ground rod can be driven into the soil in the vicinity of the test, and the positive lead connected to it. The additional resistance of the soil as the leakage current travels to the rod is irrelevant when compared to the resistance of the insulation. With motors, generators, and transformers, the negative lead is to windings, the positive to case. With electrical tools and other equipment, negative is to circuitry, positive to frame.
However, the operator has additional freedom to employ other hookup configurations. Just be careful to avoid inadvertent continuity tests when elements thought to be isolated are in fact connected. Be familiar with the basic wiring diagram of the test item. Remember, there is supposed to be an insulation barrier between the two elements to which the leads are connected. As an example, wire and cable can be tested hot-to-neutral or phase-to-phase, but don’t forget to disconnect at the other end of the circuit. Otherwise, it’s only a high-voltage continuity test, and the resultant “zero” reading will be misinterpreted as indicating faulty cable. Worse, if there is equipment left connected, you could end up sending a high voltage through its circuitry.
The operator is free to make a judicious choice whether to test the entire piece of equipment as a single test, or to sectionalize. As an example, hot and neutral conductors can be clipped together and tested to ground; similarly, with three phases. Or, each conductor can be tested separately, either to ground or between each other. The choice is largely the operator’s, but standard procedure is to do a complete test first, then proceed with sectionalized tests only if the first test resulted in an unsatisfactory reading. Remember, testing the entire piece of equipment at once yields a worst case result, because electrically, the insulation is only as good as its weakest point. If the entire piece tests “good”, its individual elements will read even higher.
Finally, many models have a third terminal. This is a guard, not a ground, as operators sometimes misinterpret from the “G” designation. Connecting it to ground will only serve to short-circuit the test and give an invalid reading. Its actual purpose is to act as a shunt circuit to remove parallel leakage paths from the measurement. If the test item has more than one leakage path in parallel, one can be shunted around the measurement circuit by connecting it to the guard, leaving a more specific measurement of the other path. Therefore, the guard acts as an extra diagnostic tool to permit more depth and scope to analytical testing and troubleshooting. A reasonably thorough knowledge of the test item is required, but when employed, the guard can yield invaluable detail.


Please let us know if you have any additional questions.

Regards,
Brian

Brian Hammerschmidt
Applications Specialist
Megger
Valley Forge Corporate Center
2621 Van Buren Ave.
Norristown, Pennsylvania 19403-1007 USA.

Crickets . . . THANK YOU for sharing this. Great info.