Megger testing technique questions - TestGuy

Originally Posted by 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 +/- VDC. If the negative lead is not connected to ground the guard would also be at +/- 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 Originally Posted by 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
Van Buren Ave.
Norristown, Pennsylvania - USA.

If I remember my IEE Regs correctly testing domestic circuits normally energised at 230 volts AC is to be done at 500 volts DC, three phase industrial circuits subjected to 400 volts (line to line) is done using the volts facility. You might not need the latter but most instruments will offer the facility – they'd be no use to a commercial electrician if they didn't. Which kinds of begs the question of the quality of an instrument which doesn't offer the facility.

Hengfeng Product Page

Insulation testing is about whether the appliance under test will kill you or not, and the same goes for the continuity tester which will likely be part of the same instrument and which performs an equally important test. At least with a calibration certificate in your hand you know if the information acquired from the measurement can be trusted.

As Emgee notes, if we could have some idea of your intended use for such measurements we could offer better relevant advice.

Seasons Greetings Simon

Hi,

Some time ago I purchased a MASTECH MS insulation resistance tester (commonly known as a megger) from Ali Express and I am very pleased with it. Good specs and a good price. Some of the cheaper units did not appear to have good low insulation resistance readings on the high voltage setting if that makes sense. So on the 500 volt setting (normal for 230-250 volt countries) the readings that are critical are in the 0.5 to 5 Megohm range as anything above this is largely academic. Some of the cheaper units did not appear to be able to display meaningful readings below 2 or 5 Megohm and this made them useless for me. Recommended unit for my general usage.

Hope this helps. Cheers and Christmas greetings, StephenS.

Thanks all for your useful and thought provoking comments.

I suppose when I think about it, it wouldn't be the most frequently used tool in my armoury. But I occasionally do small mods to my domestic arrangements and it would nice to be able to say I have made 'that final check and all is ok'.

Also what seems to be popular at the moment here is two washing dryers tripping the ELCBs due to leaky heater elements. When I check these for insulation resistance with a normal Ohmmeter you seem to be able to get any figure you want up to 20Meg depending on the time of day. I was hoping that a higher test voltage would make this kind of check more reliable. (I do hate to spend money on parts when I don't have conclusive proof that they are defective…. of course its always ok to spend on tooling )

Thanks StephenS for your positive comments on the Mastech. That looks like a reasonable device.

Posted by SillyOldDuffer on 19/12/ 18:24:26:

Is it necessary to disconnect everything before using a Megger? For example, I'd have thought putting Vdc into a anything semi-conductor like a switch mode PSU risked killing it.

Rectified single phase is around 320VDC, full wave rectified 3-phase about 720VDC. So not that far from 500V or V respectively. There are nasty spikes on the mains as well. One of the EMC/EMI mains tests applies a series of spikes on top of the mains, of both polarities, at 500V, V and V to see if anything fails. The whole test sequence can take around 2 hours. Which is long time to hold ones breath and hope the equipment under test doesn't go phut!

Insulation testers will be fairly high impedance so the currents flowing will be low. Any proper mains input equipment should have transient protection on the input against the spikes. These will cope with the low DC currents.

Andrew

If you want to know if a piece of equipment is safe to energise the insulation tester is the kit to reach for, as a low voltage resistance test using a Fluke DVM as an ohmmeter doesn't cut the mustard. Other makes are available. Quite apart from the regulations, you need that high voltage for the test to be a valid representation of the real life energisation using mains supply. So while the insulation tester isn't the most frequently used bit of kit an anyone's toolbox, when you need it, you need it.

If you want to learn more, please visit our website current leakage tester.

The same goes for the continuity tester. A simple ohms check won't pass 20 mA (the specified minimum test current through a short circuit) through the earth continuity conductor, and won't find loose screws, oxidised connections and the like.

So it's a specialised gadget for a very specific purpose, being the final checks to confirm that an appliance is safe to energise. By all means do the best you can with a digital ohmmeter but bear in mind these are not representative tests of the satisfactory state of the appliance, and if you are taking responsibility for someone else's welfare (don't we all?) the tests had better be done in the approved fashion.

Don't agree about digital vs analogue, digital is much more robust, holds its calibration better, is at least one or maybe two orders of magnitude more sensitive.

Testing stuff with semiconductors exposed to the test voltage is a whole different ball game, well beyond the scope of the casual user. The possibilities for ending up with a piece of equipment which you know is safe to energise but now needs repair are endless.

As Phil has it above, if you are testing motors and heaters, an insulation tester is going to be essential, not least because it's the absolute first question anyone using the equipment should (will) ask. Has it got a valid insulation test? Second question is what is the earth continuity conductor resistance. Without confidence in the answers (for which read sight of a valid test certificate for the measuring instrument) cut the plug off.

HTH Simon

FYI measuring something with an ohmmeter and getting a varying reading is absolutely typical of having water in the circuit. Did I mention you need an insulation tester?

Seriously this is very much an example of where the high test voltage of a "Megger" (other makes etc) gives you an answer you can believe. With the high test voltage of a proper insulation tester you will (probably) get a more repeatable answer, though of itself the fact that the measurement is not constant indicates that the equipment should be withdrawn from service, even though the range of readings is otherwise acceptable.

Seasons Greetings to one and all

Simon

Measuring insulation resistance with high DC (relative to normal operating voltage) is pointless if all you are trying to ascertain is whether the equipment is safe to energize. Take a typical domestic 240V installation with MCB overcurrent protection and 30mA RCD earth fault protection. To operate the MCB the phase to neutral or earth insulation would need to have an resistance of just a few ohms and to operate the RCD an resistance of a few Kohms. Both these values are easily detectable with a multimeter and a deciscon on the safety of energizing can be made. Once the system is energized the safety of the system and its insulation is down to its design, maintenance and operation.

However if you are trying to ascertain the condition of the insulation then its resistance (measured with high DC voltage) may be helpful but interpreting the results will be difficult.

At higher system voltages (above 11kV) there is evidence that high DC test voltages cause damage to plastic insulated cables.

My view is use a decent multimeter to check your insulation.

Michael Gilligan, yes, I noticed the lettering does seem odd on that megger, there was a mine version which was lower voltage (I have one) but they were black.I alsao have an older wooden one with "Royal Engineers" branded on it! Now we know there was also an aircraft, and a train model too! The needle position is random till the handle is wound, so could be anywhere on the scale. Sorry Simon Williams< I can't agree on robustnes, I have spent literally thousands on modern digital equipment, and have chucked it all out when it failed, and not replaced it, but I am still using an AVO8 and a megger from the early seventies! As to digital equipment being more accurate, don't believe it, check your specs and you will find that your test gear has a given accuracy at a fixed temperature only, because its electronics is sensitive to ambient changes, the AVOs and meggers use wirewound resistors which are unnafected by temperature, and anyway, the actual difference in claimed accuracy is so small as to be irrelevant, but the AVO will hold its accuracy over a much wider range of ambient temp. As to sensetivity, they all measure within the parameters of the selected scale. My problem with digital meters is that they are trying to measure values which are very often analogue! try watching a capacitor charge up with a digital meter, and looking for the little needle dips which indicate breakdowns at certain voltages, or going around tapping and rattling to find a loose connection or broken flex, ok, you can press hold and see a reading for a varying voltage, but on an analogue meter, you can watch it happen.