Insulation tester - some guidance please - Model Engineer

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.

Link to Hengfeng

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.

Contact us to discuss your requirements of 3 phase leakage current tester. Our experienced sales team can help you identify the options that best suit your needs.

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.

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mvno_subscriber

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Insulation tester / megger to test capacitor leakage

« on: March 16, , 08:46:52 am » Hello,

I'm looking around for an insulation tester / megohmmeter to, amongst other things, test capacitors and transformers. Especially, capacitor leakage current.

My question is, is there anything in particular I should be aware of? I try to find meters with high resistance readings and a good selection of voltages, but is there anything else that's important? And do you have any good recommendations within a reasonable budget ($100-$200)?

Buying used is fine.

Gyro

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Re: Insulation tester / megger to test capacitor leakage

« Reply #1 on: March 16, , 11:46:28 am » You can often pick up used  'Electrician grade' insulation testers for less than that amount. A good example is the Kewtech KT35 which will test at 250/500/V at up to 2G . They include a voltage scale too which shows the remaining voltage (decay) after test. They also include high resolution continuity test with lead resistance nulling They are good for testing low cost imported SMPS chargers etc and general PAT testing.

Important warning: It is potentially very dangerous leakage testing capacitors of any size. Although the tester current is limited to 1-2mA, the capacitor can store significant (potentially lethal) charge.


EDIT: Check out this recent thread too... https://www.eevblog.com/forum/testgear/vc60b-megger-for-wire-insulation-testing-decent/ « Last Edit: March 16, , 12:13:37 pm by Gyro » Best Regards, Chris The following users thanked this post: mvno_subscriber

threephase

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Re: Insulation tester / megger to test capacitor leakage

« Reply #2 on: March 16, , 09:59:56 pm » I will second the comment from Gyro regarding charging capacitors with an insulation tester and the stored energy that will be present. Now, the vast majority of modern insulation testers will contain a post test discharge circuit, however the discharge rate will vary a lot across the instruments. Some instruments like those from Sonel have quite a high resistance in the discharge circuit but will not display the reading until that voltage is discharged below the safety level, which is usually 50V. They cannot also lockout completely if the voltage is not discharged within a specified time frame.

I will add an attachment of the insulation testers I have reviewed that shows the discharge circuit resistances.

A lot will depend upon exactly what you are testing and what you want to achieve. Capacitors are very voltage sensitive and if you use too high a test voltage can be easily damaged. The vast majority of insulation testers will have fixed test voltages, common ones are 50V, 100V, 250V, 500V and V. If you have a 400V capacitor, then the V range will definitely damage it, the 500V range might be ok, but you are risking it a bit. The capacitor will survive the 250V range, but you will not be testing it at its working voltage. It will depend whether that aspect matters to you or not. This could be overcome with a tester that has a variable test voltage, such as the Megger 400 series, but they are usually more expensive. The only one I know of with a variable test voltage in your price range would be the Uni-T UT505A.

This raises another point in that a lot of insulation testers will have reduced resistance ranges at the lower voltages. The Kewtech KT35 is slightly different in that it appears to have 2 GOhm across the range of test voltages, a lot of instruments will only get into the GOhm on the V range, and will drop to circa 550 MOhm at 500V, 255 MOhm at 250V and so on, so I would look carefully at the range specifications in the data sheets and not just the general manufacturer's brochure statements. You also suggested measuring leakage currents, not all insulation testers have this function and not all will display the actual voltage being applied either. You may therefore be looking at using a uA meter as well. The IEC standard, specifies that insulation testers should be able to drive into a 2uF load and remain stable, so if you are testing capacitors above this, it may take a while to charge them and get a stable reading.

I will add a further couple of attachments of the meters I have tested, that list their functionality and test ranges, that may be helpful to you.

Some of my blogs of the insulation testers are on here, where they are not they will be on element14, but if you want to know about a specific instrument I have tested and cannot find it, let me know and I will point you in the direction of the blog. The following users thanked this post: mvno_subscriber

mvno_subscriber

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Re: Insulation tester / megger to test capacitor leakage

« Reply #3 on: March 17, , 02:18:03 pm » @threephase, thank you for this invaluable information! I should elaborate a bit on how I currently test leakage current - I simply hook up the capacitor to my lab supply, and add a multimeter on the µA range in series. However, my supply won't go higher than 30V, and the whole setup can be a bit cumbersome.

I guess I could add some sort of voltage multiplier and be done with it, however I don't consider that particularily safe. I have, multiple times, forgot to properly discharge the capacitor, and 30V does cause some sparks. 250V... I don't have the intelligence to manage that in this manner.

I also hope this insulation tester can be used for other purposes as well, for example testing for damaged insulation in transformers.

The UT505A really seems like a good unit, but the resolution would be limited to about 1µA. I guess that's ok, based on the available voltages. Variable voltage is also a very, very big bonus. However, I see your tested discharge resistance is 8.3Kohm - does it just discharge a set amount of time, or actually check that the voltage is low enough? I have never used an insulation tester, so all of this is new to me. I'll find that review now.

Thank you! « Last Edit: March 17, , 02:39:27 pm by mvno_subscriber »

threephase

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Re: Insulation tester / megger to test capacitor leakage

« Reply #4 on: March 17, , 06:10:56 pm » The vast majority of insulation testers will have the discharge resistance applied across the terminals when an insulation test is not being carried out. So as soon as you switch the function switch to insulation test, the discharge resistance is present. When you press the insulation test button, the discharge resistance is removed and then the instrument applies the test voltage. When the test is finished, the test voltage is switched off and the discharge resistance applied and it will stay that way until the next test is started or the function switch moved to a different position. Most testers will alarm until the voltage goes below its safety limit, usually between 30 and 50V, although the discharge resistance will still remain connected after the alarm has gone.

On most insulation testers, the test button is a momentary style, so press it in and hold to keep the voltage nom as son as you let go it switches off. Some have a lock function to keep the test voltage applied until it is stopped manually, and some have a timed function, where you can set the test to run for a fixed duration, anything from 5 seconds to 999 seconds, some even longer.

The insulation tester can be used for testing cables, motors, transformers. Those that have a continuity function can be used to test the resistance of connections, wires and windings. The handheld ones up to 1kV output usually have a short circuit capability between 1 to 2 mA, so are not particularly powerful, but the uncontrolled release of a stored charge can obviously create more significant issues, so I guess an insulation tester in that respect would offer an improvement over your current setup. The following users thanked this post: mvno_subscriber

mvno_subscriber

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Re: Insulation tester / megger to test capacitor leakage

« Reply #5 on: March 18, , 10:04:36 am » Closed shops and nothing to do easily gets the best of you in these times.. so I got an offer for a UT-505B at the same price as an A, and went for it. The B not only has 1Gohm at 50V, but also comes with remote control probe and other niceties. Hopefully it will address the shortcomings you pointed out in your UT505A review. It's perhaps overkill for my needs right now, but experience tells me one day I'll be very thankful for having upped my "investment".

Thanks for all the help!

threephase

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Re: Insulation tester / megger to test capacitor leakage

« Reply #6 on: March 18, , 06:13:29 pm » You're welcome. If the UT505B would have been available to me, I would have opted for it over the UT505A.

Hope it serves you well.

mvno_subscriber

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Re: Insulation tester / megger to test capacitor leakage

« Reply #7 on: March 23, , 09:30:30 am » Just a quick afterthought here - I see it being said that measuring leakage current and leakage resistance are two different things. But if I know the leakage resistance at a given voltage, wouldn't I know the leakage current as well? Or how do these two concepts differ?

Gyro

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Re: Insulation tester / megger to test capacitor leakage

« Reply #8 on: March 23, , 11:34:52 am » Yes, you can apply ohms law for a given voltage. I think the difference is that leakage current is often non-linear with applied voltage - it certainly is by the time you get near breakdown, any corona discharge effects etc. Best Regards, Chris The following users thanked this post: mvno_subscriber

threephase

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Re: Insulation tester / megger to test capacitor leakage

« Reply #9 on: March 23, , 02:42:49 pm » For the purposes of your tests, given how small the circuit is and what it is made up from, I would say that the leakage current will be directly related to the insulation resistance using Ohm's law.

In terms of DC insulation testing, an insulation tester displays the total current in the form of a resistance value. This total current comprises of capacitive, absorption and leakage currents. If you apply a DC test voltage for long enough, the capacitive and absorption currents decay down to zero and you are left with just the leakage current. This is the purpose of the polarisation index (PI) test and timed insulation testing in general.

How long you need to apply the test voltage for is dependent upon what it is you are actually testing and the type of insulation and ambient conditions. It may take several minutes on motor windings and cables. The larger generators I test can take up to an hour.

I presume it will only take a few seconds for your test to stabilise as the capacitor charges up, unless you are using a series resistance and are testing particularly large capacitors.

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