RoadTest: Megger Insulation Tester
Author: three-phase
Creation date:
Evaluation Type: Test Equipment
Did you receive all parts the manufacturer stated would be included in the package?: True
What other parts do you consider comparable to this product?: For some of the tests I compared the MIT420/2 to the Keysight U1461A
What were the biggest problems encountered?: Biggest problem encountered by me was connecting to some electrical apparatus that needed testing that were not compatible with the test leads and crocodile clips supplied with the instrument
Detailed Review:
Introduction
I have had the pleasure of road testing the MIT420/2 insulation tester over the last couple of months courtesy of Megger and element14. I have been looking for a new insulation tester for a little while to compliment the Fluke 1555C 10kV insulation tester that I currently utilise for testing turbogenerator rotor and stator windings. This instrument is bulky and is designed for higher voltage work than some of the testing that I carry out, I therefore wanted a physically smaller instrument that offered testing capabilities at lower voltages and potentially improved computer connectivity to aid with report writing. A while ago, I opted to purchase a Keysight U1461A, and whilst this is a capable instrument, its performance as an insulation tester on some of the apparatus I work on has been disappointing and has therefore been consigned to use more as a general purpose multimeter.
I therefore applied to review the Megger MIT420/2 offering to run it through various bench and field tests to establish is usability, functionality and reliability against some of the apparatus that I work on and I have produced a number of blogs to run alongside this official review that can be read to obtain further information. The following table provides shortcuts to the various blogs.
| Number | Title | Description |
|---|---|---|
| Blog 1 | Introduction | Desktop review of the package that arrived and the individual components |
| Blog 2 | A Peek Inside | Teardown of the instrument |
| Blog 3 | Bench Tests Part A | Calibration and testing of auxiliary voltage, resistance and capacitance functions |
| Blog 4 | Bench Tests Part B | Calibration and testing of insulation testing functions |
| Blog 5 | Safety Features | Demonstration of safety cut-out function and internal fuse protection |
| Blog 6 | Motor Control Centre Tests | Demonstration of testing 37 and 45kW motors from a control centre |
| Blog 7 | Testing at the Motor Terminals | Demonstration of testing a 4kW, 90kW and 132kW motor at the terminal boxes |
| Blog 8 | Air Circuit Breaker Testing | Demonstration of testing 400A and 1600A air circuit breakers |
| Blog 9 | Generator Rotor Winding Tests | Demonstration of testing 92MVA and 145MVA generator rotor windings |
The MIT420/2 Package
The instrument is provided in a sturdy plastic blow moulded case for which spares are available, a complete case part number 1007-169 or a pair of locking clips part number 2001-649. A control probe lead, with a GS38 shield and separate probe and two high quality silicone test leads with crocodile clips and probe tips accompany the meter and there is plenty of space within the case to store everything. Back in my first blog on the package, I had some minor reservations after reviewing the items.
The first was that it was easy to loose the smaller probe tips and crocodile clips from out of the case. To overcome this I have fitted a small piece of tool foam into the lid section of the case and made cut-outs in this for the crocodile clips and probe tips. This way, when I open the case over grated floors, there is much less chance of them escaping and getting lost. It is also now much easier for me to account for these items when working in clean conditions areas. In line with the excellent product support that Megger offer, replacement unfused lead sets part number 1002-001 and 500mA fused lead sets part number 1002-015 as direct replacements. Replacement probes / crocodile clips part number 1002-490 are available for the multi-function testers that look to be compatible with these leads, but i could not find a direct set of red and black replacement probes / clips.
I also found that whilst the supplied calibration certificate is a vast improvement over those supplied by many other manufacturers, it was missing a couple of pieces of traceability information that would likely be picked up during work place audits carried out in the industry I work in.
Whilst the leads looked to be of good quality, it was evident that the built in probe had a shroud over the standard 4mm banana plug connection, that prevented the use of a lot of other manufacturer's probe accessories that proved itself to be a bit of a nuisance as the instrument was used for testing some apparatus. The shroud however, did provide for positive grip of the accessory making it unlikely to come adrift during testing.
All of the elements supplied in the package appear to be of high quality, typical of instruments manufactured by Megger that I have used in the past. The meter itself was robust and comfortable to hold in the hand. Separate compartments were provided for the internal HRC fuse and the batteries, allowing them to be replaced without affecting the calibration status of the instrument. In Blog 2, I opened up the MIT420/2 to find a very neatly designed instrument, utilising good quality components and thoughtful design aspects. Internal safety features were clearly visible. Robust threaded inserts were utilised for compartment lids with rubberised seals to protect against the ingress of dirt and moisture. It is clear that the instrument is aimed at the industrial user market with safety and reliability seeming to be at the forefront of Megger's thoughts during the design process.
Bench Tests
I carried out numerous bench tests on the instrument to test its functionality and accuracy. Details of these tests are covered in blogs three and four for Part A tests on the auxiliary functions and Part B tests on the insulation testing function. The results of the tests were quite impressive. The accuracy of the instrument was significantly better than the specifications provided by the manufacturer for all of the functions. In particular, AC and DC voltage measurements were determined to be within 1% for the instrument that I tested, against a manufacturer's specification of 2%. The resistance and capacitance functions were all accurate and perfectly acceptable for electrical testing functions, although they cannot compete against the range and accuracy of professional multi-meters, but I am sure that Megger have not designed it to compete against such multi-meters. For me, the functions are there to provide basic electrical testing facilities to promote fault finding and supplement the insulation testing function.
The bench testing on the insulation function produced a similar set of results with a deviation of 1% measured against a specified deviation of 3%. Voltage compliance was very good, but was found to be closer to the the manufacturer's specifications for the lower voltage ranges. However, this is typical of all the insulation testers I have used over the years. The short circuit current was comparable across all the test voltages and sitting in the middle of the manufacturer's specified limits. An open circuit voltage response curve was produced using a high voltage probe and an oscilloscope that showed a rise time of between 300ms to 400ms against a consistent fall time of 30ms across all of the test voltages.
A number of things were also noteworthy during the bench tests.
The one downside found during the bench tests was that the Megger leads could not be plugged directly into the decade boxes and insulation tester calibrator that I used without using some adapters, or swapping out the leads completely for a more standard set of safety test leads.
The final element of the bench tests was to review the safety functions in Blog 5 that Megger have introduced into the instrument. Some of these were found during the tear down, such as varistors, discharge tubes and the internal 500mA fuse. It was this latter safety item that was of particular interest to me.
Operating the instrument without the fuse in it revealed that only the voltage measurement function would operate correctly. The resistance and capacitance functions would read over range and display a blown fuse symbol. The insulation testing functions would cut-out and display the blown fuse symbol as seen on the right.
A voltage detection safety feature is also integrated into the MIT420/2 to prevent inadvertent operation of the instrument should a live voltage be detected when the instrument is selected to any of the other test functions other than the voltage function. The voltage setting of the cut-out could be adjusted in the settings menu to one of three predetermined values.
Personally, I have not come across another insulation tester with such a range of safety functionality and thought gone into the design and I am deeply impressed with Megger for this design aspect.
Working with the Megger MIT420/2
Several elements were tested with the instrument across several blogs to see how it would perform under real world test applications. In general the meter performed very well, not only just as an insulation tester, but also showed the capability to measure AC and DC voltages with sufficient accuracy for my electrical test needs. The one area where I did tend to struggle was making connections to the items requiring testing. Whilst testing the outgoing circuits of motor controllers in Blog 6, some issues were found when making winding resistance checks in small panels where the meter had to be held in one hand. This is peculiar to a specific type of swing out direct-on-line starter, eventually it can be overcome using the supplied test leads, but it took me over 3 minutes to make three phase to phase resistance measurements. This could have been made easier if Megger supplied a magnetic hanger such as the Fluke TPAK system to allow both hands to work with the probes. There appears to be a hoop on the back of the instrument for such an accessory, but I could not find one offered by Megger, the Fluke system may fit, but I do not have one to check this out. As an alternative, I have some magnetic test probes, which I could have used, but they are not compatible with the Megger leads.
I also had another panel available to me that was a star delta starter circuit of the largest low voltage motor available to me. I did find that the crocodile clips supplied with the MIT420/2 were at the maximum opening to connect to the M12 bolted connections on the contactors. If you work on starters larger than this one, then you may struggle with the leads supplied and have to resort to 3rd party leads and crocodile clips.
The MIT420/2 is supplied with a bespoke control probe that can be used to activate the instrument when carrying out insulation voltage testing. It is not a facility that I will use a lot of, but I found an old protection panel with a terminal rail full of standard SAK4 terminals. In this scenario, the control probe comes into its own, as a series of tests can quickly be performed. However, it is not a concept unique to Megger. The control probe has a GS38 compatible sleeve over its standard 4mm tip end that prevents connection into the SAK4 terminal. It can be removed, but doing so defeats the GS38 protection. However, Megger have provided a GS38 compatible probe accessory to overcome this issue and allow connection into small terminal screws, yet another nice touch from them. I have also found that the GS38 sleeves on the control probe are available as a pack of five spares, part number 1000-562.
This was also a test where the variable voltage function was used to select a test voltage between the standard 100V and 250V ranges, that was closer to the 125V operating voltage of the panel, without over-stressing the insulation of old obsolete components but applying enough voltage to ensure that there was no breaking down of the insulation at the working voltage of the circuit.
In Blog 7, testing at actual motor terminals, the results were much more successful. I tested a range of motors from 4kW up to 132kW, using both the dielectric absorption and polarisation index test functions with no issues. The 250V and 500V standard test voltages were both used for testing the anti condensation heaters and the actual motor windings. The resistance range performed exactly as expected when measuring the phase balance of the motor windings. I did get caught out with the automatic retention of the lead nulling, that gave an almost short circuit reading on the second motor after the leads had been nulled when testing the first motor. To be honest I am not sure what value the retention of the lead null has, it is certainly not something I have come across on other instruments, that reset when turned off, so the leads have to be nulled every time. I guess it is just getting used to a slightly different way of working. However, the automatic operation of the resistance function worked without any issues.
The video below is around 9 minutes long and covers the complete testing of a 132kW motor with an anti condensation heater. All the insulation tests were conducted for a standard 1 minute duration but I have cut out the majority of the test and just included the start and end of each test.
I did find that on larger motors, the Megger crocodile clips did start to reach the maximum opening when finding earths on the motor cases. This was never really a major problem, just a minor hindrance, that could usually be overcome due to the variety of areas to clip the probe, or they could be held on for the duration of the tests just to prove the earth connection.
The next apparatus to be tested were a few air circuit breakers that ranged from 400A to 4000A current carrying capacity. New style ACBs did not present any issues, as they are a much smaller, compact design and test connections could be easily made. Some connection issues were found with older ACBs with a higher amperage rating when the crocodile clips were found to be too small to clip around the primary connections. The crocodile clips supplied with a competitors instrument did fit, but were not compatible with the Megger leads, so the whole lead and clip had to be swapped. For the highest 4000A rating, only the large crocodile clips from my Fluke 1555 HV insulation test set would fit, but again were incompatible with the Megger leads. The alternative of using tinned copper wire to tie onto the primary contacts for the crocodile clips to attach on to, was an easy resolution, as long as you have a real of tinned copper wire with you of course.........
The testing of the air circuit breakers allowed the 1000V insulation test function of the instrument to be utilised. On new style breakers, the value of the high quality test leads supplied by Megger was evident, allowing the test resistance value to reach the maximum 200GOhms rapidly and remain stable throughout the 1 minute test time.
Using the 1000V test range also introduces a further safety feature as when activating the initial 1000V test, an extra series of beeps are emitted, giving the user time to release the test button if they decided they did not want to apply 1000V. This feature is only active for the initial test after turning the instrument onto the 1000V range, any subsequent tests are performed without a delay.
As part of the test scenario, the ACB is closed, to reduce the number of insulation tests carried out. The resistance range of the instrument was used to verify that the contacts have closed. It would never be accurate enough to conduct a contact resistance measurement, but it was an example of how the auxiliary functions can be utilised, instead of having to have a standard multi-meter available.
Throughout the tests, the instrument performed impeccably once the connections issues had been overcome.
The final apparatus to be tested were two generator rotor windings in Blog 9. This was the ultimate test scenario for the MIT420/2, utilising all of its test functions apart from the capacitance function. As a basic insulation test, the instrument again performed without any issues. In the past, I have come across these smaller style insulation testers would cut-out when testing on large inductive windings. The variable voltage function again provides a useful facility to apply a more controlled test scenario without over-stressing old insulation on strategic plant.
It was also particularly pleasing to see that the automatic resistance range yet again performed without fault, when measuring the connection resistances through the rotor windings for the RSO test aspect. I had concerns that the higher current operation than a normal ohmmeter may be affected by the large inductive circuit, but this was not the case. In fact, the 200mA test current offers quite a benefit over a standard ohmmeter test.
The DC winding resistance and AC impedance tests required voltage measurements to be made and the voltage function on the MIT420/2 was used to carry out these measurements instead of the more usual multi-meter. The results of these tests were reliable and comparable to previous test measurements carried out, again another pleasing result that shows the added value of the extra functionality offered by the MIT420/2.
Throughout all of the tests, the save function was utilised to record the test results. It was easy to use, displaying the record that the data was saved to. This could then be retrieved from the folder function on the rotary switch, using the function buttons to page to the desired record and then the OK button to select it. The information retrieved is the same as the results displayed at the time of the tests. During a polarisation index test, the dielectric absorption ratio value is not displayed and so this must be recorded manually or a separate test carried out. This is a function that I have on the Fluke 1555 and I missed it when testing with the MIT420/2.
You can also not download the saved data from the instrument, which I find a bit of a failing in this day and age. The top of the range instrument in the family, does offer this function, and whilst I have not utilised this aspect, I believe it just downloads the data saved at the time of the test and does not offer any further functionality. For my role, the download facility is something that I really miss, and I don't see the value of just be able to save the data to meter memory. It would be a great improvement to me if Megger offered the basic download facility on this model and gave the ability to download DAR and PI plot data into their PowerDB software to the top of the range instrument.
The only function, I did not really utilise was the capacitance measurement function. Certainly in electrical apparatus we can come across capacitors in lighting, motor and battery charger applications. For smaller lighting applications, I think the function is perfectly acceptable. For single phase motor run capacitors and some charger applications I have come across, I find that a 10uF limit on the capacitance measurement is limiting and it needs to be extended up to 100uF.
As an example of this, I visited the workshop and found three capacitors to test that appeared to be from sodium / mercury discharge lighting fittings. Two were 20uF and the third was 13uF - all three out of the range of the MIT420/2 but could be measured by the Keysight U1461A.
Capacitors from smaller fluorescent style light fittings are likely to be under the 10uF limit and so could be tested by the MIT420/2 insulation tester. For example, the power factor correction capacitor from a light fitting in my garage was found to be 0.047uF, well within the capacity range of the instrument.
Scoring
I decided to apply some poetic license to the scoring categories on offer, as for me, they don't quite match the item under review. Therefore, I will explain my scoring methodology;
Product Performed to Expectations - I knocked a point of in this category due to the limitations of the test lead connectivity to standard 4mm probe accessories.
Specifications were sufficient to design with - I knocked a point of in this category, as in my experience the capacitance range is a bit limited at 10uF.
Demo software was of good quality - I knocked off one point in this category as the instrument does not have the facility to download the data saved.
Product was easy to use - Undoubtedly a maximum score in this department, there were never any issues with the meter functionality and operation.
Support was available - The information supplied on disc was exceptional, along with the available spares. Again a maximum score.
The price to performance ratio was good - I knocked one point of in this category as a combination of all of the inconveniences for me devalues an instrument from a premium manufacturer.
Summary
This is an excellent insulation tester with some more added functions that greatly enhance the value of the meter. The probe design is a bit of an inconvenience and can easily be overcome, but for me, there is room for improvement to either offer an adapter that permits standard 4mm safety accessories to be used, or improve the current offering for their accessories. Undoubtedly the clip design provides a secure connection unlikely to come apart during a test, and I have had leads fall out of their accessories during tests in the past. This was only a real issue if you test high current apparatus, if your role is based around more common low voltage apparatus, it is likely that the supplied leads and accessories will be suitable for your needs.
A lot of thought has gone into the instrument design, it oozes quality and numerous innovative safety features. For me, I think the operation of the lead nulling, the capacitance range and the save /download facility need some more thought, but this is based around my specific needs for an insulation tester.
Would I purchase this instrument? Personally, I would be more inclined to move towards the top of the range model to obtain the download functionality, even though it appears to be a little limited. If this isn't required, then I would be inclined to edge towards the MIT410 and utilise a standard multi-meter for a wider capacitance range.
This completes my review, I hope that it, along with the blogs, provide the element14 community and the manufacturer with the information they need, but if anyone needs any clarifications, then please feel free to ask in the comments below and I will do my best to answer.
Update 5th January 2018
I have added another blog after carrying out a retest on the generator rotor winding to see if the bedplate heaters had successfully driven out the moisture from the winding and returned it to a healthy insulation resistance.
Blog 10 120MW Generator Rotor Winding Retest
Throughout the test the MIT420/2 performed reliably as before with no concerns or issues being found. Battery life of the unit appears to be good and continues to give satisfactory service.
Top Comments
Just a quick compliment for a really good review (as in you did a good job of reviewing
)
I was most impressed by the real field work.
MK
Updated Road Test today after returning to site and carrying out a further test on the generator rotor.
Kind regards
Very good test report.
You did an excellent job of showing the device in use for real projects.
Well done.
DAB