Extech MG302 Review

Introduction

Continuing on from the disappointment of the Flir IM75 I decided to move away from looking at specific insulation testers and looked more towards an insulation multimeter on a par with the Keysight  U1461AU1461A Hence the MG302 from Extech ironically a Flir company

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I have come across Extech instruments in the past, I have a borescope from them that has served me well over the years. So it seemed reasonable to me to see what Extech could offer in the form of an insulation tester that may meet my needs.

As it happens, Extech make a number of capable insulation testers, from an old analogue unit through to a digital 5kV tester. There were two units of particular interest to me. The MG325, their top of the range instrument with data logging capability and a 200GOhm measurement range, limited to 100GOhm at a 500V test. The MG320, another insulation tester, cheaper than the MG325, but only with a 20GOhm capability, limited to 500MOhm at 500V range - but I seem to have collected a few meters already with that capability level. The downside to both of these units was that whilst they offered the ability to save data, they had no download facility.

Enter the MG302, although limited to a 4GOhm insulation measurement, across all the voltage ranges, it offers the ability to download saved data and to connect to a computer to send live measurements to, in a similar manner as the CA 6526 insulation tester. This instrument is also a fully fledged multimeter offering voltage, current, resistance, capacitance and temperature measurements alongside its insulation testing capability. The unit is offered with a 915MHz wireless connectivity for use in the USA and a 433MHz wireless unit for use in Europe and the rest of the world. Its seemed like a good compromise choice.

Comparison to Keysight / Megger

Function wise the MG302 stands up to the  U1461AU1461A quite well it matches it for all the multimeter functions even offering current measurement up to 10A against the 440mA limit of the Keysight It also offers a bespoke 4 to 20mA range particularly useful for me for testing voltage current and power transducers used throughout switchgear In terms of insulation testing the MG302 misses out on a variable test voltage function ramp DAR and PI functions It does have a lock facility for the test mode that allows timed functions to be made manually It also only has a standard resistance mode and not an earth bond test with a 200mA test current available on both the Megger and Keysight units It doesn't have a 50V test voltage so those working in telecoms networks may not find the MG302 a good option and the 100V test voltage is replaced with 125V not too much of an issue for me

The unit compares well to the Keysight and Megger insulation testers, it is slightly shorter but does feel heavier than the other two. It has a more traditionally shaped meter body built into an orange rubberised coating.

Specifications wise, with a 4GOhm test limit across the whole voltage test range, the MG302 will not be able to match either the Megger or the Keysight, but it will do a lot better than the offering from its parent company. Voltage, current, resistance, capacitance and temperature ranges are all comparable to the Keysight.

Package Contents

The MG302 is purchased in a kit, arrives boxed in a hard plastic case. Inside there is a foam moulding in both the bottom and top halves of the case. The bottom half contains the meter, wireless module, temperature probe, test lead set and batteries. The instruction booklet, software disk and an advertisement card for Flir Instruments, were located behind the foam filler in the top half of the case. This leaves plenty of room to add some items in the top foam or rearrange the case using your own foam cutouts.

Test Leads Supplied

The test leads are not the best. They come as a set of three leads, a red and black pair with moulded probes. These probes have removable GS38 compliant protective caps. A third black lead is supplied with a crocodile clip wired onto the end. A pair of safety plugs for jacks not in use complete the kit.

I have come across this type of crocodile clip and lead before on old analogue insulation testers. I was not too fond of them then, the major issue is a safety concern as the safety cover fits very loose over the actual clip and has a tendency to slip down the lead revealing the clip, not too much of an issue for insulation testing as I will test for dead first, but if measuring voltages there is the potential to expose a live connection. Luckily enough, I had a pair of insulated crocodile clips from RC Components designed to fit onto test probe tips and these fitted fine, providing a suitable alternative.

The leads are quite thin and feel more PVC than silicone type insulation, they are also a little shorter than the standard leads supplied with the Keysight and Megger units. Tested against the MG302 on its 1000V range, the leads went over-range and should not affect any of the readings taken, as long as they are kept clean. However, repeating the 1000V test using the Megger, produced a different result, an insulation value of only 9.7GOhms reveals the low quality lead that has been supplied.

Interestingly, I found that the leads supplied with the MG302, although standard 4mm, would not go do directly into the Megger, nor would they fit directly into the Keysight unit. The plastic safety shroud seems to be a slightly different size to standard 4mm leads I have from other manufacturers.

This produced issues whilst carrying out the tests as it was found that standard 4mm test leads did not fit very well into the socket on the MG302 unit. The first picture below shows a blue test lead made with standard Hirschmann 4mm safety plug tested on the Keysight meter and reads 0.036 ohms. The same lead plugged into the MG302 shows an open circuit reading for the same lead, something to be very aware of when using this instrument if the supplied leads are not used. The Keysight supplied leads however, did make connection when used with the MG302.

Bench Performance Tests

Insulation accuracy tests were carried out across the test voltage range using the Time Insulation Resistance calibrator. The reduced resistance ranges means that only 100 tests could be conducted instead of the 200 tests on the Megger and Keysight units.

Accuracy across the test ranges was seen to be very consistent As with all the insulation testers the reading accuracy was much better than the specified tolerances Over the 100 tests the MG302 achieved+0.07 accuracy just a little higher than the accuracy of the  U1461AU1461A and MIT420 over their complete ranges and was actually better if those two meters are restricted to the same test range as the MG302

However, the voltage regulation was not anywhere near that offered by both the Megger and the Keysight. Surprisingly with a 1.5mA load placed across the output, the MG302 still managed to reach up to 520V, higher than any of the other units tested. Despite this, the data shows a relatively flat plot with the voltage only reaching up to 528V maximum.

Looking at the open circuit and short circuit test data reveals the reason behind this.

The open circuit tests show that the maximum voltage reached on any of the ranges is reasonably well controlled. The manufacturer specifies a +10% tolerance for the output voltage and the instrument falls just within this specification with the 125V range showing a +9.6% output level. As the voltage level is increased the tolerance becomes better dropping down to 4.6% at 1000V. Typical of most of the insulation testers of this style.

The short circuit test reveals the answer. The MG302 has a huge 13mA short circuit current capability. This was very surprising, generally, units of this size and capacity will have a short circuit current in the region of 1 to 2mA. It also isn't comparable to the manufacturer's specification as seen below.

To me, the specification reads as a short circuit current of less than or equal to 1mA. This doesn't match the requirements of the IEC 61557-2 standard for insulation testers that requires a short circuit current of between 1mA and 15mA. It also doesn't make much sense, as if the manufacturer specifies a 1mA current at 500kOhm load, also a requirement of the IEC standard, then it will definitely be capable of putting out at least 1mA on a short circuit.

I did repeat the measurements just to make sure I hadn't gone cross-eyed at any point and came to the conclusion that the specifications in the manual probably need to be revisited by Extech. I have queried this with Extech technical support, and in general they have been very responsive. However, they have not advised what the short circuit current should be which they have been specifically asked. Whilst they have sent a calibration process to adjust the meter, this does not affect the output current capability and includes no check for it.

With such a current capability, there was no surprise that it could pass the 1mA load tests without any issues.

With such a high short circuit current, I was interested in the rise time of the MG302 against the Keysight and Megger units. The two plots show the output voltage of the MG302 captured on an oscilloscope using a high voltage probe and a 57.5MOhm test load.

A very fast rise time of 2.15ms was recorded in comparison to the 338ms rise time of the Megger. A slight overshoot of the test voltage was observed, that peaked at 572V before it settled down to 534V test voltage. The overshoot is possibly due to the fast rise time of the output voltage, but is not excessive and should not cause any issues.

The next set of tests related to the battery performance. A load measurement of each function was made using the Rigol power supply. The biggest load on the battery was from the insulation test with a 1mA load applied, this naturally went up significantly for short circuit conditions as seen in the photo above when the load on the battery is 1.916W during a short circuit. The rest of the functions put very little load onto the battery in comparison. Even the backlight and wireless functions were reasonable.

It was noted that the MG302 does not have a battery level indication. The low battery symbol is displayed at 6.80V, but there is still plenty of life left in it after that. At 4.10V, the backlight ceases to function but the display is still readable. At 3.30V, the display becomes unreadable, but is still quite usable down to a battery voltage of 3.50V. That is quite an impressive battery usage compared to the other instruments, down to 46% instead of the 60% to 70% typically seen.

Build Quality

Given the relatively low price and high specification of the Extech MG302, I was keen to take a look inside to see if any compromises had been made. The case has an orange rubberised moulding glued around it. Probe holders are moulded into the rear of it, as standard with most multimeters these days along with a slot for a hanging clip. The tilt stand almost goes back to a right angled and gives the meter a slightly more laid back position than its competitors.

However, it does impede removal of the top screws in the battery cover, ideally a long reach screwdriver is required for these. The four screws are proper machine screws that go into threaded inserts in the rear case, they are also captivated into the battery cover so they do not get lost easily. Due to the battery cover being recessed into the rear case, once the four screws are removed, I found the battery cover a little awkward to open up as I couldn't get my fingers in easily.

Removing the battery cover allows reveals the 6 AA battery pack and the 400mA fuse. The tilt stand to be removed as well if required.

There are six self tapping style screws holding the front and rear halves of the case together. The bottom two are hidden underneath the tilt stand. Once removed the case can be prised apart to reveal the main high voltage components, input circuit and the 400mA fuse, now accompanied by the larger 10A fuse. The battery pack is hard wired onto the PCB using two fly leads.

There looks like a few varistors, PTCs and dropper resistors packed in next to one another to form the input protection circuitry for the voltage input jack and insulation test positive output. This is connected to the PCB via a short wire with a ring terminal crimped onto it and held onto the input jack with a nut and shake-proof washer. Both fuses were SIBA manufactured and rated for 30kA breaking capacity - essential for the areas where I work. Just below the two fuses is the 10A current shunt next to its input jack.

At the top end of the meter, the main HV transformer and wireless communications module were found separated by a conductive shield.The aerial is soldered on to the board at one end and is then floating and can be heard to vibrate if the meter is knocked or shook.

The HV board is held in place by six small self tapping screws and the wire connecting to the input jack. Some of these screws are located underneath some of the components and a little care is needed to remove these. The HV board is removed to reveal a further board below that has the main processor and rotary selector switch. The latter has contacts on both sides and flipping over the HV board into the back case shows the contact pad arrangement for the rotary switch and an insulating sheet between the upper half of this board and the processor board. There appears to be a small square EMF shield over a few of the components on the main board.

This shield does look a bit battered, but is soldered in place, so I did not remove it.

The rotary switch is held in place by a single nut and shake-proof washer. Removing it reveals further contact pads on the processor board. Looking around the top end of the board reveals a lot of calibration potentiometers, so it looks like the meter has an open case calibration methodology.

The boards seem to be clean, with no real signs of solder residue and I didn't see any evidence of any rework on either of the boards. The wireless transmitter board, may have been hand soldered onto the main board.

The main processor board can be removed to reveal the inside of the front case with the push button contacts and rotary switch detents. A little debris was found on the inside of the front case that seemed to be from the orange rubberised coating.

Overall the build doesn't seem to be too bad, but there is a marked difference between this style of unit and the more expensive units manufactured by Keysight and Megger and it is evident where design choices and compromises have been made on the cheaper unit, which I guess, is to be expected.

Software

The MG302 is supplied with Windows software that provides a wireless connection to the instrument to record data live or receive data previously stored on the instrument. The instrument is connected to the computer using the wireless USB adapter supplied with the instrument. The software and driver loaded without fault first time and the meter connected by activating the wireless communication button. Data was then transmitted to the computer at 2 readings per second.

With the software running, the instrument can be displayed on the left hand side of the screen. The rotary switch moves and corresponds with the meter setting when the wireless communication is established and the screen updates with the reading on the instrument. The software is only a viewer and does not have any control over the instrument. All function selections and tests must be made using the instrument itself.

Software functions are also quite basic. The scale and graph colours can be customised to suit the user to a certain extent. A zoom box function is also available to allow the plot to be centred or a specific are to be selected and zoomed in. The final button provides a cursor to place along the graph and a small pop-up box is displayed with the details of the reading at the cursor. This however, is only momentary and both the cursors and the pop-up box disappear when the cursor is moved away from the plot line.

Testing the motor simulator, seen in the two screenshots, it was observed that the instrument tends to initially overshoot the insulation value before dropping back down and then start its steady rise, simulating the polarising of an insulation system. A similar behaviour was also observed when testing the meter with the insulation calibrator resistance decades.

The alternative to a graphical display is a complete data list of the readings in a separate tab of the software. No manipulation of the data can be carried out in this table. Once the data collection is over, it can be saved to the computer using the normal windows file save methodology. The slight difference is that the data is automatically saved as both a 'recorder' file, for the MG302 software and an 'xls' file that can then be opened in Microsoft Excel or MacBook Numbers.

The great thing with being able to export the data into Excel allows for custom trend graphs to be created. So a three phase motor with independently tested phases could be all plotted on the same graph for comparison. This cannot be achieved using the MG302 software as it will only support one data collection at a time.

The onboard storage facility has quite a unique concept to it. The instrument can be set up to automatically save readings at a designated rate that can be set by the user between 1 to 255 seconds. Alternatively an individual reading can be stored to memory with every press of the store key, if the automatic save is left set to zero seconds. I found this memory saving facility very flexible and adaptable to my specific needs for an insulation tester.

Whilst the memory storage will work on all of the meter functions, a slight quirk was found with the insulation testing functions in that the insulation testing had to be started first before the memory saving function was activated. For all the other functions, the meter reading is automatically updated when in memory saving mode.

Sending the stored data to the computer from the meter is not obvious, and I probably wouldn't have worked it out without reading the manual. Accessing the stored data on the meter is made using the 'recall' key, which makes sense. But to then send them to the computer, the 'peak hold' key is held down until the meter starts to transmit. Unfortunately, it was at this point that the wireless function on the meter stopped working and I could not download any data.

All the usual efforts were tried to get it to work again, software installed and re-installed, latest drivers downloaded and installed and then the software installed on another computer, all to no avail. Several emails swapped with Extech technical support, who were quite responsive and helpful, did highlight some issues, but this still did not get it to start to communicate again. The conclusion was that it was likely to be a hardware fault, this was confirmed later, when I obtained a DiLog DL9307 instrument that would communicate using the Extech software - it was at this point that Extech support stopped talking to me, so the MG302 has never been made to function again.

The DL9307 is I believe, the same instrument as the MG302, just under a different brand name and I believe the instrument is actually manufactured by CEM Instruments as the DT9985.

A further issue with the software has also occurred. On one of the computers I run SimplyPats software that contains all the electrical safety checks carried out on my mains powered portable equipment. This software uses an AciveX control 'mscomm32.ocx'. Installation of the MG302 software over wrote the registry settings for this file and prevented the SimplyPats software from running.

A few questions at Google revealed that the MSCOMM32.OCX is an activeX control that can be accessed by programmers to provide Windows standard functions in their software. It would appear that both the MG302 software and the SimplyPats need access to this file in order to run.

It would appear that SimplyPats has its own version stored in the common files directory. The MG302 software appeared to come with a new version of the file, but I could not find where it had been installed.

Eventually, the error was resolved by copying the new version of the file into the Windows\System32 directory and re-registering the MSCOMCTL.OCX within a Windows command prompt box. Both programs then run happily together on the same machine.

Conclusions

I do like this meter a lot, you do get an awful lot of meter for the price of it. It may not have the finesse of the Keysight unit, but it is a very capable meter within its own right with features relevant to the kind of work that I do.

The 4GOhm limit is a little low, but it is interesting that the manufacturer has chosen to run the limit across the complete range of test voltages. This enhances the use of the unit at the 500V test range, which is the range I am predominantly interested in. If the manufacturer could increase the resistance reading up to 40GOhms across the 250, 500 and 1000V test voltages, that would be brilliant. It also lacks a variable voltage test function, that would enable me to get closer to the rated voltage of the generator rotor under test.

The onboard memory function has been very cleverly implemented. The ability to select the desired rate at which to save the readings allows me to get just the right amount of data for the PI and DAR tests.

The 4 to 20mA current range is another great addition to this meter and a function that I have a lot of use for. Due to the failure of the wireless system, I didn't test this function out too much, but would like to go back to it again with the DiLog unit.