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RoadTests & Reviews

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You will all be thankful to know that I have finally reached my last blog in this road test review of the Megger  MIT420/2MIT420/2 Previous blogs have covered bench reviews and tests a teardown and insulation testing from electrical panels on motors and air circuit breakers In this blog I will cover the testing of generator rotor windings from a 92MVA and a 145MVA turbogenerator.

 

Insulation Tester Models

 

This is the main function of my job and the reason why the road test was of great interest to me. I currently use a Fluke 1555C insulation tester, with a 250V to 10,000V output capability to carry out rotor and stator winding tests on turbogenerators. However, this instrument lacks a 100V output sometimes specified for insulated bearing tests and struggles with voltage compliance at the lower output levels. It also comes in a large padded case and takes up a quarter of the car boot space when travelling. Due to logistics and costs of testing generator stators,I test more rotors and taking the Fluke 1555C around with me is a bit of a burden.

 

Whilst the 1555C has some download capabilities it does not download a complete polarisation index test and I wanted to improve my reports so i opted for a Keysight  U1461AU1461A that on paper seemed to meet most of my needs and would also provide me with meter functionality for RSO and AC Impedance tests Unfortunately the  U1461AU1461A does not appear to have the capacity to test generator rotor windings so has been consigned to use as a multimeter most of the time.

 

This brings me to the road test of the Megger  MIT420/2MIT420/2 It is a smaller more managable unit than the 1555C and has a low enough output voltage for my requirements It does have some multimeter functionality that looks promising enough to meet my needs I am aware that this particular version does not have a results download capability but the next model up in the range the  MIT430/2MIT430/2 does have this capability So this was going to be a learning curve to see if further investment would be beneficial.

 

This will be the most onerous test conditions for the  MIT420/2MIT420/2 that I can offer and will test multiple aspects of its functionality

 

1) The variable insulation test range will be utilised to prevent over stressing of the rotor winding

2) The resistance range will be utilised to verify test connections during recurrent surge oscillograph tests

3) The voltmeter function will be used to measure AC volts during AC Impedance testing

4) The voltmeter function will be used to measure DC volts during winding resistance tests

 

Both the generator rotors tested are of a brushless design, with a rotating diode pack that must be disconnected to allow the insulation tests to be carried out. The rotor winding is, therefore, a DC rated component and considering that it would cost around £600,000 and take 3 months to rewind a rotor, care needs to be taken when applying test voltages to them, especially as the units age and become more contaminated with dirt.

 

Rotor Winding Insulation Test

 

For the 145MVA unit the rotor winding is designed to work at a maximum of 360V DC under short circuit conditions of the stator output. The  MIT420/2MIT420/2 has nominal test voltages of 250V and 500V. If I use 250V then I am not testing the rotor under its worst operating conditions. If I use the 500V range then potentially I am stressing an old winding beyond any voltage level it is likely to see in service. The 92MVA unit has a maximum rotor voltage of 143V. The  MIT420/2MIT420/2 offers nominal ranges of 100V and 250V around this value, with a similar scenario to the larger rotor.

 

The answer is to use the variable voltage function of the  MIT420/2MIT420/2 to set the output voltage to match the maximum in service conditions the rotor will see. This is the safest way I can test an old rotor winding, testing it to its full service voltage without over stressing it and risking damage. The variable voltage function defaults to 10V and is easily set to the required value using the settings function this also prevents inadvertent adjustment of the test voltage during use. Once set the range functions exactly like all the other insulation ranges with timed DAR and PI test setups and the ability to save any results obtained.

 

Throughout the tests on both rotor windings the  MIT420/2MIT420/2 performed without any issues, connections were easily made and the tests conducted. Unfortunately neither rotor was in too good a condition. The 92MVA rotor was tested on day 1 and had a low insulation value with no DAR. Overnight the heaters were turned on and the unit retested again in the morning. A doubling of the insulation resistance was seen but no DAR was obtained. The 145MVA generator was much worse and the insulation tester output voltage could not be maintained on the first test. No heating was available for this unit, so only a marginal improvement was seen the next day probably due to the lower ambient humidity. I hope to return to this rotor in the next two to three weeks after the heaters have been put back into service.

 

Rotor Winding IR Test Results

 

Rotor Winding DC Resistance

 

I do have a micro ohmmeter available to me to test winding resistance at 10A. The instrument however is not the best and will cut-out due to the time taken to allow the reading to stabilise due to the inductive nature to the winding. As an alternative, I have constructed a 100A variable DC supply and utilise a 4 wire methodology to measure the winding resistance at 5 different current values and then take an average as per the IEEE118 standard. The supply contains a 100A to 50mV current shunt that I connect a mV meter to, I then utilise the  MIT420/2MIT420/2 as a DC voltmeter connected directly across the rotor winding

 

DC Winding Resistance Test SetupDC Winding Resistance Test Connections

 

Both rotors tested reasonably well. For comparison on the 92MVA rotor, I repeated the test with the standard multimeter usually used in place of the  MIT420/2MIT420/2.

 

The  MIT420/2MIT420/2 produced a test result to -0.73% of the nominal DC winding resistance corrected to the specified temperature where as the usual test setup produced a result to -0.10% The specification is to be within 2 of the corrected value and so whilst the  MIT420/2MIT420/2 was not quite as accurate as the usual setup it still returned a result within tolerance

 

MIT420/2 DC Resistance Test ResultsU1282A DC Resistance Test Results

 

Recurrent Surge Oscillograph Test (RSO)

 

The RSO test is a specialist test on a generator rotor winding to detect inter-turn faults within the winding. It works on time domain reflectometry. Pulses are injected alternately from the two ends of the windings and the reflected waveforms across impedances connected to the windings are compared. If the two traces overlap, then no fault is present. A fault is seen as a deviation in one of the waveform traces.

 

RSO Test Principles

 

Details of rotor reflectometers and the test can be obtained from Rowdtest Ltd who manufacturer the units and provide the diagram above of the principle of operation.

 

For this test I utilised the  MIT420/2MIT420/2 to measure the resistance of the test connections to the winding and to earth For an ideal RSO test there should be a maximum connection resistance of 1 Ohm across the winding and to earth This would usually be measured using the ohms function of a standard multimeter but I was curious to see if the automatic function of the resistance range on the  MIT420/2MIT420/2 would operate correctly across the inductive load of a large rotor winding As seen in the photo below the instrument function perfectly well recording a 0.26 Ohm resistance reading through the winding whilst sourcing a 200mA current.

 

RSO Connection Test

 

Rotor Winding AC Impedance Test

 

The final test on the rotor winding is an AC Impedance test. This is an alternative test methodology to the RSO test for detecting inter-turn insulation faults. Where as the RSO test only applies 12V, in an AC impedance test, a voltage of up to 110V AC is applied across the winding, therefore applying more voltage across the inter-turn insulation. The test is carried out in the same manner as the DC winding resistance test, with the voltage across the winding and the current through it are measured at 10V steps and then the impedance calculated afterwards.

 

 

A good rotor winding will produce a plot of impedance against voltage with a slight upward trend. An inter-turn fault in a winding is seen as a step change down in the impedance along the plot. There is also the opportunity to carry out a long term historical impedance trend of the winding at a specific voltage, nominated as 50V during my tests.

 

AC Impedance Test Result Plot

 

AC Impedance Trend Plot

 

As can be seen from the plots above for the 92MVA generator comparable test results were achieved using the  MIT420/2MIT420/2 in place of a standard multimeter. Again the instrument has proved that it can be utilised in place of a regular voltmeter for the kind of tests that I carry out increasing its value and flexibility.

Up to this point the on site tests carried out with the  MIT420/2MIT420/2 have been based around motor tests either from a control centre or direct at the motor terminals Unsurprisingly the instrument has performed very well with the only issues really being around making connections to specific items during the earth test on a large motor and when using the supplied test lead to short out the winding after the tests These are all minor issues and relatives easy to overcome

 

This blog introduces the testing400A Air Circuit Breaker of air circuit breakers (ACBs). These are large three phase, low voltage breakers design to switch high currents. Although only rated to 690V AC, due to their robust construction and the types of insulation utilised, we often test them at 1000V, which is a test range that has not been utilised out on site yet with the instrument.

 

The added benefit of the 1000V range on the  MIT420/2MIT420/2 is the it will measure up to 200GOhm resistance value which will be more likely to pickup carbon contamination from arcing as the breaker operates

 

If this was routine planned maintenance being performed on the ACBs then a suite of visual inspections mechanical measurements electrical and timing tests would be completed but as this is mainly for demonstrating the use of the  MIT420/2MIT420/2 I will only be checking that the main contacts have closed using the continuity resistance function and checking the insulation resistance from phase to earth phase to phase and across the open contacts using the 1000V range

 

As the type of insulation and assembly methods differs to that of a motor or a cable, a dielectric absorption or polarisation index is not applicable and only a 1 minute timed test is conducted, the reading will be expected to ramp up to its final resistance value within seconds, and stay there for the duration of the test.

 

I have two types of ACBs available to me to test. One is a relatively modern design and is more compact and includes a built in protection relay. The second one is a much more older design and sits in a much larger metallic frame with greater clearances. The principle of their operation however, is the same, and they are tested with an insulation tester in exactly the same manner.

 

The first aspect of testing an ACB, is to make it safe. Electrically, the ACB is isolated by removing it from its carriage where it can then be placed on the floor or a maintenance trolley. As the ACBs use a powerful spring to close and trip the main contacts, a significant amount of stored mechanical energy remains within the unit that is discharged by closing the breaker. I can then make the breaker safe by inserting a safety pin into the ACB frame to lock the tripping mechanism for carrying out the first set of insulation tests, to earth and between phases, with the contacts closed. This is done so that I am insulation testing the complete primary circuit of each phase and do not have to test each contact individually. Once these tests are complete, I can remove the safety pin and trip the ACB open. This will remove all stored mechanical energy in the ACB and the mechanism will be safe to work on.

 

The following video shows an older style 1600A air circuit breaker being tested. As you will see I had a slight issue with the crocodile clips on the Megger leads that would not grip around the primary contact, so I had to resort to using the leads and crocodile clips from another manufacturer. Other than this slight issue, the tests went well with all results at the 200GOhms maximum reading, showing that the insulation is in excellent condition and there is little contamination.

 

Crocodile Clip Connection Issues

 

A test on a 4000A breaker shows a different story, with a much lower reading being achieved, indicating that there was contamination across the contact in this breaker. With the primary contact having an even bigger diameter than a 1600A contact, I had to resort to the large crocodile clips from my Fluke 1555C insulation tester to make the connections, but the photo below shows an alternative arrangement using tinned copper wire to wrap around the contact and twist tight. The Megger crocodile clip can then be clipped to this which gives a perfectly adequate test connection for a voltage test with a low output current.

 

4000A ACB Insulation Resistance Test

 

This concludes the blog, the instrument has performed perfectly well and continues to be reliable and easy to use.

 

The next blog will be the final installment on testing a generator rotor winding.

In the previous blog some motors that had been refitted after maintenance were tested from their respective control panels I found that the  MIT420/2MIT420/2 had a few quirks whilst using it but nothing that became a major issue or prevented the instrument from being used In the next blog I am off to the other end of the cable and test out some motors at the actual terminal box Actually I am cheating a bit as I have access to motor stores so I tested a new 4kW motor a refurbished 90kW motor and two 132kW motors removed for utilisation at another plant In general testing at the motor terminals is much easier than testing at the motor control centre so I am not expecting any issues with the use of the  MIT420/2MIT420/2

 

The first motor is a 4kW star configured motor. This is e4kW Motor Test at Terminalsasily verified by the shorting strips seen across the three terminals at the top of the block. This means that I will connect the tester to the terminals at the bottom of the block.

 

The same test procedure used during the test from the panel are used. A phase balance test across each pair of phases and then a single insulation resistance test at 500V. As the motor is new and the store is heated, I am not expecting any issues.

 

4kW Motor Test Results

 

As the table above shows, the phase were well balanced and the 1 minute insulation reading was high with a dielectric absorption ration above the 1.2 minimum. With such a high 1 minute insulation value and the fact that the motor is relatively small, cheap and random wound, I would not progress onto carrying out a polarisation index test on this motor.

 

90kW Motor Tests

 

So onto the next motor. This a much larger 90kW motor. Looking at its terminals, it can be seen that the shorting bars are now going across the block indicating that this motor is actually connected up in a delta winding configuration. This is typical for larger motors that are predicted to have long running cycles as the motor runs more efficiently, but takes more power to start. The testing principle is still the same with the phase balance measured across either the top set or bottom set of terminals and then a single insulation resistance test to earth.

 

The insulation test was carried out as both a DAR test and then a PI test to record both values.

 

As this motor is a much larger motor a much lower phase balance reading is obtained unfortunately I got caught out with the retention of the lead null and the initial readings were 0.01Ohms The lead null was reset and then the readings repeated with 0.04Ohms now being obtained to verify the accuracy a phase balance reading was also taken with the Keysight  U1461AU1461A which came out as an average of 0.053 ohms with the lead resistance subtracted Given that this is such a low resistance reading with only a two wire method the results seem comparable to me

 

Again a good 1 minute insulation value was achieved, however, the DAR value was a little low if going by recommended guidelines. Reviewing the PI ratio result, also indicates a low value, which ideally should have been above 2.0. However, as the 1 minute insulation value is high, the DAR and PI values are not as important and this motor is still acceptable for installation should it be required. This motor is also random wound design and may also be impacting on the PI ratio value.

 

The insulation readings were recorded manually at 15 second intervals for the first minute and then at 60 second intervals for the remaining 9 minutes to produce the insulation resistance plot seen next to the results table. This shows a rapid rise of insulation resistance over the first minute that then flattens out and shows very little rise.

 

 

90kW Motor Test Results90KW Motor Insulation Plot

 

 

Music Credit: 'Roadtrip' by Nicolai Heidlas Music

 

The final pair of motors are part of an old vacuum pumping system that has been decommissioned and removed from one site to another for potential installation. The preservation testing of these motors are more critical, as they would have seen deterioration from use due to thermal, electrical and mechanical stresses during operation, with also the potential to have suffered from dirt ingress over the years. These are also in an non-heated store and therefore have the potential for lower resistance values.

 

132kW Motor testsAt 132kW rating, this is a much larger motor would be powered from a star / delta starter and not a direct-on-line starter. As a consequence no shorting bars are installed across the motor terminals, meaning that each phase can be tested individually. There is also the added bonus of an anti-condensation heater, that can also be tested.

 

An expected resistance value for the anti-condensation heater can be calculated based on the rating on the nameplate which was 487.5 Ohms, this can be compared to the measured value and was found to be a good match for both motors. The insulation resistance of the anti-condensation heater is then measured at 250V with all three phases of the motor shorted to earth, as the heater is actually a cable taped to the windings so could short direct to earth, or could short to one of the motor windings. Therefore, shorting the motor windings to earth saves testing the heater against each phase individually.

 

The motor winding resistance is measured across diagonal terminals for each individual phase and was found to be balanced on both motors, but with motor A showing a slightly higher resistance value. In hindsight though, as with the 90kW motor, I may have been caught out by the instrument retaining the lead null setting from previous tests.

 

For me, the retention of the lead null settings is proving a bit troublesome. It is the only meter that retains the setting have changing functions or switching of the instrument. I am not sure what the value of this function is.

 

132kW Motor Phase Resistance

 

The dielectric absorption ratio tests were all initially conducted at 250V as the condition of the motors is unknown, so a lower test voltage is used initially to reduce the likelihood of damage to the winding if it is in a poor condition. As can be seen in the tables below, all the motors had relatively low insulation values and poor DAR.

 

As W Phase on motor B had the best insulation value, it was decided to increase the test voltage on this phase to see if the insulation value reduced significantly which would be a further sign of deteriorating insulation. Not much of a reduction was seen, 60MOhms at 500V in comparison to 61MOhms at 250V.

 

As the motors displayed no DAR ratio it was pointless proceeding to a PI ratio test and it was decided that the best course of action would be to power up the anti-condensation heater on one of the motors and leave it for a couple of weeks before returning to carry out another test and see if an improvement in insulation resistance is obtained I plan to return later November December to recheck the motors to see if the insulation resistance has improved I will also take the Keysight  U1461AU1461A with me to verify the phase resistance readings

 

Vacuum Pump A Test ResultsVacuum Pump B Test Results

This concludes the blog on testing at the motor terminals There were no major concerns found with the use of the  MIT420/2MIT420/2 all tests were conducted safely and effectively It was found that whilst clipping to the motor case to prove the earth connections the crocodile clip would sometime be at its maximum opening and didn't grip very well and I did get caught out with forgetting to reset the lead null function

 

In the next blog I will move on to carrying out insulation tests on air circuit breakers.

As a Roadtester for the Raspberry Pi 3 & MathWorks Learn-to-Program Pack, I've encountered some bumps in the road due to the MathWorks documentation and examples being somewhat out of date.  I'd intended to blog earlier as I expect the other roadtesters have probably also encountered similar issues.  I'll do a full discussion in my review, but in case it might help anyone - the Raspberry Pi support packages for Matlab and Simulink in r2017b do not currently support Raspbian Stretch.  It will give you a build error when you try to create a raspi object.  I ran into this issue because I setup my RPi3 before I tried to install the Matlab software.  The RPi3 hardware in the kit comes with a 16GB micro SD card with NOOBS pre-installed.  All this works great, but when you install Raspbian using NOOBS it will install the latest version of Raspbian which is Stretch.  The Matlab hardware support package allows you to modify an existing OS or to write a new OS image onto an SD card.  Since I had already configured the RPi3 (ssh, wifi, ipaddr, etc), I chose to modify my existing OS.  At that point I couldn't create a raspi object to connect to my RPi3, so I couldn't continue.  I tried building a new OS image and that actually worked because the image was built with Raspbian Jessie Lite.  To their credit, MathWorks support did provide me a workaround that allowed me to run Matlab on Stretch.  I am waiting for a similar workaround for Simulink.

 

I also have encountered a problem specific to my application in that Matlab's IP camera module does not support H264 streaming only MJPEG streaming.  Unfortunately, the specific camera that I'm trying to use only supports H264.  I'm trying to get a third party module, HebiCam, to work.  I'm getting errors rtsp streaming with H264.  I'll post again if I get that working.  Otherwise, I may have to change the project that I was going to do for the roadtest.

 

Good luck to all the other roadtesters.

The bench tests and desktop review have all gone well for the MIT420/2 Insulation Tester. In the next blog, I carried out some phase balance and insulation resistance tests from a low voltage (400V) motor control centre (MCC) for a gas turbine. The motor control centre is a large panel with two main feeders and then split down into multiple direct-on-line starters for various pumps and fans that the gas turbine requires to operate. The motors had been removed for maintenance work and then refitted and reconnected, so a quick test of the motors was required to show that they were safe to reinstate ready for a direction of rotation check.LV Motor Control Centre

 

In all three motors were tested;

  1. A 415V, 30kW Exhaust Ventilation Fan Motor
  2. A 415V, 37kW Cool Air Fan Motor
  3. A 415V, 45kW Exhaust Frame Cooling Fan Motor

 

Each motor has two tests carried out;

  1. A phase balance test using the resistance / continuity function of the MIT420/2
  2. A 1 minute insulation resistance test using the 500V range on the MIT420/2

 

The objective of the phase balance is to ensure that all three phases of the motor have been connected correctly and there is not cable damage or loose terminations.

 

Obviously the insulation test is done to ensure that there are no earth faults on either the cable or the electric motor.

 

As the motors are all fed from direct on line starters, the phase balance test is carried out first to ensure the motor connections are good and then a single insulation resistance test is done, as the phase balance test has shown that the phases are all interconnected.

 

The first motor control centre is a swing-out direct-on-line starter used for smaller motors. All of the controls are mounted onto the door frame and I gain access to the motor contractor by opening up an inner door. This makes measuring the phase resistance a little bit of a challenge as the probes have to be held in one hand across the contractor terminals whilst pushing the MCC apart to gain access. Mean while the instrument is held in the other hand. The meter will read the phase resistance automatically, but pressing the save button was more of a challenge as the meter is held in my right hand, I cannot reach across to the save button. As the panel is 5 foot from ground level I cannot rest the instrument on the ground. I would have really benefited from a magnetic style loop accessory so I could have left the meter hooked up onto the next panel to release my right hand to help take the measurements and operate the save function when required. The instrument looks like it it has a fixture on the back for such an accessory, but I have not been able to find one yet.

 

30kW MCC Swing Out Starter PanelPhase Balance Test in 30kW MCC Starter

 

The 1 minute insulation test went much easier as one of the crocodile clips could be used to clip to the metal chassis of the MCC. As the instrument has the built in dielectric absorption ratio timed function that could be set up before applying the leads, it was an easy one handed operation to operate the instrument. On insulation testing, the reading is also stored on screen at the end of the test, so, unlike the phase balance test, the save button could be pressed without the probes connected. Had I wanted to carry out a polarisation index test that would have required manually writing down the readings every minute, I would have had a very hard time without having an assistant or utilising some other form of connection for the leads.

30kW MCC Starter Insulation Test

 

The test results can be seen in the table below. The phase balance was stable and equal across all three phases. The one minute insulation value was 5.8GOhms which is excellent for a combined motor and cable test of all three phases together.

 

The dielectric absorption ratio was 1.36 which is above the minimum threshold value of 1.2.

 

The tests show that motor was connected correctly, with no earth faults and was ready for a run to prove the direction of rotation.

 

 

30kW Motor Test Results

 

The next MCC panel was a fixed starter panel and slightly easier to work in than the swing-out starter. This type of panels are for slightly larger motors with direct-on-line starters and the largest motors with star-delta starters. The same test procedure is utilised, however ad the contractors in this type of starter have unshrouded connections, the crocodile clips can be utilised to make the connections which removes all of the fumbling around when trying to use the probes in one hand and operate the instrument in the other.

 

37kW MCC Starter Phase Balance Tests37kW Starter Insulation Resistance Test

37kW Starter Motor Test Results

 

The phase balance test results from this motor were unsteady. This sometimes happens when fan motors are tested and the fan is free-wheeling in the air flow turning the motor. This results in small voltages being induced into the motor stator windings that can prevent an ohmmeter from making reliable readings and is not a problem with the instrument. This can be seen in the high U-V reading compared to the other two phases. A bad connection would have given a steady high reading and not a reading cycling from low to high.

 

As with the other motor, the insulation test readings were very high and the dielectric absorption ration above the minimum accepted value.

 

The final motor to be tested was a 45kW motor, this was also a fixed starter type panel and the leftist motor of the three to be tested. For this motor I also decided to carry out a polarisation index test to test this aspect of the MIT420/2 that had not been tested on the other two motors.

 

For this test I tried to make a video, but I apologise for the audio quality, as it was drowned out by the switchroom and panel fans, so I have tried to do some voice over which doesn't explain things quite as well.

 

 

There were no problems carrying out the tests using the MIT420/2 as the crocodile clips could easily be used with the open terminal style contactor installed. At the end of the PI test, it was noted that the instrument had gone over the 100GOhm resistance limit for the 500V range, so internally no PI ratio was recorded. For the purposes of the data table and the insulation resistance plot, I used the 100GOhm figure.

 

45kW Motor Test Results45kW Motor Insulation Resistance Plot

Despite going up into high GOhm resistance, the test results clearly show DAR and PI ratios, this is likely to be due to testing from the MCC that includes the motor and the cable which the latter would generally exhibit a capacitance trait when connected to a DC supply.

 

This completes this blog, the sensible thing now seems to go to the other end of the cable and test out some motors on their own.

Hello and welcome to this tech review of E36312A- A triple output programmable bench power supply from Keysight Technologies.

 

E36312A

 

Recently, I won a triple output bench power supply from Keysight Technologies.

It’s been more than 3 weeks I’m exploring this bench power supply and this blog post reveals my experience with this product so far.

 

Let’s start with the appearance and the front panel of the supply. It has a nice and big colourful LCD, 2 separate control knobs for

voltage and current adjustment, a keypad to enter the desired values of the voltage and current,

colour coded push buttons for each channel along with colour coded outputs (yellow, green and blue) and

few other push buttons for quick actions like tracking and print screen, meter view etc.

It also has a USB port for data logging and screen shot purposes. Overall the build quality is really good.

 

The three outputs are independent of each other, output 1 gives 6V, 5A and other two outputs give 25V, 1 A each.

Each output is colour coded and hence is easy to distinguish between different outputs. Each output has a independent

colour coded buttons to switch on/off and a master switch for all outputs.

 

The unit powered on and the LCD showing the outputs which are colour coded

 

And now the most amazing feature of this power supply- You can combine the outputs of port 2 and 3 in series or parallel just by pressing a button!

It has internal switching which uses relays. You can get a max of 50V output at 1A from output 2 by using series connection and a max of 2A current

at 25V by combining port 2 and 3 in parallel.

This feature is not available in any of the bench power supplies I have used till now.

 

I have used programmable power supplies from multiple vendors in the college labs, this being my first owned bench power supply.

It surely beats all other bench power supplies in the market.

 

Overall, the build quality is great. The LCD visibility is too good. Again a feature not available in most of the bench power supplies in the market.

It also has data logging capabilities and a meter view which displays the voltage, current and power measurement of the device under test.

 

Meter view showing the voltage, current and power consumption of the DUT

 

I would highly recommend this bench power supply to research labs, universities and makers. It is a one time investment but worth it.

Really helpful for testing your electronic designs.

The only downside of this product is that is a bit costly for individuals.

Hi Roadtesters and element14 members,

 

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It's a short survey regarding test equipment suppliers you are familiar with. Survey's estimated amount of time to complete is around 15-20 minutes.

 

If you complete the survey, you will be entered for a chance to win one of two $1,000 Amazon e-Gift Card. Sounds like a good deal for a few minutes of your time.

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Randall Scasny

RoadTest Program Manager

The last two blogs covered the bench tests applied to the  MIT420/2MIT420/2 and found that it performed better than expectations with all values obtained significantly within the manufacturer's specified tolerances However working on high energy electrical circuits has many safety implications so before I take the  MIT420/2MIT420/2 out into the field to do some real work I wanted to take a quick look at some of those safety features

 

As standard with most electrical instruments these days the terminals on the  MIT420/2MIT420/2 are 4mm safety socket designs with compatible plugs on the leads to prevent any inadvertent contact with the output of the instrument I also like the location of the voltage function in between the off position and the insulation test functions Merger make it absolutely clear that it does not constitute as an appropriate testing for dead procedure and I fully agree with that But for me it adds an extra safety feature as pausing on the voltage function allows for a last double check to ensure that no live voltages are present before moving to an insulation testing function or that the circuit has been discharged when switching back having completed the insulation testing

 

MIT420/2 Safety SocketsMIT420/2 Function Switch

Some more safety features whilst using the  MIT420/2MIT420/2 for insulation testing are the extra warning beeps before a 1000V insulation test is started two handed operation of the test lock function and the cut-out function that prevents the starting of an insulation test if a live voltage is present These features are demonstrated in the short video below

 

 

The remaining safety feature is the internal 500mA HRC fuse that I looked at during the teardown. There was a PCB track that bypassed the fuse via a 10MOhm resistor so I decided to remove the fuse from the meter and see what the response was.

 

The outcome was impressive, the fuse protects all functions within the tester except for the voltage function which still measures a voltage present even with the fuse removed. On switching to the resistance or capacitance function, a small flashing fuse symbol is displayed and the instrument reads over range.

 

On switching to an insulation testing function nothing is immediately obvious The test button is pressed and the meter initially responds but goes over range and no test voltage is displayed After a couple of seconds the function cuts out and the fuse symbol is again displayed However if a live voltage is present when on an insulation range with the fuse removed the cut out function still worked and the instrument alarmed with the voltage indication Pretty impressive The next video shows the operation of the  MIT420/2MIT420/2 with the fuse removed

 

 

Overall Megger have made an excellent job of adding safety features to the  MIT420/2MIT420/2 it is evident that a lot of thought has gone into the design of this instrument

 

With the instrument review and bench tests all completed, it is time to go and find some work for the instrument to do and see how it preforms out in the field.

Hi Road Test Community,

 

I have a fully-featured Rohde and Schwarz RTE1204 oscilloscope on my hands at the moment, and I thought there may be some interest in reviewing it in the style of an official Road Test. I have been a somewhat passive member of this community for a few years now, so hopefully this will allow me to hone my review skills and provide a unique look at this professional-quality oscilloscope.

 

A bit of background: I had ordered a Rohde and Schwarz RTB2004 COM4 scope back when their amazing launch deal was on, but unfortunately there was an error in the processing of my order. Luckily, the folks at Newark and Rhode sorted it out for me, and in the meantime provided this to me as a loaner scope to allow me to complete an ongoing project. I am not being compensated in any way for this review -- it is all on my own accord, and I will be candid when discussing things I like and do not like. I have used this scope in a professional setting alongside similar Keysight 4000X scopes, so I hope I can bring a realistic user perspective.

 

RTE1204 Front DisplayFig 1. Front view of the RTE1204 Scope, showing one of the two logic probes connected via the rear

 

I propose to complete this review in three initial parts, with a fourth and final section focused on questions and tests requested in the comments.

      • Part 1: Introduction. From a user perspective, I will assess this oscilloscope in terms of the typical metrics:
        • Specs and comparison to other scopes in the same category. Explore what makes this scope unique in the marketplace from its data sheet.
        • Physical and Graphic user interface. What is it like to sit in the drivers seat? Is the button/knob useful and intuitive? Same for the software and GUI -- customization, responsiveness, etc.
        • Misc: noise, form factor, display, build quality and accessories.
      • Part 2: Putting the scope through its paces (analog)
        • Analog channel analysis of various signals (sine, modulated, PRBS, etc.). Measurements, markers, math functions, and different scope settings (including 16 bit "HD" mode, and various built-in filters). I will use some 1+ GHz sources to do eye diagrams and test masks. Different triggering options
        • Spectrum analysis (FFT). Determine the usefulness of the FFT function of this scope compared to an actual spectrum analyzer. Look at THD of an amplifier, spectrogram of a modulated signal (maybe as a signal monitor for a UHF receiver), and an EMI test utilizing the gated FFT functionality.
      • Part 3: Putting the scope through its paces (mixed-signal domain)
        • Break out the logic probes and decode some serial data.
      • Part 4:
        • Remote control and automated measurements
        • Remaining tests suggested in the comments
        • Conclusions

 

I will also post a separate review of my RTB2004 when it arrives, and compare it to its "big brother" RTE1204. It seems like there are a lot of reviews already floating around, so I will hold off on this until after the promised firmware update from R&S.

 

I will be actively checking the comments, and I am happy to update my plan or add to the reviews with any suggestions or ideas (I will surely miss a thing or two). I should mention that I unfortunately do not have access to high-speed active or current probes, which prevents me from testing some of the power analysis features.

 

With that, let the "Rohde Test" begin (sorry... couldn't help it).

 

-James

Part A of the Bench Tests reviewed the performance of the auxiliary functions on the insulation tester and found them all to be well within the specified tolerances. In this second part of the Bench Tests, I will concentrate exclusively on the insulation testing function.

 

The MIT420/2 has five fixed voltages of 50V, 100V, 250V, 500V and 1000V and then a variable output function from 10V up to 1000V. This is not a voltage ramp function, the output voltage is set in the settings menu to the desired level and the function switch turned around to the variable voltage function to initiate the test. A desired change in the voltage level means that the user has to go back to the settings menu to adjust the voltage and then back to the variable voltage function again to carry out the test. The instrument will remember the last voltage setting even after it has been switched off. However, it reverts back to the default 10V if the batteries are removed.

 

For the purpose of testing an insulation meter, I have a bespoke insulation tester calibration box from Time Electronics. I carry this box around with me when I go to a site to verify that the insulation tester is function correctly before carrying out the tests on generator rotors, mainly due to the value of the rotor and the costs involvMIT420 and Calibratored in carrying out the tests. This instrument has the ability to measure the open circuit voltage, the short circuit current and then apply a range of high voltage resistors across the instrument via 4 decade switches to verify the resistance readings from 10MOhm up to 90GOhm. I also carried out an extra test to verify that each insulation test range could output a 1mA current and maintain the specified voltage level as specified in the BS EN 61557 standard.

 

The calibrator has 4mm safety sockets to plug the insulation tester leads into, but unfortunately due to the ridge on the Megger leads, I had utilise an uninsulated 4mm adapter to allow them to connect to the calibrator. Nothing more than a slight inconvenience, but one example of the disadvantage of the test lead approach by Megger.

 

Test Lead Adapter

Insulation test of leads

 

Despite this, the leads offered by Megger are of excellent quality and to show this I set up the calibrator to an 80GOhm load and tested the insulation tester against this using both the Megger test leads and the test leads I have from a multimeter. These leads are rated at 2000V but are manufactured from PVC and a lower quality.

 

The leads were set up twisted together, which is a poor test set up, but is being done to illustrate the quality of the Megger test lead set. The insulation tester was set up to 1000V for each test.

 

With the Megger test lead set a reading of 80GOhm was achieved and was found to be stable. With the PVC set, the reading was found to be much lower and quite erratic. It ended up as 6.9GOhm which is a significant difference to the 80GOhm setting on the calibrator. This kind of behaviour would seriously impact on the DAR and PI ratio tests the MIT420/2 is capable of carrying out.

 

The outcome is, if you use alternative test leads to the Megger set, ensure they are good quality silicone leads and pay attention to the test set up.

 

On to some insulation testing! The MIT420/2 has a base tolerance of +/-3% +/- 2 digits with an additional % tolerance per MOhm or GOhm dependent upon the resistance value and the range. To test the insulation accuracy each range of the instrument was tested against each decade value. The table below details the results found and the chart summarises the range of deviation seen from the actual resistance applied. All readings found to be significantly better than the base tolerance values with an overall deviation of +1% to -0.5%.

 

Insulation Test ResultsInsulation Test Deviation Range

 

The next few tests were based around determining the performance of the measurement process for the insulation resistance. The first aspect was to measure the open circuit voltage of the instrument for each testing range. To do this, the output of the MIT420/2 was connected to a high voltage probe with a 1GOhm input resistance. This is done to reduce the load on the output of the insulation tester and allow the voltage to rise to its highest. If the output of the MIT420/2 was connected directly to a DC multimeter or bench meter it would have a 10MOhm load applied to it, that would increase the current being drawn and lower the output voltage.

 

The manufacturer specifies the voltage output to be between 0% and +2% + 1 digit. The results in the table show that the voltage output remained within specifications. The first test was carried out using the Time Insulation Calibrator but as the voltmeter on this only has a tolerance of 1%, the tests were repeated with the HV Probe and a bench meter that gave an overall tolerance of 0.15%. It was interesting to see that the instrument drifts further from the nominal voltage values at the lower end, but tends towards a tighter tolerance as the output voltage is increased. This is typical of insulation testers I have used over the years.

 

Open Circuit Voltage Test Results

The circuit voltage tests are then repeated with a 1mA load applied to each range. As can be seen in the table below, the increased load on the output reduces the voltage closer to the nominal values but at no time did the voltage drop below the nominal values.

 

Voltage Output Tests with 1mA Load

The next test was to verify the short circuit current capability of the unit. The standard permits a maximum output current between 1mA and 15mA, typically instruments of the type have an output of around 1.5mA and Megger specify the short circuit current to be between 1mA and 2mA. The tests conducted showed this to be the case. Whilst carrying out the tests, the uA-s-V function switch can be used to cycle the secondary display through to read the output current.

 

MIT420 Short Circuit Test MKT420 Short Circuit Test Results

 

I then carried out a response test of the output by monitoring the voltage with a Picoscope 3404A via a high voltage probe. Due to the input voltage limitation of the Picoscope, the measurement was made via the high voltage probe, utilising a custom probe setup within the Picoscope software to create the correct voltage range. This test was done more out of interest rather than determining if the meter was performing to a specification. The results can be seen in the table below alongside a photo of the setup utilised.

 

MIT420 Output Response Test ResultsMIT420/2 Output Response Test Setup

The rise time of the output varied dependent upon the voltage range, except for the variable output range that was tested at 390V and then 760V with both returning a comparable rise time. The fall time on each range was similar each time, between 20 to 25ms. A sample plot can be seen below on the left that shows the voltage rise as the test button is operated. A small overshoot is occasionally seen, but it is small and causes no concern. The plot on the right shows the ripple of the output by zooming in on the waveform.

 

MIT420/2 Output Voltage PlotMIT420/2 Output Voltage Ripple

 

 

The final element of the test was to verify the accuracy of the timed tests offered by the MIT420/2. Three types of timed test are offered, the first is a simple duration test that is set between 1 to 10 minutes in 1 minute intervals in the settings menu. There is no duration shorter than 1 minute, which could have been useful for testing portable appliances or simple checks on smaller motors and transformers.

 

The second type of timed test is a dielectric absorption ratio (DAR), this is the ratio of the 60 second value divided by the 30 second value and is a representation of the quality of the main insulation as the applied voltage charges the circuit similar to a capacitor. A good insulation system will have a DAR ratio above 1.2.

 

The final timed test is a polarisation index (PI) which is the 10 minute value divided by the 1 minute value. This is more of a measurement of leakage current over the insulation surface mainly affected by moisture and dirt. For a Class F insulation system a ratio value above 2 is acceptable.

 

Not all apparatus will have a DAR or a PI ratio, it depends on the type and construction, hence why Megger have added a starlight timed test function. The IEEE 43 standard for AC and DC dielectric testing also advises that when a 1 minute insulation value exceeds 5GOhm, the DAR and PI values may not be representative of the insulation condition.

 

To measure the time function, I utilised the variable voltage test function, setting it to its minimum of 10V. This was applied across the input of a counter / timer unit set to the interval function. The manufacturer, does not specify a tolerance for the time function, so this was also done more out of interest. The values determined were all within 1% of the nominal value. Interestingly, the timed function gave values less than the set duration where as the DAR function always gave a time duration slightly over the 1 minute. Due to the instrument operation and the test setup on the DAR and PI tests, only the final 1 minute and 10 minute values can be recorded.

 

MIT420/2 Timing Test SetupTiming Test Results

 

This concludes this blog on the insulation resistance bench tests. Throughout, the instrument performed flawlessly, it gave accurate results and had a fast response. The display is crisp and clear to read. Adjustment of voltage output and timing durations was easy to carry out and switching them to the setting menu means that they would not get adjusted easily by mistake.

 

In the next blog, before I take the instrument out into the field, I will review some of the safety features that have attracted my attention.

In Blog 2 I opened up the  MIT420/2MIT420/2 insulation tester and found a well designed instrument with good quality construction and design methods to improve the robustness and safety of the instruments In this blog I will carry out some bench tests on the instrument to ascertain its functionality and accuracy This blog will concentrate on the auxiliary continuity resistance capacitance and voltage functions Any of the manufacturer's tolerance I have quoted in this blog can be found from their data sheet on the Megger Website.

MIT420/2 Battery Load Tests

 

For the first test, I removed the batteries and hooked up a DC power supply to measure the load that the instrument puts onto the the battery pack and to checkout the internal battery monitor.

 

The battery monitor was found to have four states from fully charged down to empty. Dropping below the empty symbol, the meter produced a 'Low Battery' text message on the screen and then the instrument switched itself off a few seconds later.

 

The loading tests were carried out for each function on the rotary switch. For the first test, the load was measured with the function in its quiescent state. The backlight for the display was then activated to observe the increase in the load. Finally a measurement was made on each function performing a test, again to observe the increase in current from the supply.

 

All the insulation test ranges were found to draw the same current, two performance tests were conducted, the first with the insulation test providing a 1mA fault current and a second with the insulation test de-loaded to 0.1mA fault current. There was a 10mA load difference between these two tests indicating that internal circuitry was also drawing power from the battery pack.

 

The resistance range was tested at two of its output currents, a 200mA and a 2mA test current. For both tests, the meter was found to be drawing approximately an extra 50mA for its internal use. Naturally the 200mA continuity test was found to have the highest loading on the battery pack of 271mA.

 

The test results can be seen in the two tables below;

 

MIT420/2 Battery Load Test Results

 

The capacitance function on the insulation tester has a range of 0.1nF to 10uF. However a tolerance is not specified for capacitance values less than 1nF, values above 1nF have a tolerance of +/-5% +/- 2 digits. To test the capacitance, I have a Time Electronics capacitance decade box. All the readings taken were found to be within tolerance and the instrument was quite rapid to respond to the changes in capacitance values applied to it. I am not sure why Megger have added a capacitance range to the MIT420, obviously single phase motors and light fitting all have capacitors in them. If the instrument is aimed at diagnosing on this kind of apparatus, then in my experience, a maximum 10uF reading seems to be a little short and the range could benefit from being taken up to 100uF as a lot of motor run capacitors will be well above the 10uF value. The test results can be seen below;

Capacitance test results

The next range to test was the continuity / resistance range that has a 0.01 Ohm to 1000 kOhm capability at varying current levels. Like the capacitance function, the reading is automatic and the test current is also adjusted automatically as the resistance value increases. A Time Electronics resistance decade box was used for this test, but I also opted to put an ammeter in circuit to measure the actual test current being applied. Prior to starting the tests, the instrument was nulled with the test leads plugged into the decade box. The function has a variable tolerance specified, from 0.01 to 100 Ohms the tolerance is +/-3% +/- 2 digits, from 100 Ohms to 500 kOhms it is +/-5% +/- 2 digits and above 500kOhms, no tolerance is specified. Obviously a 1 MOhm resistance range is quite a low specification, but if the instrument is intended for electrical apparatus maintenance then it is likely to be more than adequate. Again all measurements made were found to be within the specified tolerances as can be seen in the next table;

Resistance test results

I will now move on to the voltage measurement function. On switching to this function, the instrument switches to a dual AC/DC mode. This can be changed to DC or true RMS AC mode using one of the function buttons just below the display. This function is quite interesting to me as during the testing of generator rotor windings, as well as an insulation resistance and polarisation index test, winding AC impedance and DC resistance are also measured. This is carried out using a 4 terminal test methodology, so I therefore have a need to measure AC voltage up to 120V and DC voltage up to 15V reasonably accurately. Most of the tests therefore revolved around these ranges.

Voltage Measurement Test Setup

 

The instrument will measure up to 600V AC or DC The AC tolerance is specified as +/-2% +/-1 digit the DC as +/-2% +/-2 digits On the AC/DC range it is specified as +/-5.1%. When reading AC voltage the  MIT420/2MIT420/2 will also read the frequency from 15 up to 400Hz with a tolerance of 0.5% +/-1 digit for frequencies below 100Hz Above 100Hz the tolerance is unspecified To test out the voltage and frequency functions I have a single phase Megger SMRT-1 injection test set that is accurate to 0.1%.

 

Basic AC and DC voltage readings were taken from zero up to 300V as this is the capability limit of the SMRT-1. For the frequency measurements, I adjusted the frequency between 50 and 400Hz whilst maintaining the voltage at 110V AC. To investigate the DC voltage function a little more, I then carried out a DC linearity test from -18V through to +18V at 1V intervals.

 

As observed in the tables that follow all of the voltage measurements were within the tolerances specified by at least 50% I was pleased with this result and it means that it will be worthwhile me testing the  MIT420/2MIT420/2 on the generator rotor winding AC impedance and DC resistance tests in comparison with the usual multimeters that I utilise.

Voltage Measurement Test Results

This concludes the bench tests on the auxiliary functions, all the results were found to be within the specifications detailed in the manufacturer's data sheet, which was to be expected, since the instrument came with its own calibration certificate. What was particularly pleasing was that the voltage measurements were found to be considerably better than the specified tolerance, which means that it may be more useful than just as a voltage check function, but testing out in the field will prove this.

 

In Part B of the Bench Tests I promise I will get round to actually carrying out some insulation tests.

In the last blog I took an overview approach to the  MIT420/2MIT420/2 Insulation Tester package that I have been sent to review In this blog I plan to concentrate on the instrument itself to review its build quality and design features

 

To gain access inside the meter, the battery and fuse compartment lidsRemoving MIT420/2 Case Screws are removed. As these are subject to more wear and tear, Megger have used steel screws captivated within each compartment lid that screw into brass inserts in the meter body which should provide a high level of reliability. There are four screws in the corners of the top section of the instrument and removal of the batteries reveals a fifth. These are self-tapper style screws, that screw into plastic holdings and are not as robust as the compartment lid screws. But as the case will be rarely opened, I don't view this as being an issue.

 

The top and bottom parts of the case are separated by pulling them apart. There is a bit of resistance when doing this as there is a lip style seal around the whole of the case to protect against moisture and dirt ingress. Once the seal is split, the case comes apart easily. The battery connects to the board inside via two spring tabs so there are no wires to disconnect. Interestingly there is no shielding inside any of the case components.

 

With the bottom case removed, the internal electronics is spread across three PCBs giving a modular design. The PCBs connect together electrically through header connectors and are mechanically joined by plastic spacers that push through holes to clip the boards in place. The complete assembly is held into the top case via a single screw just below the function buttons. Removing this allows the PCBs to be removed.

 

The top case contains a clear perspex screen to protect the display, the rubberised function buttons and the rotary selector switch mechanism.

 

MIT20/2 Dismantled

 

 

 

 

 

MIT420/2 with back case removed

 

 

The display is mounted on the front PCB via clips and is connected electrically via a ribbon cable onto the rear of the PCB. The processor electronics appear to be underneath the display and whilst the display unclips from the PCB easily enough, I couldn't release the ribbon cable from its socket and didn't want to use too much force incase clumsy thumbs broke something.

 

The rest of the front face of the PCB is fairly open. Directly beneath the display are the PCB tracks for the function buttons that are the silicone membrane variety with the carbon contact pad. Beneath these, the main test button was found to be an actual PCB mounted push switch, presumably to give more reliability for the button that will be see the highest number of operations. The OK button beneath this was found to be the track pad design as per the other function buttons.

 

The opposite side of this board contained a lot more electronics, but as I am not much of an electronics engineer and even less of a metrologist, there is not much on there that I recognised.

 

The next board in the stack appeared to contain the main high voltage amplifier circuit along with some selection relays. The main PCB track that carries the high voltage to the output terminal seems to be well spaced from the other components and tracks on the PCB but did not have any isolation slots around it. The track does go to an eight way PCB header that formed the connection through to the small output board. Only two other pins were used on this header at the opposite end for the control probe connections.

 

It was also noted that the rotary selector switch was a manufactured coded rotary hexadecimal switch, again presumably chosen to giver a higher robustness to the design instead of the more standard PCB track design function switches found in multimeters.

 

 

Front PCB Display SideFront PCB Component Side

 

 

High Voltage BoardHV Board - track side view

 

MIT420/2 Output terminal board

 

The final board is much smaller than the other two boards and is segregated from the middle board via an insulation sheet that locates around the header connectors and PCB spacers. This board primarily contains the 500mA HRC fuse and the 4mm safety connectors contained within a plastic moulding. The plastic moulding has location lugs to go onto the PCB and then the sockets are soldered onto the PCB through a right-angled pin. This design seems to offload a lot of the mechanical pressure into the plastic moulding when inserting and removing the leads, this should provide protection for the actual solder joints and shows good design by Megger.

 

The 500mA HRC fuse was of great interest to me due to the high energy circuits that I work on Tracing through the tracks on the PCB showed that the fuse is directly in circuit with the positive terminal of the insulation tester There was a thinner track between the fuse and the terminal that led to a 10MOhm resistor that bypasses the fuse I couldn't work out what the function of this was but I got the impression that the fuse protects all of the functions on the tester If so this is quite impressive as I am not aware of any other insulation testers that have this level of fusing I will investigate this more when the  MIT420/2MIT420/2 is reassembled There also appears to be space on the PCB for a second fuse presumably for the three terminal insulation tester offered as part of the range that adds a guard terminal to the instrument

 

Programming Connection

During the reassembly of the instrument, I noticed a cutout in the battery case compartment covered over by a sticker. This cutout lines up with eight pads on the rear of the high voltage board. Unlike other manufacturer's, I have never seen a service manual for an instrument from Megger, to find out what the calibration procedure is. There does not appear to be anything obvious within the setting menu, so seeing this cutout made me wonder if there is some bespoke test system that connects to the instrument through this cutout to allow the meter calibration to be adjusted. If this is the case, and I am not certain of this, this would limit places where the instrument can have its calibration adjusted. Granted, the majority of calibration laboratories could check that the instrument is working within tolerances, but maybe it has to go back direct to Megger, or one of their partners, to be adjusted.

 

Overall, the meter construction is of good quality with some interesting safety features. The instrument has a good modular design, allowing for replacement of individual PCBs. There is no evidence of post design modifications on any of the boards. The instrument case is strong with a built in rubberised coating. Robust design features such as threaded inserts and manufactured switches are utilised in areas where reliability through extensive use may become an issue.

 

In the next blog I will move on to carrying out some bench tests with the  MIT420/2MIT420/2

I have been selected to road test the Megger MIT420/2 for the element14 community and this first blog is an introduction to the various elements of the package that has arrived and what I intend todo for the review.

Insulation Tester Package

 

The first item, often overlooked, is the case. For an engineer, such as myself, that travels from site to site, protection of the instrument during transit is extremely important. The meter sells for around £500 in the UK, not including VAT, so damage would be an expensive experience.

 

The case provided with the MIT420/2 is a 100% plastic blow molded case however, it appears to be very robust and strong and will provide plenty of protection to the meter. The hinge is a molded pin design and looks to be solid and long lasting. The catches are also of a hinge pin design, but are much thinner. They also only open up 90 degrees and could be susceptible to damage if they got caught. Should any damage occur, Megger provide, not only a complete case as a purchasable spare, but also a pair of catches that can be clipped into place of the old ones.

 

MIT420/2 and accessories in case

Internally within the case, there is a shaped section for the meter and a pocket for the documentation and CD ROM that arrives with the package. The leads, probes and clips are contained within two open pockets and do rattle around inside the case. They are not likely to cause any damage but my preference is to have the items secured as it reduces the likely hood of loss when working up on platforms and grilled floors and allows items to be easily accounted for when working in clean conditions areas. Over time, I plan to obtain some tool foam to overcome this issue.

 

The documentation provided with the meter consists of a warranty extension period when ownership of the meter is registered with Megger on their website. Two safety sheets care included, one covering the safety elements of carrying out insulation testing and working on electrical apparatus and a second on the use of fused test leads. Megger offers the latter in both 500mA and 10A HRC fused variants as purchasable accessories.

 

A quick start guide is also included. This covers the basic functionality of the insulation tester. It appears to be a common sheet covering the various testers within the family, therefore some of the elements within the guide are not applicable to this specific tester.

 

Calibration CertificateA manufacturer’s calibration certificate is also included within the package. Megger, should be praised for this, the certificate contains full calibration data, with measured values and acceptable tolerances. As I work in an industry that consists of traceability and audits requirements, there are two slight improvements that would benefit me. The first is guidance from Megger over the duration of the calibration period. The certificate is dated for the actual calibration checks and a blank box is on the certificate for the owner to add in the date of ownership. However, no guidance was found on when the tester would be due for re-calibration.

 

For traceability, the certificate lists the test apparatus used for the calibration, but does not include a calibration date or certificate details for this item. The certificate would benefit from this to meet the full requirements of audit.

 

All other documentation is contained within the CD-ROM, including a full manual for the meter and also the document ‘A Stitch in Time’. This is the guidance document from Megger on how to carry out the various aspects of insulation testing. It is an excellent document and a good read for someone wishing to know more about the subject. It is a great touch for Megger to include this. For those interested, the document can be obtained by registering on the Megger website.

 

A set of test leads has been supplied with the MIT420/2 insulation tester.

 

A control probe lead is supplied with a bespoke connector that plugs into the positive terminal on the MIT420/2. A built in probe is permanently wired to the other end that houses the control button and a built in probe tip, this has a plastic cover that makes the tip GS38 compliant but can be removed to reveal a standard 4mm plug that can be used with other accessories. Megger have included an insulated probe tip as one of these accessories.

 

A pair of leads with built on probes are also supplied, one red and one black.  These plug into the insulation tester outlet terminals but neither connects to the control button terminals. The probes have a standard 4mm connection plug for accessories however, the safety shroud is bespoke to Megger and means that only their accessories can be utilised with the probes. Megger have provided a pair of crocodile clips and short probe tips within the package but other third party clamps and probe tips will not always fit. I hope to expand on this aspect during the testing phase of the review.

 

This isn’t much of a problem for experienced engineers that are likely to have other leads and clips within their toolbox to cater for unusual connection requirements that may arise on the odd occasion. Less experienced or those just setting out may find themselves caught out.

 

As the tester will take standard 4mm safety leads, a third party set of leads can easily be used when required. However, the quality of the leads must be right. The insulation tester has an output of up to 1000V and can read up to 200 gig-ohms. At this level of insulation values and voltages small current leakage between the positive and negative leads can have varying impacts on the insulation reading obtained. This will also be dependent upon the type of testing carried out. The MIT420/2 has the capability to carry out dielectric absorption ratio and polarization index tests. These types of tests can lead to high resistance values but leakage through poor quality leads would reduce the values obtained and give lower ratio values giving an impression that the apparatus being tested may need attention or have a fault. Again, I plan to expand on this aspect during the testing phase.

 

Control Probe Test LeadStandard Test Lead SetBespoke Crocodile Clip Attachment

 

Finally to the MIT420/2 insulation tester itself.

 

MIT420/2 next to Keysight U1461A

The instrument is from a family of seven testers with varying functionality. Two of the instruments are aimed towards the communication sector and another offers a high voltage output of 2500V DC for more specialist applications. This leaves four insulation testers of immediate comparison. The MIT420/2 is the third in this line of testers offering a capacitance function, a variable voltage output and the ability to save test results. The top of the range instrument adds the ability to download the saved results to a computer via bespoke software and a rechargeable battery option.

 

This is also an updated series of insulation testers. Compared to the original version, the MIT420/2 offers a variable voltage output and a single resistance measurement function up to 1MOhm that was previously split across two ranges. The voltage measurement function has also been moved to the first position on the rotary switch on the latest design. Although not a recognised testing for dead procedure, it does allow a final check for voltage absence before moving to an insulation testing function and that the circuit has discharged when switching back from insulating testing to the off position. I like this touch as an additional safety feature not present on other insulation testers on the market.

 

Physically, the MIT420/2 is of a similar size to the Keysight U1461A but weighs around 100g more. Its shape is unique to Megger and is comfortable to hold in one hand, however I found that I could not reach all of the function buttons below the display when holding in one hand.

The MIT420/2 does not have a separate protective boot; the black rubberised coating is permanently fastened to the hard gray plastic case. The display is large and clear, with a combined analogue and digital display. A backlight facility is included that enhances the display greatly for use in poor lighting conditions.

 

Battery and Fuse Compartments Opened Up

The battery compartment has a separate cover with an integrated stand and is held securely by a single captive screw and a metallic threaded insert giving good durability. There is also a separate compartment that contains a single 500mA HRC fuse for meter protection. Both of the covers have an integrated seal to protect against dust and dirt ingress.

 

The basic function of the MIT420/2 is selected using the rotary switch on the front of the meter. Optional functions are then selected using the push buttons just below the display. This area also houses the save button to record the current test result to a memory location, the display backlight button and beeper volume button.

 

Retrieval of the saved records is made by selection of the record function on the rotary switch. The next position on the rotary switch allows for specific records or all of the records to be deleted from memory.

 

A final position on the selector switch allows for the various instruments settings to be adjusted such as the variable voltage level, timer adjustment and voltage safety cut out level.

 

 

 

 

Below is a brief description of the functions and how I intend to put them to use;

  • Voltage Measurement Function – I plan to test this out whilst carrying out rotor winding resistance and AC Impedance tests that will require measuring both AC and DC volts.
  • 100V Insulation Test – This would be used to test insulated bearings in generators and some motors; unfortunately I am struggling to plan this kind of work in.
  • 250V Insulation Test – Utilised to test single phase AC circuits, I plan to use this whilst testing out some motors that have anti-condensation heaters installed.
  • 500V Insulation Test – This will be the most utilised function for testing various three phase motors and panels.
  • 1000V Insulation Test – This will be utilised for testing air circuit breakers that are currently in storage.
  • Variable Insulation Test – I plan to use this function to test some generator rotor windings that are DC rated. The variable function will allow me to select a test voltage closer to the working voltage of the winding.
  • Timed Insulation Test – Each test voltage has the ability to record a dielectric absorption or polarization index ratio, I aim to use these functions when testing motors and generator rotors.
  • Resistance Measurement – I will use this during motor testing to verify phase balance and also rotor winding testing to verify connection resistance for specific tests.
  • Capacitance Measurement – Presumably this has been added for single phase motor start / run capacitors and lighting circuit capacitors. This is another function I probably won’t use too much.

 

This concludes the blog, in the next blog I plan to take a look inside the meter to assess the build quality.

This was supposed to be my 4th blog. Discussing advanced topics such as triggers, filters, multiple messages.

But this is when the Microchip CAN Analyzer didn't perform to its advertised specs.

 

Good Basic functionality

The device performed perfectly when doing basic analysis. It performed well on logic level and on physical bus level. Receiving, Sending and logging work just fine.
Some additional features such as terminating the line via software, changing frequency and set the device to listen-only mode are a big asset. So far so good.

It's perfectly usable in that context. Not a single failure. Not a single let-down.

 

Advertised Functionality not Implemented

The disappointment comes when moving to the advanced functions. They are not implemented.

It feels like an abandoned device. Functionality that's key to make it useful as a real can bus development and test utility (group message) are announced as "to be implemented as a future functionality" in 2010.

That's a pity but acceptable. No commitments were made, so I can't complain.

Functionality that's mentioned on the product page and user manual (triggers, filters in the GUI) are not implemented. I have no words for that.

Microchip has had 7 years to work on that or remove the features from the product page and User Manual. Customers have been asking them since 2010. They gave up in 2012.

 

This is a Road Test, so I hope that Microchip will chime in and correct me. I will check - hope I stand corrected ...

 

 

Road Test Blog
part 1: First trials
part 2: Inject CAN Messages
part 3: Analyzer as Test Tool
part 4: Advanced Functions
Related Blog
part 1: tryout
part 2: Communication between 2 Devices
part 3a: Design a Bus Driver PCB
part 3b: Design a Bus Driver PCB - Schematics and Custom Components
part 3c: Design a Bus Driver PCB - Layout

I'm reviewing the Microchip CAN Analyzer for a Road Test.

It'll be used in a CAN Bus that I'm designing. This blog shows that it is useful in a firmware design cycle.

 

The Analyzer as a Development Helper

 

In a previous CAN blog I wrote about configuring a Hercules microcontroller's CAN module. My setup is using interrupts to react on traffic and capture only relevant data.

The trigger should only fire when a message with ID == 1 appears on the CAN bus. Any other ID should be ignored. The CAN module should not interrupt the ARM controller.

An ideal scenario for the CAN Analyzer. I can use it to send a payload and choose an idea. It allows to generate messages to be captured and also ones to be ignored.

 

Here's the service routine:

 

#pragma WEAK(canMessageNotification)
void canMessageNotification(canBASE_t *node, uint32 messageBox)  
{
/*  enter user code between the USER CODE BEGIN and USER CODE END. */
/* USER CODE BEGIN (15) */
    if((node==canREG1)&& (messageBox == canMESSAGE_BOX1)) // transmit box
    {
      tx_done=1; /* confirm transfer request */
    }

    if((node==canREG1)&& (messageBox == canMESSAGE_BOX2)) // receive box
    {
     while(!canIsRxMessageArrived(canREG1, canMESSAGE_BOX2));
     canGetData(canREG1, canMESSAGE_BOX2, rx_ptr); /* copy to RAM */
     rx_ptr +=8;
    }
/* USER CODE END */
}

 

These lines of code are of interest here:

 

    if((node==canREG1)&& (messageBox == canMESSAGE_BOX2)) // receive box
    {
     while(!canIsRxMessageArrived(canREG1, canMESSAGE_BOX2));
     canGetData(canREG1, canMESSAGE_BOX2, rx_ptr); /* copy to RAM */
     rx_ptr +=8;
    }

 

canREG1 is there because we're working with CAN module 1 of the microcontroller (it has 2 of them).

canMESSAGE_BOX2 is the message box of that CAN module that we've configured as an inbound box,  listening to ID 1 only.

Test Scenarios

 

I've tested the following cases:

  • trigger should not fire if ID is different than 1 (because I haven't defined a trigger for any other ID in the controller configuration).
  • trigger should fire if ID is 1.
  • if triggered, the following condition should evaluate TRUE:

 

if((node==canREG1)&& (messageBox == canMESSAGE_BOX2)) // receive box

 

  • the payload should be copied to the controller's memory by the service routine
  • send multiple messages.

 

All of the cases passed. Here's the result of a breakpoint firing after the first arrival of a message with ID == 1.

 

In the CAN Analyzer GUI, I set the ID to 1, the size to 8 bytes, and no repeats.

You can see that the Hercules stopped at the service routine's breakpoint, and copied the payload to the RAM:

 

     canGetData(canREG1, canMESSAGE_BOX2, rx_ptr); /* copy to RAM */

 

I'm showing the content of the first 8 bytes of that RAM buffer in the upper right of the capture. You can see that the values 1 -> 8 that I sent via the Microchip CAN analyzer are now in the RAM buffer.

In the past I have been testing these scenarios by programming another CAN enabled microcontroller.

The Analyzer makes this easier.

 

Road Test Blog
part 1: First trials
part 2: Inject CAN Messages
part 3: Analyzer as Test Tool
part 4: Advanced Functions
Related Blog
part 1: tryout
part 2: Communication between 2 Devices
part 3a: Design a Bus Driver PCB
part 3b: Design a Bus Driver PCB - Schematics and Custom Components
part 3c: Design a Bus Driver PCB - Layout

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