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.
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;
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;
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;
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.
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.
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.
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