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
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.
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