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Legacy Personal Blogs Keysight U1461A Insulation Multimeter Test Fault
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Engagement
  • Author Author: three-phase
  • Date Created: 5 Jan 2018 9:56 PM Date Created
  • Views 1341 views
  • Likes 6 likes
  • Comments 6 comments
  • mit420/2
  • test fault
  • megger
  • insulation test
  • keysight u1461a
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Keysight U1461A Insulation Multimeter Test Fault

three-phase
three-phase
5 Jan 2018

As part of my road test review of the Megger  MIT420/2MIT420/2 insulation tester, I have carried out some comparison of this instrument to the Keysight  U1461AU1461A insulation multimeter and have commented that I have had issues with the  U1461AU1461A on occasion. During the last test of a generator rotor winding I recorded some data videos on the  U1461AU1461A and this blog is an explanation of the problems I have had.

 

Now before I get accused of assasinating the  U1461A,U1461A, I do accept that I carry out an unusual role within the electrical industry. The machinery I work on is of the larger variety and has onerous and exacting test requirements that can take the functionality of some test instruments to their limits. I have had issues with a number of instruments over the years, from simple cheap voltage sticks and their proving units, up to specialised secondary injection test sets worth tens of thousands of pounds. Sometimes unforseen issues occur during tests and it is accepted as being just part of the job.

 

I stress that the  U1461AU1461A has worked fine carrying out insulation tests on general electrical apparatus and does offer improved accuracy and reporting functions over a lot of other insulation testers on the market for general purpose work.

 

Just to prove that statement below is a video of the Keysight  U1461AU1461A carrying out an insulation test on a 90kW three phase induction motor. The video lasts just over one minute.

 

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Keysight had some reasonable reports that I could produce by either linking the  U1461AU1461A to a laptop using the USB adapter, or via a wireless link to an iPad. Both produced similar reports as seen below on the left for the PC Logger Software and on the right for an iPad Insulation Test Report. Ideally I would have liked to have been able to export the data out to Excel to allow comparison to be made back to a baseline insulation resistance plot but I never got round to figuring out if I could do this.

 

imageimage

 

Background

image

 

I purchased the Keysight  U1461AU1461A over 18 months ago now to use as an alternative to the Fluke 1555C insulation tester when testing out generator rotors. I also had the intention of using it for RSO winding resistance and impedance measurements where I need two multimeters to record the current and voltage at the same time. The longer term intention was to purchase the  U1115AU1115A logging module to simultaneously record the voltage and current to improve accuracy during the test and the quality of the report to issue to the site. I never got this far due to the issue I encountered

 

This was all part of my drive to make testing the generator rotors easier, safer and more efficient.

 

I have tested three generator rotors so far with the  U1461AU1461A and all have produced a similar result

  • Rotor 1 - Static excited 412MVA hydrogen cooled generator
  • Rotor 2 - Static excited 210MVA air cooled generator
  • Rotor 3 - Static excited 161MVA air cooled generator

 

In all cases the  U1461AU1461A produces a very erratic insulation resistance reading switching between MOhm and GOhm values rapidly. This was only ever noticed when the rotor insulation was in good condition and the current supplied by the instrument would have been quite low. When a rotor with a bad insulation reading was tested the  U1461AU1461A would produce an insulation resistance value comparable to the reading from other instruments.

 

Rotor 3 Tests

 

When testing the last rotor with the Megger  MIT420/2MIT420/2 a 1 minute insulation value of 1.8 GOhms was obtained with DAR factor at 1.6 and PI factor at 3. It was therefore known that the rotor insulation was in good condition. The Megger  MIT420/2MIT420/2 is also a comparable meter to the Keysight  U1461AU1461A offering the same insulation test ranges with a similar sized package and construction.

 

Time to see the Keysight  U1461AU1461A in action. The video shows the problem with the meter and lasts just under three minutes.

 

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In the video you can clearly see the insulation value on the Keysight  U1461AU1461A bouncing around, where as when using the Megger unit the reading shows an increasing trend over the duration of the test as would be expected for a rotor with good insulation condition. In the end the  U1461AU1461A fails the test before the one minute duration and displays an error message, no DAR value is calculated and the impression is that the rotor winding has an insulation fault on it. This has proven not to be the case for this rotor and the other two that were tested.

 

The conclusion I therefore came to was that some element of the large rotor winding was affecting the operation of the Keysight  U1461A.U1461A.

 

To try and gather some more information on a potential cause of this error, I hooked up one channel of an oscilloscope via a high voltage probe to measure the voltage applied to the rotor and then a differential probe to the second channel on the oscilloscope via a shirt resistor in series with the rotor winding to measure the current applied. The video of these tests is below in two parts, first part is the voltage and current, lasting three minutes and then the second part is the voltage only and lasts two minutes.

 

 

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I had issues with recording the current, for some reason the polarity would always go negative despite reversing connections through the shunt, which didn't change the polarity and then on the differential probe, which had no effect either. The second issue is that the insulation value recorded on the instruments was steady at around 2MOhms, so I assume that the input impedance of the differential probe was affecting the overall test circuit.

 

Despite these issues the only difference obvious at the time was the faster application of the test voltage when using the Keysight  U1461A.U1461A. However both the voltage and the current from both instruments appeared steady on the oscilloscope screen through out the tests.

 

imageimage

 

I therefore currently have no conclusion as to the cause of this and how it could be rectified. So at the moment, the Keysight is marked as unusable for rotor winding insulation tests, which is a huge disappointment both in terms of the money spent and the loss of the potential improvements to the testing methodology and the reporting facility.

 

Unfortunately the Megger  MIT420/2,MIT420/2, whilst it can conduct the tests without a problem, it does not offer me the downloading and reporting facilities I desired. I do have my eye on another insulation tester that I need to do a little more research on to try and ascertain its suitability. In the mean time I have the option to use the Megger  MIT420/2MIT420/2 and manually record the results or use the oversized Fluke 1555C to obtain an element of the reporting option but live with lugging around a large case.

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Top Comments

  • mcb1
    mcb1 over 5 years ago in reply to three-phase +5
    three-phase If I'm not mistaken I think the relationship between element14 and Keysight is fairly good. e14phil may have a contact in keysight that can get you two together. I don't imagine they have a…
  • jw0752
    jw0752 over 5 years ago +4
    Thank you for sharing your research. I enjoyed learning more about how these instruments are applied to real world problems. I have found that some manufacturers are too internal. They test using their…
  • three-phase
    three-phase over 5 years ago in reply to jw0752 +4
    Interesting information, I don't have much of an idea how manufacturers go about testing new instrumentation. In Keysight's defence, I don't imagine they have a 36 tonne generator rotor sat on a shelf…
  • three-phase
    three-phase over 5 years ago in reply to mcb1

    mcb1

    You are probably right, when I have needed support for a Keysight counter I have it came via Farnell and was quite a rapid response and did give the solution.

     

    Unfortunately when dealing with them directly myself in my country I haven't even been able to get a quote for calibrating the counter......

     

    I have pointed them to this blog so we will see what happens.

     

    Thanks for your insight

     

    Kind regards

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  • mcb1
    mcb1 over 5 years ago in reply to three-phase

    three-phase

    If I'm not mistaken I think the relationship between element14 and Keysight is fairly good.

     

    e14phil may have a contact in keysight that can get you two together.

     

    I don't imagine they have a 36 tonne generator rotor sat on a shelf

    image ... given what you've identified so far, I would imagine they'd jump at the opportunity to try it and work it out, or work with you.

     

    Especially since the competition seems to work properly.

     

     

     

    Mark

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  • three-phase
    three-phase over 5 years ago in reply to jw0752

    Interesting information, I don't have much of an idea how manufacturers go about testing new instrumentation. In Keysight's defence, I don't imagine they have a 36 tonne generator rotor sat on a shelf for them to test.image But whether or not they approach motor and generator manufacturers too arrange to test their meters on a range of their equipment, I don't know. I guess there is also the question of if they would make the realisation that someone may use their meter on such a large piece of apparatus?

     

    Kind regards

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  • three-phase
    three-phase over 5 years ago in reply to mcb1

    As of yet, I haven't discussed anything with them as I hadn't collected any data or evidence, just observed the issue.

     

    I may send them a link to this post but in my experience companies aren't to keen to discuss issues with their equipment, especially in open forums.

     

    Kind regards

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  • jw0752
    jw0752 over 5 years ago

    Thank you for sharing your research. I enjoyed learning more about how these instruments are applied to real world problems. I have found that some manufacturers are too internal. They test using their own labs and test jigs and do not spend enough time in the field talking to the end users. I would think that Keysight would find your tests and input very valuable.

    John

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