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Evershed and Vignoles 500V hand-cranked megger

  1. Jul 24, 2017 #1
    image.jpg

    I saw this beautiful old machine on eBay, and couldn't resist it. Comes in a brass case with carry handle, and is encased in walnut-effect Bakelite. The screws and terminals are solid brass, and it actually smells of pipe tobacco.

    I have tested it, and wonder if anyone can verify it really is working correctly:

    Open circuit voltage, crank speed 120 rpm: 400-420V. However, if I do 1ns peak detect, the voltage max is 561.5V. Needle reads very close to, but not quite, infinity.

    Short circuit current: 6 mA DC. Needle reads dead-on 0 ohms.

    Attaching resistors across output from 10K to 10M results in a reliably accurate reading.

    Scope shot:
    IMG_0379.JPG

    2ms/div.
    20V/div x10.
    DC coupled.

    Sorry for the wobbly shot - it's hard to capture while cranking!

    Do you think it's 'calibrated' and working OK? I'd rather not open it, as the original lead metal plugs are still present, with a maker's stamp, suggested it's not been opened since manufacture, approx 1940. If there is a fault then I'll open it for service.

    Serial number is 282427.

    Oh, for the days when one could chuff away on a pipe, cranking a Megger and testing installations.
     
  2. jcsd
  3. Jul 24, 2017 #2

    scottdave

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    I put this out to the US Navy Firecontrolmen group on Facebook, to see if anybody there had seen one. So far, I have 10 replies back in 1 hour. It is a closed group, so you won't be able to see the posts, unless you are a member, unfortunately. Several of the people, who replied, had actually used one. The main purpose listed was for inspecting and finding faults in insulation (wiring and safety gloves). The maintenance sheet would tell how many megOhms the particular insulation should read, at a high voltage (like the 400+ Volts that this is capable of generating). Nobody had any tips on calibrating, yet. I hope this helps.
     
  4. Jul 24, 2017 #3
    Yes, it is the precursor to the Meggers in use today for insulation resistance testing. A rare story of a company founded in the late 19th century which has managed to span the entire 20th century and is still going strong today.

    As far as I know, the guideline for sub-500V installations is at least 1 megohm of insulation resistance. However, anything between 1-2 megohms invites investigation as the expected value for good cabling is tens or hundreds of megohms.

    Serendipitously (eh?) I have just used it to diagnose a low insulation fault at my friend's farm - his well pump is switched by a float switch in a header tank up the field. The cable to this switch is buried near a fence and has a leak to earth. His entire house is protected by one RCD (GFCI) which kept tripping.
     
  5. Jul 24, 2017 #4

    scottdave

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    One person who responded, stated that it was good for testing long lengths of wire. Another stated that when testing, a reading of about 10 or more megohms is the expected insulation resistance.
    What is the voltage level with one of your resistor loads? You said it is around 6 mA when the leads are shorted.
     
  6. Jul 24, 2017 #5
    I'll have a play and get back to you on that.

    I think, because of the limited current, that the voltage will simply drop lower the lower the resistance. I have, of course, grabbed onto the leads and cranked it to see what it's like - unpleasant.

    The movement is very interesting - the needle is not biased, and when not reading it just flops around. Internally, on the moving part, there's a current coil and voltage coil, set perpendicular to each other and wound in opposite directions. A permanent magnet sets up a field across these. When cranking/measuring, the mechanism appears to compare voltage and current:

    1. Open circuit = high V, low I. Reads infinity.
    2. Shorted = low V, high I. Reads 0.

    The resistance reading seems to result from the vector sum of the opposing influences of the current and voltage coils.
     
  7. Jul 24, 2017 #6

    scottdave

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    Based on your short circuit current, we could estimate a Thévenin Equivalent circuit of 420V voltage source, in series with a 70 kΩ resistor.
    With skin resistance of say around 100 kΩ, this gives around 2.5 mA current if you are holding it. Considering that it is sort of an AC waveform, that would feel worse than equivalent DC voltage.
     
  8. Jul 24, 2017 #7
    According to the Megger, my skin resistance was 50 kohm with slow cranking, and decreased to 10 kohm with fast cranking. And yes, I could feel the 'vibration' of the AC-on-DC. At slow speeds I could feel every commutator bar.
     
  9. Jul 24, 2017 #8

    jim hardy

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    What a beauty !
    And SOOO practical . No batteries, no bleeping computer, no problem !
     
  10. Jul 25, 2017 #9

    scottdave

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    Ha. A few others admitted to have used it to test their "resistance". It sounds like that it serves the dual purpose of testing electrical resistance, as well as resistance to pain. :woot:o_O
     
  11. Jul 25, 2017 #10

    Baluncore

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    No calibration needed. No batteries to go flat. More strength to your arm. They are an excellent instrument.
    I have several scattered around here. Meggers come in different insulation voltage ratings, usually 500V or 1000V. The voltage is often printed on the scale, but yours appears to be 500V, cast into the Bakelite case.

    For the principle of operation see; http://www.richardsradios.co.uk/megger.html

    The meter needle has no control spring for setting zero calibration, so it appears to be faulty and usually settles wherever. That can reduce the price when you find one. I was at an Electricity Authority disposal sale looking at two with high price tags on them. I picked one up, when another purchaser demanded immediately to buy the one I was holding. I told him OK, and handed it to him, pointing out that the meter was “drifting”, so he grabbed the other, “it's the same” I said. After the offensive customer left, the store keeper who had been watching, sold me the two for $5, I kept one and gave the other away.

    I also picked up a very nice 1000V E&V Bridge Megger from the same Authority. It is the same as is shown here; http://www.richardsradios.co.uk/brmegger.html
    I checked the calibration, but have never needed the 4 digit precision of it's manganin wire wound resistors, but the 1000V insulation tester gets used.
     
  12. Jul 25, 2017 #11

    dlgoff

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    :oldlove::oldlove::oldlove:

    Bold by me.
     
  13. Jul 25, 2017 #12
    I hear that being a nerd is now acceptable, even fashionable these days...
     
  14. Jul 25, 2017 #13

    scottdave

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  15. Jul 25, 2017 #14
    New versions are still made, I am not sure about the no-calibration comments however... We used to have ours professionally calibrated. THIS one on Amazon - I like the "Certification Met = certified frustration-free" -- !
     
  16. Jul 25, 2017 #15

    dlgoff

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  17. Jul 25, 2017 #16

    jim hardy

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    Just so you're aware - old timey electricians using those things crank them with great vigor. They laughed out loud at my gingerly pace.
     
  18. Jul 25, 2017 #17

    OCR

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    In my case, I have no resistance to pain, so consequently ... resistance is futile !! .. :-p .. lol
     
    Last edited: Jul 25, 2017
  19. Jul 25, 2017 #18

    OCR

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    Just reading that made me.... lmao-gif.gif



    BTW... a Megger could perform the same tests as a hipot... correct ?
     
    Last edited: Jul 25, 2017
  20. Jul 26, 2017 #19

    jim hardy

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    Both test insulation.. HIPOT tests i've seen were a bit more elaborate usually on larger machines and higher voltage, "...twice rated plus a thousand".
     
  21. Jul 26, 2017 #20

    dlgoff

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    On the order of lightning bolts? Any pictures?
     
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