Evershed and Vignoles 500V hand-cranked megger

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Discussion Overview

The discussion revolves around the Evershed and Vignoles 500V hand-cranked megger, focusing on its functionality, calibration, and historical significance. Participants share their experiences, technical observations, and insights regarding the use of this vintage insulation resistance testing device.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes the physical characteristics of the megger, noting its brass case and Bakelite encasement, and shares initial testing results, including open circuit voltage and short circuit current.
  • Another participant mentions that the device is used for inspecting insulation faults and provides insights from a closed Facebook group, indicating that several users have experience with it.
  • Some participants discuss the expected insulation resistance values for sub-500V installations, suggesting that readings between 1-2 megohms warrant further investigation.
  • Technical details are shared about the internal mechanism of the megger, including the interaction between the current and voltage coils and how it measures resistance.
  • One participant proposes estimating a Thévenin Equivalent circuit based on the short circuit current, discussing the implications of skin resistance when using the device.
  • Several participants express admiration for the simplicity and practicality of the megger, highlighting its lack of batteries and electronic components.
  • Concerns are raised about the calibration of the device, with one participant suggesting that the meter needle may not settle correctly due to the absence of a control spring.

Areas of Agreement / Disagreement

Participants generally agree on the historical significance and practical utility of the megger, but there are varying opinions on its calibration and the expected resistance values for insulation testing. The discussion remains unresolved regarding the calibration status and specific operational characteristics.

Contextual Notes

Some participants mention the limitations of their observations, such as the lack of calibration tips and the variability in insulation resistance readings based on different conditions.

Who May Find This Useful

Individuals interested in vintage electrical testing equipment, insulation resistance testing, and those seeking insights into the operation and historical context of the Evershed and Vignoles megger may find this discussion valuable.

Guineafowl
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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.
 
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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.
 
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scottdave said:
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.
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.
 
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Guineafowl said:
Yes, it is the precursor to the Meggers in use today for insulation resistance testing...

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.
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.
 
scottdave said:
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.
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.
 
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.
 
scottdave said:
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.
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.
 
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What a beauty !
And SOOO practical . No batteries, no bleeping computer, no problem !
 
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Guineafowl said:
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.
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
 
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  • #10
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.
 
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  • #11
Guineafowl said:
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.
jim hardy said:
What a beauty !

:oldlove::oldlove::oldlove:

Bold by me.
 
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  • #12
dlgoff said:
:oldlove::oldlove::oldlove:

Bold by me.
I hear that being a nerd is now acceptable, even fashionable these days...
 
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  • #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
Just so you're aware - old timey electricians using those things crank them with great vigor. They laughed out loud at my gingerly pace.
 
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  • #17
scottdave said:
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
as well as resistance to pain.
In my case, I have no resistance to pain, so consequently ... resistance is futile ! .. :-p .. lol
 
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  • #18
jim hardy said:
...old timey electricians using those things crank them with great vigor. They laughed out loud at my gingerly pace.
They laughed out loud at my gingerly pace.
Just reading that made me....
lmao-gif.gif

BTW... a Megger could perform the same tests as a hipot... correct ?
 
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  • #19
OCR said:
BTW... a Megger could perform the same tests as a hipot... correct ?

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".
 
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  • #20
jim hardy said:
HIPOT tests I've seen were a bit more elaborate usually on larger machines and higher voltage, ...
On the order of lightning bolts? Any pictures?
 
  • #21
dlgoff said:
On the order of lightning bolts? Any pictures?

I wish i did have some...
Highest voltage in our equipment was the 22KV generator, isophase and and primary side of transformers.
I don't know how the T&D folks check the high side , never got to watch them test one just saw their truck parked by the transformer..

I did see High Voltage Insulators tested once in a factory.
A. B. Chance Co has a million volt VanDeGraff generator that will flash them over.
Senior year we got a field trip to their lab in Centralia Missouri and they threw some lightning bolts for us.
Here's a picture of one from their brochure at https://hubbellcdn.com/ohwassets/HPS/pdf/BR00098E-Hubbell-Labratories-Capability-Guide.pdf
Chance_Vandegraff80%.jpg

It's an honest bolt complete with ear splitting "Crash" and the metal building reverberates .

Sorry i don't have any photos of our hipot setup. But it is very dull compared to above.

old jim
 
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