[Test-Equipment] Megger, high voltage probe

[Dave Brown]

 A megger in modern parlance is just a specialised ohm meter, 
primarily intended for measuring high resistances, (insulation 
resistances in the order of megohms and tens of megohms) and as such 
it uses a higher than normal test voltage and has high resistance 
ranges, typically from a few hundred Kohms upwards.

E&V put their tradename 'Megger' on  quite a few products that were 
not actually insulation testers- what you have is probably not a high 
voltage insulation tester.  The name in the lid almost implies this, 
but without being able to read the fine print I'm not sure.
You can check the test voltage with another multimeter to see just 
what it is. If the megohms ranges don't go up into the tens of megohms 
the test voltage may not be very high.

 The instrument you have acquired looks like  the older case style 
from that particular maker. If it was an insulation tester I would 
have expected it to have a hand cranked generator rather than a 
battery. I think that battery driven dc-dc converters for the high 
test voltage source first came out in E & V meggers in a newer case 
style than that one.


 The high resistance ranges on general purpose meggers most often use 
a test voltage of 500 or 1000 volts - it's usually written on them in 
an obvious position. Teleco versions typically use 200 volts.  Older 
meggers had a built in hand cranked generator to supply the test 
voltage-modern ones have a dc-dc converter to supply the high voltage 
from a normal battery of 6 or 9 volts etc.

Most meggers also have a low resistance range as well so that very low 
resistances, such as those encountered in the earthing bonds etc of 
electrical installations can be accurately measured. This range (or 
ranges) will typically measure resistances from 0.1 to 10 ohms. 
Modern meggers usually have a kohm range as well.  The battery usually 
provides the test voltage directly for these lower resistance ranges.

I'd be wary of using that HV probe on any really high 
voltages-definitely a 'hands off' test scenario in the first instance!

Cheers
 DaveB, NZ

----- Original Message ----- 
From: "Rasputin Novgorod" <priapulus at yahoo.com>
To: <Test-Equipment at mailman.qth.net>
Sent: Monday, July 11, 2005 7:28 AM
Subject: Re: [Test-Equipment] Megger, high voltage probe


> Hi Guys:
>
> I found some interesting things at a Hamfest this weekend,
> for which I hope you can give some advice.
>
> 1) Megger
> I got a Evershed & Vignoles, London, Megger for $30, see photo:
> http://www.ontariorocks.ca/temp/megger.jpg
>
> It takes a 4 volt (!) battery (I'll try 3 "C" cells), and
> is in a Bakelite box in a leather case. It has a "scale"
> switch (high & low) plus a trigger button on one of the probes.
> The instructions show touching the probes to the terminals
> of an electric iron (for flattening clothes) and touching the
> trigger button to read the ohms.
>
> a) What is the difference between a Megger and ohm meter?
>
> b) I've been told to use a Megger to test soil resistance
> between two ground rods in an antenna system; resistance below
> 5 ohms is good. I've also been told that you use a megger to
> test insulation resistance in cables and antennas. Can anyone
> shed some information about this?
>
> 2) High voltage probe
> I bought this high voltage probe for $20; see photo;
> http://www.ontariorocks.ca/temp/probe.jpg
>
> There are no markings on the probe, except a piece of
> tape with "137 times" written on it. I was told to swap
> this with my VOM probe and I could read high voltages.
> I was to take the VOM reading and multiply it by 137.
>
> Is anyone familiar with this thing? I'd like to measure
> a 1600 vdc powersupply and a 3700 vdc amplifier plate.
> Will the probe work on DC and AC?
>
> 3) Electro-Capacitor Sub Box
> I bought this gorgeous Bakelite electrolytic cap box, see:
> http://www.ontariorocks.ca/temp/cap.jpg
>
> No questions, just bragging; I got it for $1. It has four
> values: 10, 20, 40 and 80 uF @ 450 vdc, each individually
> switched to give a range of 10-150 uF. Given that Electro-caps
> are the first thing to go in old gear, this should be very
> useful for troubleshooting. It had 4 calibration stickers;
> what idiot would send a cap sub box like this out for calibration?
> (I'll check the caps in the box before using...)
>
> Sincerely
> /b
>
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