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Author Topic: Proper way to use a "megger"?  (Read 9478 times)
WY3X
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Posts: 768




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« on: August 12, 2003, 01:56:15 PM »

I just bought a Chinese manufactured meg-ohmmeter to check my station's ground system. Basically, it's a hand crank generator that creates a 1000V potential, and there's a meter that registers resistance in meg-ohms. What is the "proper" way to test a ground system? In other words, how far away should I place the "test" ground rod on the "plus" side of the meter from my existing ground system? I'm assuming the smart thing to do would be to disconnect all my radio gear from the ground system while the test is being conducted- so this would go without saying. Is there a "procedure" that should be followed?

On another front- since this thing generates 1000V potential, how effective do you think it would be at
driving out moles from someone's yard <SNICKER-EVIL-GRIN>? Or procuring worms for fishing? Anybody ever do this? How long does it take for the worms to start surfacing? If so- how do you use it to make the worms surface? (How far apart and how long should the rods be that go in the ground?)

73, -Web
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WB2WIK
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« Reply #1 on: August 12, 2003, 02:16:39 PM »

I think using a megger to check ground conductivity is a misapplication.

Meggers are usually high voltage hand-crank generators used to test dielectric breakdown of insulators, and not used to test conductors at all.  I would not expect to find earth conductivity to be in the "megohms."

WB2WIK/6
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WY3X
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Posts: 768




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« Reply #2 on: August 12, 2003, 02:37:49 PM »

OK- I bought this with the specific use of testing earth
grounds, and the instructions that came with it state:

4. When making "Earth" test, connect the object terminal
to "Line" and a good earth wire to "Earth" terminal.

This is as specific as it gets. It makes no mention of
the distance between the ground rods (that I would
anticipate using for this test). The seller did state
that it could be used to test station ground systems,
which is why I bought it. I hope I haven't been
mislead...?
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N4GI
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Posts: 56


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« Reply #3 on: August 12, 2003, 02:40:19 PM »

Nope.  A megger is indeed used to check ground systems.  See the link below for more info (or where to get it).

 
http://www.contractor-books.com/EX/GRT_Main.htm



73,
Blake N4GI
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W3JJH
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« Reply #4 on: August 12, 2003, 02:49:49 PM »

Drive in a ground rod and connect it to one of the megger's leads.
Drive in a second ground rod 5 to 10 m away and connect the other lead to the second rod.
Turn the crank and measure the resistance.
Compute the ohms/m.

Move the second rod to another location 5 or 10 m away from the first and repeat the measurement.

Average the values.

If you get widely varying readings, take several more data points to get a better average.
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KA5S
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Posts: 229




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« Reply #5 on: August 12, 2003, 02:56:35 PM »

The "Megger" (trademarked term) is designed to measure insulation resistance in the megohm range (thus the name and the voltages used), and NOT earth conductivity, where resistance is very much lower than what a "megger" is designed to measure.

"Jerry Sevick, W2FMI in the March 1981 issue of QST has described a simple method to measure ground conductivity, which requires a 100 watt electric
light bulb, a 14.6 ohm 5 w resistor, four small probes and an ac voltmeter. This method has been used by Doty, Frey and Mills, QST February 1983, who found problems with ground currents, which was resolved by adding an isolation transformer.  The measurement is made at the power line frequency
(50 or 60 Hz)."
--John Belrose, VE2CV, quoted in a post on the Internet

The method is described in at least some of the books of Sevick's writing, such as "Transmission Line Transformers."

THIS IS VERY DANGEROUS. Don't try to reinvent it. And when or if you do perfom this test, use an isolation transformer.

Have fun -- and stay safe.

Cortland
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WB2WIK
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« Reply #6 on: August 12, 2003, 02:56:58 PM »

Nope, you guys are still wrong, although you can believe whatever you wish!

The hand-crank megger product is intended for use as a ground fault tester, hipot (high potential) tester, or dielectric breakdown tester and definitely *not* an "earth ground conductivity" tester.  If your earth ground conductivity is in the Megohm range, you've got a very serious problem.

The instructions are referring to checking for ground fault within electrical or electronic equipment.   You connect the megger between, say, the AC input lead of a piece of electronic equipment, and its chassis (which is referred to as "earth" in most of the world, except for the U.S., which refers to it as "ground").  You turn the crank and generate high voltage, then read the leakage resistance between the AC line input circuit and chassis ground ("earth").  That *should* be in the Meghohms, for sure, otherwise a shock hazard exists within the equipment.

The standard UL 60950 test for this presumes 1500Vac or 2121Vdc is applied and it's called a "hipot" test.  This is the intended application for a megger.  The referenced website and instructions contained therein have absolutely nothing to do with a hand-crank megger or its operation.

WB2WIK/6
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WB2WIK
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« Reply #7 on: August 12, 2003, 02:58:25 PM »

Oops, I was posting while KA5S was also.

KA5S is correct.

WB2WIK/6
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K5DVW
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« Reply #8 on: August 12, 2003, 03:00:19 PM »

Even if you do measure your ground conductivity with this thing, it's not going to tell you a thing about your RF grounding efficiency.

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WY3X
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Posts: 768




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« Reply #9 on: August 12, 2003, 03:35:46 PM »

OK, thanks for all the input- I guess I have myself a relatively expensive worm-wrangling machine. This whole thing started because I was getting reports of RF on my signal. I disconnected the ground from my rig, and the problems disappeared. So I assumed that I had a problem with the ground system. I found the ground wire from my electric meter base rusted away from the ground rod recently, and put it back on with a new clamp. This may have solved my problem, but I have not tested it since. I'm going to wait for cooler weather before I check anything anyhow, so I have plenty of time, say, a couple or three months, before I dig into the problem. I also found out (after posting this) that a friend of mine who works for a tower company has some fancy clamp-on ground testing meter. I will probably beg his assistance one day to check my system with it.

Thanks, -Web
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W3JJH
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« Reply #10 on: August 12, 2003, 03:50:05 PM »

ANSI/IEEE 80, ANSI/IEEE 81, the applicable MIL-STD, and the National Electrical Code all specify that the usual means of measuring ground resistance is with a "Megger."

Normally, a low-range (10- to 100-ohm) 1000-V megger is used, but it is possible to make crude measurements with a megger insulation tester.
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N4GI
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« Reply #11 on: August 12, 2003, 03:53:08 PM »

<<Nope, you guys are still wrong, although you can believe whatever you wish!>>


My link referred to a "megger" instead of a "hand crank megger".  I appologize for attempting to assist.  Perhaps pointing out that simple error would have sufficed?  (Of course not)

This web page and its trolls never ceases to amaze me.

blake  
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K5DVW
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« Reply #12 on: August 12, 2003, 03:53:24 PM »

Sounds more like an antenna problem than a ground problem. Grounding should only be designed for lightning to have a discharge path.

RF grounding is entirely different, and depending on the type of antenna you're using, may not even be necessary. For instance, a dipole needs no RF ground, but a vertical needs one (radials).

Since you have a problem, perhaps you can tell us a bit about your antenna system? Maybe we can help with the real problem?
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WB2WIK
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« Reply #13 on: August 12, 2003, 04:37:37 PM »

KR4WM, I think a megger is a cool thing to own, anyway.  It is useful, especially in checking out older gear ("boat anchors") which are likely to have worn out AC bypass caps, etc.  Useful for checking homebrew gear, too, if you ever do any building.  I have a 30 yar-old James G. Biddle megger (I think that's the company that trademarked the name "megger"), the darned thing still brings about $300 on the used market if you can find them.

Your improvement in shack RF problems when disconnecting the ground isn't the first case I've heard of that, either!  You might want to call in the local electric utility company on this one, since they will be concerned about any problems with your service ground and will likely correct it for you for free.

WB2WIK/6
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KE4DRN
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« Reply #14 on: August 12, 2003, 08:51:37 PM »

Hi,

From http://www.uspto.gov

the Trademark search engine

Word Mark MEGGER
Goods and Services IC 009. US 026. G & S: ELECTRIC TESTING APPARATUS FOR TESTING INSULATION AND OTHER RESISTANCE. FIRST USE: 19030525. FIRST USE IN COMMERCE: 19030525
Mark Drawing Code (1) TYPED DRAWING
Serial Number 71017497
Filing Date March 3, 1906
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