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Old 01-11-2009, 10:57 PM   #1
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HI;

Sounds like were going to have someone come in that can do the megger testing for my dad, just to make sure nothing else was damaged.

However, I'd love to have a megger tester for myself. I found some meters like this on e-bay:

http://preview.tinyurl.com/9m4qwb

This is another meter I found that is much less expensive than the fluke megger, and I wonder if it might be useful or if you see any major problems with the inexpensive meters like this.

http://preview.tinyurl.com/757eu9
Would a megger like that allow me to do insulation tests on most residential wires? I am unclear at this point as to what voltages you want to be able to run down romex or THHN for the megger test.

Can you steer me in the right direction for more information about meggering?

Thanks

Jamie

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Old 01-12-2009, 12:20 AM   #2
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I hate digital meggers\

but definately hang on for other opinions.

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Old 01-12-2009, 01:53 AM   #3
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The term Megger (originally a Brand name) and Hi Pot testers do the same thing. Hipot - Wikipedia, the free encyclopedia

Jamie, I couldn't open your links. Did you mean the Fluke 1577?


Harbor Freight does have a Cen-Tech 42397-1VGA Digital Peak-Hold Clamp Meter that claims a 500V Insulation test.
HF Clamp Meter w-insul test


Let us know if you find a megger at a reasonable price.
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Old 01-12-2009, 07:46 AM   #4
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Just keep in mind a megger isn't the last word on insulation. There can still be latent risks and failed insulation that is just waiting to be wiggled or heat up which shifts it into an arc fault. I know this isn't helping you sleep any better at night but I figured I should mention it.
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Old 01-12-2009, 09:58 AM   #5
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I like the AVO meggers.
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Old 01-12-2009, 01:09 PM   #6
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Quote:
Originally Posted by PaliBob View Post
The term Megger (originally a Brand name) and Hi Pot testers do the same thing. Hipot - Wikipedia, the free encyclopedia

Jamie, I couldn't open your links. Did you mean the Fluke 1577?


Harbor Freight does have a Cen-Tech 42397-1VGA Digital Peak-Hold Clamp Meter that claims a 500V Insulation test.
HF Clamp Meter w-insul test


Let us know if you find a megger at a reasonable price.

http://cgi.ebay.com/New-Megger-insul...3%3A1|294%3A50


And

http://cgi.ebay.com/Digital-Insulati...3%3A1|294%3A50


Are the 2 links. I hope those work for you.

I have that harbor freight meter. After reading through the manual, I found the line "with optional insulation test accessory" However I am unable to locate such an item in harbor freights inventory.

The 1577 was one of the fluke meters I was taking a look at.

Thanks
Jamie
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Old 01-12-2009, 07:01 PM   #7
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Megger


Quote:
Originally Posted by jamiedolan View Post
I have that harbor freight meter. After reading through the manual, I found the line "with optional insulation test accessory" However I am unable to locate such an item in harbor freights inventory. Jamie
I have a very similar meter: HF - Clamp-On-DMM
Mine was $9.99 on sale. I bought it because it has two switch positions labeled External Input for model 261 Insulation Tester. I could not find the option. When I saw the specs on yours I thought that for the extra bucks HF would include the option. The HF spec sht claims 500V Insulation Test, but as you found out they are wrong.

My guess is that they must have had a working 500V accessory but the HF Lawyers nixed the Sale.

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Old 01-12-2009, 08:16 PM   #8
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Megger


I like the analog meggers much better. A bottom line one (which will work just fine for your application) will run around $150-200.

It might be possible to rent one though.

Rob
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Old 01-12-2009, 08:37 PM   #9
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If you are going to get a meter with the optional insulation test function then I would stay away from a meter without a CAT3 or 4 rating for safety purposes. Cheap meters could get you melted.
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Old 01-13-2009, 02:29 PM   #10
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and you can solder, you can make a megger with a 120v/240v supply and a 1/8A or 1/16A fuse.
If the fuse blows when this tester is connected to the (open-circuited) cable under test, the insulation failed to withstand 170v/340v peak voltage.

You can get more sophisticated by putting an AC milliammeter in series with the fuse. >1 megohm insulation resistance should pull <120/240 microamps.
If your meter won't read that low, put a 100k resistor in series with the fuse and measure the AC voltage across the resistor with a DVM. In this case, less than 11v/22v means >1 meg.

If you read zero and you're not sure whether it's due to good insulation or a test problem shunt the cable with a 10 megohm resistor and check for 1.1v/2.2v across your 100k resistor.

With a neon sign xformer or an electronic air cleaner power supply, you can go to even higher voltages (but you want less than 600v peak). An HVAC contractor will probably give you a scrapped air cleaner supply.

Above 600v can puncture your skin.
Been there, done that, it smells bad and leaves a tiny hole that eventually heals.

Last edited by Yoyizit; 01-13-2009 at 03:03 PM.
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Old 01-13-2009, 04:14 PM   #11
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It would be hard to get a neon sign transformer to run as low as 600v.

You'd do better using a 24v transformer run backward. If its insulation can take it, 120v in gives you 600v out. Probably want to feed it with a voltage divider to cut it down to 500v and limit current, as well as all the other safety precautions a project like that would require.
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Old 01-13-2009, 09:02 PM   #12
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Quote:
Originally Posted by Gigs View Post
It would be hard to get a neon sign transformer to run as low as 600v.
You'd need a dropping resistor.

You'd do better using a 24v transformer run backward. If its insulation can take it, 120v in gives you 600v out. Probably want to feed it with a voltage divider to cut it down to 500v and limit current, as well as all the other safety precautions a project like that would require.
Yeah, I think the insulation of any power xformer is rated for 1500vrms.

This has got to be the cheapest and simplest megger design ever, and most likely to work as soon as it's built.

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Old 01-13-2009, 11:17 PM   #13
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If you put 120 volts across the 24 volt winding of any transformer, it'll burn up in just a couple of seconds. The reason is because there isn't enough iron in the core to handle the voltage applied. The core saturates, and can't generate enough counter-EMF, so it heats up rather quickly....POOF!

The 1500 volt rating is called a BIL (basic impulse level) rating. It is the highest spike that the transformer can take without sparks jumping to adjacent winding turns.

Rob
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Old 01-14-2009, 02:00 AM   #14
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Quote:
Originally Posted by micromind View Post
If you put 120 volts across the 24 volt winding of any transformer, it'll burn up in just a couple of seconds. The reason is because there isn't enough iron in the core to handle the voltage applied. The core saturates, and can't generate enough counter-EMF, so it heats up rather quickly....POOF!
It's something to watch out for, but we only need a small amount of current, that's why I recommended a resistive voltage divider in front of it, which will also limit the current and prevent any overheating.

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