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Old 06-29-2007, 06:39 PM   #1
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Default Megger how to

Well, I finally broke down and bought a megger. From what I have heard from you all, its a necessary troubleshooting tool. From what Marc and Brian have posted, I have some of the basics, but does anyone have any tips for a new user? Any recommended reading?

I will be using it mostly residential and some commercial.

Extech 380260

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Old 06-29-2007, 07:32 PM   #2
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Wow, nice megger.



A couple of quick things off the cuff:

In commercial, use it to megger out a new feeder you just pulled in. Measure each wire to each of the other wires, and each wire to the metal conduit. You want basically infinity readings.

If you're reusing something like a disconnect or a section of busway, megger each lug or buss tang to each other and to the case of the equipment.

Since yours has a selectable voltage, use twice operating voltage for your test, but no more. On the 277/480 equipment, use the 1000v scale, and on 120/240/208 stuff use the 500v scale.

You can meg motors from each motor pigtail to the motor case. Anything less than 20 megohm is a junk motor, and anything below 50 is on its way out, in my opinion.

Make sure you remove any loads on a circuit before you megger any circuits hot to neutral, lest you blow something up.

If you have a nuisance tripping GFCI, you can remove the load side conductors and megger the hot to neutral and hot to ground to see if you have "leakages" that are tripping the GFCI for good reason. Same with AFCI's

Think of the megger as a fancy ohm meter that throws some juice to the lines. Anytime you're having troubles that might be an intermittant short (such as with motors) or low level shorts that only show up in the presence of voltage (such as damp wiring that will nuisance trip GFCI's), the megger is the tool to prove that out.

There's a million and one uses, and my list is by no means complete. Maybe I should write a short essay on using one for troubleshooting one day.

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Old 06-29-2007, 07:43 PM   #3
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As I recall, you need to remove any dimmers, GFCI receptacles, etc before testing. Unplug everything . Do you remove lamps from fixtures?

What spurred this is an intermittent AFCI trip. (Vacuum) I'd like to show this HO that our wiring is good... or find the problem!
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Old 06-29-2007, 08:27 PM   #4
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Quote:
Originally Posted by JohnJ0906 View Post

What spurred this is an intermittent AFCI trip. (Vacuum) I'd like to show this HO that our wiring is good... or find the problem!
The vacuum one will be easy, I think. I've found a few faulty vacs already. Take the vac cord and megger from the hot pin to the ground, and record the reading. Measure from the neutral pin to the ground, and record the reading. Vacs have "universal motors", Lamb style motors, with carbon brushes. They get carbon tracking inside. I'm sure you'll find a problem when you check the vac that way. DO NOT megger from hot to neutral at the vac cord, or you might mess the vac up.
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Old 06-29-2007, 08:32 PM   #5
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No ground pin.
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Old 06-29-2007, 08:33 PM   #6
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And what should the readings be?
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Old 06-29-2007, 08:35 PM   #7
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Quote:
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And what should the readings be?
Greater then 100 megohms
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Old 06-29-2007, 08:41 PM   #8
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I was hoping to post some pics BUT my high speed is runing slower than dial up at the moment.

Go to the AVO International web site, and see if you can still download the publication a Stich-In-Time.

The biggest thing I see with new megger owners is:

OK get it over grab both leads and turn it on, now that you have been shocked, be careful and don't do that again.

If the readings seem low or a dead short investigate the circuit for connected loads, UTILITY metere are often a problem (I have sen this time and time again.)

If the readings seem too good make sure the leads are fully intact and plugged in. Seen this also.

Test your megger, leads apart and leads together.

Use paper and scribble with a soft lead pencil on the paper and megger the paper. Yopu'll have to play with this, with different amounts of lead and scribbles you'll get varying readings. Be careful doing this under a smoke detector.


Use a good case.


FINALLY GOT IT, double lick on the pic

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Old 06-29-2007, 08:45 PM   #9
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Thank you both!

Of course, it's your fault anyway, you both keep mentioning meggers, so I just had to go get one!

When I get it (ordered it on-line) I'm going to play with it a little before I try it on the job.
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Old 06-29-2007, 08:47 PM   #10
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Quote:
Originally Posted by brian john View Post
If the readings seem low or a dead short investigate the circuit for connected loads, UTILITY metere are often a problem (I have sen this time and time again.)
Never really thought about that. Thanks!

I've never run into that myself, because when I'm meggering out a circuit from the panel, I lift all the conductors from where they're connected in the panel and do them that way. I guess maybe that's why I never ran into the meter causing a problem.
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Old 06-30-2007, 03:36 PM   #11
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One other thought John. Keep your fingers away from the terminal ends. 'OUCH!


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Old 06-30-2007, 04:55 PM   #12
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Every good fisherman needs a megger to use when the dynamite is wet and the old crank phone is at home.
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Old 07-01-2007, 08:27 AM   #13
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knothole:

There is a stream near Ft Belvoir, Virginia (where I grew up) where they are shocking everyday. SNAKEHEADS 100's of the fish which seem to be prolific.

Marc: In commercial properties around here tenant owned sub meters are often tapped ahead of the main (NEC violation, no OCP).
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Old 03-22-2010, 02:07 AM   #14
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When i find myself trying to megger something i always verify my ground against two different points to insure i have a good ground. then test the circuit. Make sure you know exactly what you are testing you can actually damage some solid state devices......

Typically when you pull the megger out you have a pretty good idea of what you want to test... I always make sure i isolate everything..

but what do i know
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Old 03-22-2010, 08:10 AM   #15
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I just bought the extech as well. Haven't read anything under infinity yet. Been going around house testing things. Have some old 40's wiring to check this week, I'm sure I'll read something bad on that job.
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Old 03-22-2010, 08:40 AM   #16
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A good practice for a new house would be.

Megger each individual branch circuit after switching and plugging (disconnects smokes and be sure all appliances are off or disconnected). Megger the neutrals to ground with the neutral bond disconnected
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Old 03-22-2010, 12:55 PM   #17
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Thats alot of money to spend on a tool that gets used mostly in industrial work. Motors to be exact. Nice megger though.
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Old 03-25-2010, 01:40 AM   #18
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Thats not true a megger is used for other things besides motors.
It will be used alot more in industrial but it is still a good tool
Bus work.. Feeders.. cords..
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Old 12-06-2010, 01:42 AM   #19
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I'll be investing in a megger soon also. Never used one since I am a resi service tech but the more in your arsenal the more options you have. Thanks for the good reading guys.
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Old 12-06-2010, 02:09 AM   #20
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Quote:
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Thats alot of money to spend on a tool that gets used mostly in industrial work. Motors to be exact. Nice megger though.
Arent you guys required to megger new residential installations...???

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