# Megger how to



## MDShunk

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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## JohnJ0906

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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## MDShunk

JohnJ0906 said:


> 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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## JohnJ0906

No ground pin.


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## JohnJ0906

And what should the readings be?


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## MDShunk

JohnJ0906 said:


> And what should the readings be?


Greater then 100 megohms


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## brian john

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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## JohnJ0906

Thank you both! :thumbsup: 

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

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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## MDShunk

brian john said:


> 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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## frank

One other thought John. Keep your fingers away from the terminal ends. 'OUCH!


Frank


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## knothole

Every good fisherman needs a megger to use when the dynamite is wet and the old crank phone is at home. :laughing: :whistling2:


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## brian john

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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## kidone

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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## Mike Guile

*Megger*

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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## brian john

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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## John Valdes

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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## kidone

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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## electrictim510

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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## Frank Mc

John Valdes said:


> 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...???

Frank


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## macmikeman

Frank Mc said:


> Arent you guys required to megger new residential installations...???
> 
> Frank


Not generally. I'll check anything suspect though. And it is a good resi troubleshooting tool too, not just motors.


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## electrictim510

Frank Mc said:


> Arent you guys required to megger new residential installations...???
> 
> Frank


Not here.


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## Bkessler

Frank Mc said:


> Arent you guys required to megger new residential installations...???
> 
> Frank


Nope, A city official comes and looks at all the work and approves it based on a quick walk through before the drywall is up, and then again before the dwelling is occupied. I had electrician from Oz help me on and off for a year or so while he was here on a two year vacation. It's was a lot of fun learning the difference between the way we do things. And I got a great worker who worked for beer money and just out of his curiosity on how we do it here. After he finished his apprentiship in Melbourne he worked in a canadian oil field for six months, saved up money and came to CA for a year, worked with me and surfed, went to London worked for six months at down under electric and came back and worked with me again. Then took a hot mexican girl back with him to Oz. What a life, I was jealous.


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## Frank Mc

Bkessler said:


> Nope, A city official comes and looks at all the work and approves it based on a quick walk through before the drywall is up, and then again before the dwelling is occupied. I had electrician from Oz help me on and off for a year or so while he was here on a two year vacation. It's was a lot of fun learning the difference between the way we do things. And I got a great worker who worked for beer money and just out of his curiosity on how we do it here. After he finished his apprentiship in Melbourne he worked in a canadian oil field for six months, saved up money and came to CA for a year, worked with me and surfed, went to London worked for six months at down under electric and came back and worked with me again. Then took a hot mexican girl back with him to Oz. What a life, I was jealous.


LOL!!!......I would imagine half the battle for him would have been the terminology for different things...??..Here in Oz we are required to do several tests before energising circuits...
Frank


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## brian john

Frank Mc said:


> LOL!!!......I would imagine half the battle for him would have been the terminology for different things...??..Here in Oz we are required to do several tests before energising circuits...
> Frank


 
They do testing here it is called a smoke test.


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## Bkessler

Frank Mc said:


> LOL!!!......I would imagine half the battle for him would have been the terminology for different things...??..Here in Oz we are required to do several tests before energising circuits...
> Frank


Yup, learning all the "bits and pieces" was the tough part for him. That and being on time when there was good surf in the morning.


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## macmikeman

Bkessler said:


> Yup, learning all the "bits and pieces" was the tough part for him. That and being on time when there was good surf in the morning.


Damn them surfers :laughing: When I used to have a crew and it was a big winter day on the North Shore, I'd paddle out at Sunset only to find about half the crew was already out there playing hookey. It was an unspoken ok thing to do with me, cause after all thats why we all were living there for in the first place.


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## Bkessler

macmikeman said:


> Damn them surfers :laughing: When I used to have a crew and it was a big winter day on the North Shore, I'd paddle out at Sunset only to find about half the crew was already out there playing hookey. It was an unspoken ok thing to do with me, cause after all thats why we all were living there for in the first place.


It was like that when I lived in the USVI, surfers could come in late when there was good surf with no questions asked.


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## electrictim510

Whats the deal with the Extech 380260 vs the NIST certified one? Whats the big difference?


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## Big John

electrictim510 said:


> Whats the deal with the Extech 380260 vs the NIST certified one? Whats the big difference?


 NIST certified shows the tester meets a standard of accuracy. 

If you're doing compliance testing, insurance inspections, anything that might have to hold up in court, you want to go with NIST.

-John


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## wildleg

Frank Mc said:


> Arent you guys required to megger new residential installations...???
> 
> Frank


nope


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## Zog

electrictim510 said:


> Whats the deal with the Extech 380260 vs the NIST certified one? Whats the big difference?


95% of my customers will not accept a test report without the test equipments NIST calibration certificate attached.


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## jefft110

electrictim510 said:


> 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.


If it already isn't, with all the new afci requirements, a megger is going to be a Resi service tech's best friend.


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## Flytyingyaker

I bought the same megger that the OP posted. I have played with it a little but I really want to learn how to use it proficiently before I try to trouble shoot a customers home or business. 

I have a few questions and I can t find the answers on here. I have looked. 

Say you are megging a 12-2 uf circuit underground feeding a light pole. What should you readings be and is there a chart or something for readings of different wire types and sizes?

When I meg a piece of 12-2 thhn to ground and it is undamaged the reading is like 78Mohms whenI meg a messed up piece it is 68 Mohms. Also when I do this the number flashes on the screen and then disappears very fast. Is that normal. 

Could someone help me out thanks.


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## Wilson Lighting

Yes I would like to know what some of these readings should be


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## brian john




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## green light

brian john said:


> *Nominal Rating *
> *Of Equipment in Volts *
> *Minimum Test Voltage, DC *
> *Recommended Minimum *
> *Insulation resistance in Megohms *
> *250 *
> *500 *
> *25 *
> *600 *
> *1,000 *
> *100 *
> *1,000 *
> *1,000 *
> *100 *
> *2,500 *
> *1,000 *
> *500 *
> *5,000 *
> *2,500 *
> *1,000 *
> *8,000 *
> *2,500 *
> *2,000 *
> *15,000 *
> *2,500 *
> *5,000 *
> *25,000 *
> *5,000 *
> *20,000 *
> *34,5000 and above *
> *15,000 *
> *100,000*


 Table 100.1


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## Zog

green light said:


> Table 100.1


And 100.14, reading are pretty much worthless if they are not tempature corrected, something that is often overlooked.


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## green light

Zog said:


> And 100.14, reading are pretty much worthless if they are not tempature corrected, something that is often overlooked.


 Ive looked through the 2011 MTS and I cant find anything about obtaining temp readings. Is the info there and Im not seeing it?:blink: Is it as simple as using a thermometer you buy from the drugstore, or does the thermometer need to be calibrated just like everything else?


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## jefft110

Page 200 in this....

http://www.scribd.com/doc/57348424/Neta-Testing-Specifications


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