# OK, I'm getting a megger



## gilbequick (Oct 6, 2007)

You guys have me convinced this is a valuable tool to have. Looking back it would have helped out with troubleshooting many times.

The VAST majority of the work I do is residential. Do I need a megger that will do 250v, 500v, 1000v? Using NM, which voltage would you use? Why would you use different voltages for the same type of wire? What features do you look for when purchasing one of these?

There was a megger in another threat that was about $200 that was recommended. Is this a good all around unit? What would a nicer megger do that this one can't? http://www.professionalequipment.com/extech-autoranging-digital-megohmmeter-380260/megohmmeter/

If you can't tell I'm pretty oblivious to these things, so any other little helpful bits of information would be great!


----------



## itsunclebill (Jan 16, 2007)

Meggers are somewhat expensive tools that do a single task in many cases. People in many areas of the electrical industry use them and it makes sense to design one tool that will test the insulation used in several kinds of applications rather than a single one. 

Since NM-B and THHN (and similar) wire has insulation rated at 600 volts you would use the setting closest to this without exceeding the rating of the insulation - 500 volts with the meter in the link.

If all you are doing is troubleshooting residential work and the occasional commercial problem you likely won't need a meter that has the ability to print graphs directly or from a computer interface. Techniques of testing include timed tests in some cases and how much you spend will make the difference between watching the seconds on your watch and setting a timer to start and stop the test. I have an old clunker that tests at 500 volts and runs off 2 big lantern batteries. I'd still be using it on jobs but have gotten spoiled by the smaller sizes of newer equipment and the somewhat more accurate readings of a digital readout.

Given the hourly rates you can justify a $200.00 meter for your purposes easily. I'd be hard pressed to justify a $3500.00 meter to do the same job. Whole 'nother deal if you have customers requiring documentation.


----------



## TOOL_5150 (Aug 27, 2007)

gilbequick said:


> You guys have me convinced this is a valuable tool to have. Looking back it would have helped out with troubleshooting many times.
> 
> The VAST majority of the work I do is residential. Do I need a megger that will do 250v, 500v, 1000v? Using NM, which voltage would you use? Why would you use different voltages for the same type of wire? What features do you look for when purchasing one of these?
> 
> ...


\

I think I am repeating myself [probably that other mentioned thread] but the megger you have a link to is great for cable insulation verification. I own it and have used it a few times. The little manual that comes with it tells you how to do the regular tests easily and safely. Im happy with it and it even comes with a case so you dont have to worry about your investment getting messed up while in the tool bag.

~Matt


----------



## ElectricianJeff (Nov 19, 2007)

gilbequick said:


> You guys have me convinced this is a valuable tool to have. Looking back it would have helped out with troubleshooting many times.
> 
> The VAST majority of the work I do is residential. Do I need a megger that will do 250v, 500v, 1000v? Using NM, which voltage would you use? Why would you use different voltages for the same type of wire? What features do you look for when purchasing one of these?
> 
> http://


My situation is identical to the above except I already own the meter referenced. Wife bought it for me for my birthday when I couldn't come up with anything else I needed.

I was alittle disappointed with the lack of specific information in the manual. I would be very appreciative of any specfic advice on the use of the meter when testing or troubleshooting that might be offered here.

Thanks,
Jeff


----------



## Mountain Electrician (Jan 22, 2007)

Most of what you will be doing is go/no go testing, i.e. determining if insulation has failed. Very simple, quick and easy. This link is to Fluke's application notes, which are an excellent source of information. Click on "Insulation Resistance Testing" to read their training article. It gives you what voltages to test at, different testing methods, etc. Good info for anyone.

http://us.fluke.com/usen/support/appnotes/default?category=AP_INSUL(FlukeProducts)&parent=APP_NOTES(FlukeProducts)#

A megger is a valuable tool in any type of electrical work, and used properly can save you much time and many headaches.


----------



## John Valdes (May 17, 2007)

Jeff, What options do you have on the face of the megger? Voltage and ohms right?
What are you planning to use this megger for. Are you in industry or construction?
Meggers are a very valuble tool. However, in the construction industry they are rarely used. In industrial work it is a must have tool.
Practice makes perfect as they say. Use the voltage settings for the wire voltage, or if for equipment use caution as you can do harm. The megger does output the voltage selected, so be careful as to what you meg out. If you are just looking for insulation breakdown make sure you remove (unhook) the load before applying the megger voltage. If your megging a motor, use the motor nameplate voltage. Don't meg control circuits unless you are certain that sensitive equipment will not be damaged.
Basically all a megger does is measure voltage between the conductor and ground. Infinity is what everyone wants to see. But in 30+ years I have never seen infinity. Close, but not infinity.


----------



## Mountain Electrician (Jan 22, 2007)

John Valdes said:


> However, in the construction industry they are rarely used.


I disagree. In residential construction they are rarely used, but in many commercial and most industrial construction jobs it is normal to meg all feeders and motor circuits and record the results. A couple of times, I've even had to meg the wire when it arrived from the supplier, and then again after it was installed.


----------



## bobelectric (Feb 24, 2007)

Mountain Electrician said:


> I disagree. In residential construction they are rarely used, but in many commercial and most industrial construction jobs it is normal to meg all feeders and motor circuits and record the results. A couple of times, I've even had to meg the wire when it arrived from the supplier, and then again after it was installed.


 How would you meg out wire on a spool, just shipped in?


----------



## Mountain Electrician (Jan 22, 2007)

bobelectric said:


> How would you meg out wire on a spool, just shipped in?


It was a hi-pot test. Installing shielded 5KV cable for 4160 motors, we would have to preform a field acceptance test of new cable before it was signed for. Somewhere around 20KV for 10 to 15 minutes, I think. After it was pulled, another test. The company we were working for at that time used both shielded and non-shielded cable for their 4160 motors, although I understand now only shielded cable is allowed over 2KV.


----------



## ElectricianJeff (Nov 19, 2007)

John Valdes said:


> Jeff, What options do you have on the face of the megger? Voltage and ohms right?
> What are you planning to use this megger for. Are you in industry or construction?
> Meggers are a very valuble tool. However, in the construction industry they are rarely used.


John,

My meggers in the truck but I beleave voltage is my only option. (250/500/1000) I do mainly service upgrades and troubleshooting residential. Most of the homes I'm in are 40 years + and many are even a 100+. I encounter K & T wiring.

Just yesterday I looked at a property with some wiring that was about 60 years old. The HO is looking to me to clean up some messes he has created (flying splices, etc.), replace wire as needed, add receps, a couple 3-ways etc. I'm headed back there on the 17th to do the job and trying to figure out how to use the megger to test the insulation on the conductors other than my usual "eyeball" method.

I doubt I will be using mine to test equipment but you never know. Thanks to everyone that has responded, I'm new to this forum. I will check the link that was provided and see if that helps.

Jeff


----------



## frank (Feb 6, 2007)

Just my 2 penny worth.

Could not live without a Megger. First thing to do to any cable fault - is test the cable circuit. With the results you have you know what to look for.

When job done - test again at 500/1000 volts and if all clear on all cores it's safe to go home. Easy Peasy. Nice as pie.

I use mine for all insulation testing and for continuity tests for earth connections to equipment via the earthing/grounding point or socket input.

I look for not less than 10 Mohms in the first instance and less than 0.9 ohms in the second.

Frank


----------



## brian john (Mar 11, 2007)

Been gone all week so short response if you need more information post back.

Meggers SHOULD be in every electricians tool box...

I own several 
10-100 VDC
50-1000 VDC Battery
25-1000 hand crank
500-5000 VDC Battery/ AC

600 VAC cable is meggered at 1000 VDC.

Megger a single conductor without a shield is meaningless.

Try AEMC
AVO INternational
Fluke


I can answer any specific questions.


----------



## HighWirey (Sep 17, 2007)

brian john said "600 VAC cable is meggered at 1000 VDC"

In my orbit the 'rule of thumb' for the DC test voltage applied to a cable (120vac to 35kv) or individual conductors was 1.5 x the maximum design rating of the cable, _unless overruled by specification or NETA standards._

brian john further said, "Megger a single conductor without a shield is meaningless"

We were once supplied via GFE (Government Furnished Equipment) a beeg spool of new, dry, fresh, 24 conductor, 14AWG, stranded, non shielded, PVC insulated cable. Profiting from our contractors 'hard learned experience' we megged it at 500 volts _on the spool_, and many of the conductors woefully failed the specified minimum test (some less than 1 megohm, conductor to conductor!) There was nothing else to meg to here gents, no shield, no conduit, no water, no nuttin' honey. This was a class 2, cheeper, faster, better, ty-rap install mentality. We RFI'ed the government about the failed spooled cable, govt contacted their supplier, supplier responded with some *obfuscation, which amounted to a complicated, extrapolated, way over my head, mathematical formula. Their response satisified our engineer (the engineer had his papers, we had our papers, and we were all good-to-go!) _And the supplier did not have to return that beeg spool of cheep cable, which was all he was concerned about!_ Our engineer responded with "install the stuff". The installed cable megged about the same as it did on the spool. Must be about whose ox is being gored, huh? 
Far as I know nothing has burned dup, yet! 

Did not want to get off on a tangent here, however a megger's readings can cause a novice to chase his tail. Meggers are great instruments. I would _never energize a feeder _before megging. 

Just beware how you interpret your results. It takes experience, just like all the stuff an electrician deals with everyday.

Nuff said about wire on a spool . . .

*obfuscation - I stole this word from someone here. I like that word. It fits me. Thank you.

Merry Christmas Everyone, and Best Wishes 
__________________
Be Safe Out there


----------



## gilbequick (Oct 6, 2007)

brian john said:


> 600 VAC cable is meggered at 1000 VDC.
> 
> Megger a single conductor without a shield is meaningless.
> 
> I can answer any specific questions.


What's a shield?...a ground?

On Fluke's directions it says for a go/no go test to take about twice the cable's rated voltage plus 1000 volts. For NM thats 600v X 2 + 1000 = 2200v. Why would you suggest 1000v? Ohhhhh wait, nevermind it says that for AC not DC.

I think I just need to get one of these tools and play with it a bit. 

There are some interesting megger videos on youtube of some fine Brits showing how they can give you a good zap. I wonder how my neighbors dog that likes to poo all over my yard would like it:clap:


----------



## brian john (Mar 11, 2007)

UMMM let me see



> beeg spool of new, dry, fresh, 24 conductor, 14AWG


24 conductor does not qualify as a single conductor, in order for meggering reading to be meaningful you have to have more that one conductor, SINGLE CONDUCTOR with shield MI cable.......


As for 1.5 X

MOST meggers that the STANDARD NORMAL electricians utilize are not variable but fixed voltage the standard for 600 VDC cable is 1000 VDC. Normal settings are steps of 250, 500, 1000 or some 500, 1000, 0r fixed at 1000, few electricians I deal with own meggers that have voltages above 1000 VDC.

As for megger readings causing someone to chase their tail. READ, LEARN, test and you'll be much further ahead than those that don't.


----------



## John Valdes (May 17, 2007)

Jeff, If you only have voltage options no problem. I never use the ohm setting either on my AVO megger.
So you want to verify cable integrity? Great. 
To see what a direct short looks like on your megger, connect both megger leads together and push button. 
Just attach one megger lead (red) to the wire to be tested and the other (blk) lead to the grounded conductor in the cable or conduit. Set the voltage on the megger to approximate wire rating. Press button. The megger is going to measure in megohms. Check to the conduit body and other grounded parts of the system too.
Here is where it gets a little tricky. What is considered acceptable and what is not? As I said in my previous post that infinity (measured when your test leads are not connected to anything) is the desired reading. But not realistic. Even reading open air has some resistance, however miniscule it may be. Good luck and don't touch the leads, wire or conduit when testing


----------



## gilbequick (Oct 6, 2007)

Just picked one up. Well, not really picked up more like ordered and waiting for it to be shipped. Here's the link: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=350003998499&ssPageName=ADME:B:EOIBSA:US:1123 
It the same one as linked to above, just a different color. It looks like Extech has them made in China and these guys are using the same design. EXACT same specs on everything, size, weight, voltages, resistances, display, even the same sound levels. 

Can't wait to test it out on the neighbors dog!


----------



## TOOL_5150 (Aug 27, 2007)

gilbequick said:


> Can't wait to test it out on the neighbors dog!


EEP!  :laughing: 

~Matt


----------



## ElectricianJeff (Nov 19, 2007)

John Valdes said:


> Jeff, If you only have voltage options no problem. I never use the ohm setting either on my AVO megger.
> So you want to verify cable integrity? Great.
> To see what a direct short looks like on your megger, connect both megger leads together and push button.
> Just attach one megger lead (red) to the wire to be tested and the other (blk) lead to the grounded conductor in the cable or conduit. Set the voltage on the megger to approximate wire rating. Press button. The megger is going to measure in megohms. Check to the conduit body and other grounded parts of the system too.
> Here is where it gets a little tricky. What is considered acceptable and what is not? As I said in my previous post that infinity (measured when your test leads are not connected to anything) is the desired reading. But not realistic. Even reading open air has some resistance, however miniscule it may be. Good luck and don't touch the leads, wire or conduit when testing


Thanks, John

That's what I was looking for. A couple of questions:

1. I have read where you lock and test for 2 minutes. Why, I would think a propblem in the circut would be readily apparent.

2. Just what is considered acceptable or not. Do you take readings on known good conductors of similar circuts for instance as a comparsion?

Still confused but making some headway,
Jeff


----------



## brian john (Mar 11, 2007)

One reason for a timed test is due to system capacitance. Long runs of cable and busway take time to fully charge and arrive at an acceptable value. 

1. It depends on the type of testing you are going to do acceptance testing or go no go.
a. In acceptance testing you should have a standard time frame for testing each phase and neutral to ground.
b. With a go no go test it still takes time (USUALLY) for the meter to stabilize.
2. One item to keep in mind is with conductors especially buried conductors needle swing from low to high are usually indicative of moisture in the conduit or cable jacket.
3. Acceptable readings vary with the item under test.
a. With new equipment or conductors one would expect the readings to be full scale, or infinity which means maximum reading for your megger. With another megger with a higher scale this reading may not mean infinity. For example most inexpensive meggers have a maximum reading of 1000 or 2000 megohms, where as some of the more expensive meggers read into the 1000’s of teraohms.
b. Readings should be similar 3 conductors in a conduit should be similar, say your readings are A phase 250 megohms. B phase 260 megohms, C phase 70 megohms. All of these readings are acceptable but C may have issues in the future. Nothing to do now, but this would be noted in any reports.
c. A rule of thumb is a minimum reading for 600 VAC distribution equipment should be 50 megohms. But some circumstances require energizing conductors with lower readings, such as 208/120 VAC at 10 megohms or 480/277 VAC underground coonductors at 5 megohms or above. This is done ONLY in certain circumstances, when the customer is aware of the issue and willing to accept responsibility and KNOWS IMMEDIATE REPLACEMENT IS NECESSARY. But this only comes with experience and as a rule of thumb I would stick with the 50 megohms.


A good start is to turn on the megger, separate the leads and the reading should be full scale/ infinity/ maximum reading, as air is a good insulator (at this voltage) and distance between the test leads. Now touch the leads this is a dead short. Lastly get some paper and a #2 pencil (carbon) scribble on the paper and then megger the paper across the scribbles. You may get varying readings and a possible fire, if not thinner paper or more scribbles.

1.Meggers can give a jolt/shock be careful.
2.If your readings are a dead short before condemning the item under test look for possible loads on the system under test, loads are a dead short.
3.Sensitive electronic loads can be damaged by high voltage.
4.If you megger has several voltage levels start low first.
5.Motors when connected are a dead short from phase to phase if not they have an open winding, you can only megger anyone phase to ground.
6.On new services you should remove the neutral ground bond and megger neutral to ground.
7.With transformers (Delta Wye) you can megger primary to secondary, primary to ground, secondary to ground ONLY if you remove the neutral/XO/grounded conductor to ground bond, sometimes a factory connection sometimes a filed installed connection per the NEC.
8. Always test your tester dead short test open/infinity test.
9. Not sure question your tester and test methods.
10. See chart below picture


DOUBLE CLCIK ON THE PICTURE









*Term**Abbreviation**Value (Scientific)**Value (Normal)*
TeraT1 x 10121,000,000,000,000
GigaG1 x 1091,000,000,000
MegaM1 x 1061,000,000


----------



## brian john (Mar 11, 2007)

More on meggers I wrote this sometime ago and some of this may mirror above

There are a variety of test that can be performed with meggers, but the primary test most electricians utilize is a go no go test, connect the megger record the readings and make a determination if the readings are in an acceptable range

Busway:

Busways should be meggered piece by piece as it comes off the truck. Then piece by piece as it goes up, the assembled busway should be meggered as each piece is added and lastly the completed busway should be meggered.

Feeders:

Feeders should be meggered as soon as they are pulled and after termination.

Switchboards and MCCs:

Switchboards should be meggered as soon as they arrive on site, after installation and prior to energizing. In addition The neutral disconnect link should be removed and the downstream neutral meggered to ground, this verifies the downstream neutral is clear from shorts to ground avoiding net current issues and the resulting EMF, additionally should you lose the neutral connection to the main switchboard the downstream connections will carry the full neutral load, which can be a fire hazard.

Panels:

Panels should be meggered when they are installed and prior to energizing. The branch circuit neutrals should be isolated from the feeder neutral and meggered to verify there no branch circuit neutral shorts to ground. AVOID NET CURRENTS and EMF

Transformers:

Like all equipment it should be meggered prior to energizing. You can not megger phase to phase as this is a direct connection through the winding. Megger phase to ground on the Primary, Phase to ground on the secondary (with the neutral ground bond lifted) and primary to secondary.

Circuit Breakers:


Large frame circuit breakers should be meggered in particular after a fault when the CB has operated. Megger line to load across each phase, phase to phase line and load and phase to ground. A simple method to do these test is to open the CB, place jumpers from Line B phase to A and C phase load side then jumpers from B phase load to line side A and C. Place your megger leads on B line and B load record your readings> No close the CB and megger to ground.


ATS Automatic Transfer Switches:

Care should be taken with ATS’s the control panel must be disconnected to avoid damaging the control panel.

Motor: 

Motors should be meggered prior to energizing, remember that phase to phase SHOULD read a dead short you only need to megger to ground.

Obviously and cables, busway, switchboards, motors and any distribution or utilization equipment suspected of having a short should be meggered.

With Electrical Preventative Maintenance (EPM), large distribution equipment should be meggered prior to and after service. Should there be a drastic change in readings you will realize you have possible made an error in your service or if the switch board has an existing problem you avoid being blamed for affecting the integrity of the switchboard. I am sure all of us have heard at least once “It worked when you got here”.

Prior to megging at a high voltage it is wise to either megger at 100 VDC or use an ohm meter to verify there are no loads on the equipment to be meggered, Protective relays, metering hidden bus taps, or customer utilization loads. If you obtain a reading LOCATE the load, sometimes this takes some perseverance.

What you should see:

We like to see full scale readings, 2000 megohms use to be the standard for most meggers but now meggers come with any were from 1000 megohms to 1000 gigohms full scale.

A simple test is to turn the megger and grab the test leads (NO JUST SEEING IF I HAVE BORED YOU TO SLEEP). With the megger on leads separate push the test button, air is an insulator and the reading should be full scale, touch the leads dead short megger should read zero (0). Take a piece of paper the thinner the better, using a pencil make a scratchy drawing all over with two heavy squares on the edge about 4” apart. Connect your megger and megger the paper at 1000 VDC, you should see arcing and may set the paper on fire or the arcing may burn the short free.

While some standards list .5 megohms (500,000 ohms) as acceptable, we like to see at least 5 megohms (5,000,000 ohms), 50 megohms is preferable (50,000,000 ohms) But if this is new equipment one would expect to see full scale readings, often refered to as infinity. It really not infinity your meter just does not have high enough resolution.

If the reading on A phase to ground is 50 megohms and B and C phase are 1000 megohms, the A phase should be checked for issues. With all things the same Temperature, humidity, age of equipment, and environment (dirt dust ECT) readings should be close.

Readings are taken A to B, B to C, C to A, and A, B, C and sometimes neutral to ground.
It is also a common practice to ground the phases not under test. Except motors and transformers.

Long cable runs and busway may take a while for the test voltage to charge the conductors under test. The conductors are a large capacitor and when the readings stabilize and max out the readings is recorded. This conductor MUST BE DISCHARGED to ground after test unless you want to receive a nasty shock.

Megger readings that do not stabilize, but fluctuate are indicative of moisture in or on the conductors and should be investigated. Sometimes this reading will continue to climb as the moisture evaporates due to the test voltage.

When meggering one must be careful with the voltage levels you are trying to prevent future electrical damage to cause it.


I use 5 different meggers, two are pictured below. All the meggers we use are digital some with an analog scale. We calibrate all our test equipment and meters yearly.

The f1st megger is a simple compact 1000 VDC go, no go megger for day to day use.

The 2nd megger I use has 50, 100, 250, 500 and 1000 VDC. I like this meter because of the 50 and 100 VDC settings.

The 3rd megger I use a hand crank model, I keep this on my truck for emergencies (LIKE DEAD BATTERIES).

The 4th megger is a 5KV (5000 VDC) with 1000, 2500 and 5000 settings, this meter is a battery and 120 VAC powered, the advantage is this meter hasa run timer for different test that require time test. Some spec. jobs require a one minute test for feeders.

The 5th megger is a ESI (Electric Static) tester with 10 and 100 VDC settings and a built in hygrometer and thermometer.

The last meter shown is a thermometer/hygrometer for measuring temperature and humidity


----------



## gilbequick (Oct 6, 2007)

Very informative, thanks Brian.


----------



## lbwireman (Jan 22, 2007)

There's a nifty little 8 page "application note" by Fluke. It gives an excellent comprehensive run through on theory, application and safety. It's entitled "Insulation resistance testing" and downloads as a .pdf file. I couldn't get the link to work but if you go to the Fluke website and search that title you should be able to pull it up. Hope it's helpful to those of you with questions. 
http://support.fluke.com/


----------



## lbwireman (Jan 22, 2007)

OK, I now feel officially stupid.:icon_redface: As I read further upthread, I see that Mountain Electrician has already posted the very link I was trying to post (see Post #5). It really is an excellent tutorial, thanks Mountain.:thumbup:


----------



## brian john (Mar 11, 2007)

The original publication for megging AVO International "A Stitch in Time"


----------



## lbwireman (Jan 22, 2007)

Brian. Found it. Ordered it. Thankyew verra much. BTW, we're using a Fluke 1587. Nice. Versatile.


----------



## HighWirey (Sep 17, 2007)

brian john:

I really was agreeing with everything you previously posted, even though it may not have clearly come across that way! The only addition to my post might be "a megger's readings here could also cause a professional to chase his tail".

And I stand by everything that I stated. Every project in my 22 year orbit had pretty much the same specifications, down to the megging requirements.

Re: the beeg roll of 24C,14AWG unshielded project.
Having nothing to reference the 'other megger lead to', our standard proceedure was: clear the far end cable, bunch (short) all the near end conductors. Remove one conductor from the bunch. Now we have something to meg to! Test conductor one to the bunch. Record. Return conductor one to the bunch, remove conductor two. Test conductor two the bunch. Record. You know the drill . . . Once the cable was installed, the validation procedure was the same, except the 'bunch' was connected to facility steel.

Each of the 24 conductors on that 'beeg roll project' had so many poor readings (none were near the 500V,100 megohm minimum) that it had us chasing out tail. I still think it was crappy cable. 

"I can answer any specific questions".

The vendor explained it away ! Can you, Brian?

Thought that I had "READ, LEARN, test and you'll be much further ahead than those that don't " over the years. Maybe not?

I know this post seems a little off topic, but we are all here to learn something, and all of our orbits are not congruent. Thank you.

Merry Christmas Everyone, and Best Wishes


----------



## lbwireman (Jan 22, 2007)

HighWirey said:


> brian john:
> 
> 
> 
> ...


----------



## HighWirey (Sep 17, 2007)

gilbequick 

"What's a shield?...a ground?"

Looks like no one answered your question. 

The very short answer is: a shield normally is a conductive overwrap on one or more insulated conductors on a cable. The shield itself may be insulated or not, may be grounded or not.

You can receive more information about the reasons for a shield, why a shield is used, and how to connect the shield, than Carter has Little Liver Pills, once you Google 'cable shield', and 'Carters Little Liver Pills', then you can report back here, Grasshopper, a much wiser man than myself !

Merry Christmas Everyone, and Best Wishes


----------



## John Valdes (May 17, 2007)

Jeff, Read over Brian John's post's. he is very bright and he covered most anything you should encounter.
Like he said if you ever have to meg a motor, make sure it is disconnected and you read from lead to ground. Each lead. Then you can read ohms from lead to lead. All ohm readings should be close, but not shorted. Smaller motors produce a higher resistance than larger motors. It is difficult to test motors with a multimeter. The megger is king in motor issues.


----------



## ElectricianJeff (Nov 19, 2007)

John Valdes said:


> Jeff, Read over Brian John's post's. he is very bright and he covered most anything you should encounter.
> quote]
> 
> I did and its very informative. Just what I was looking for!
> ...


----------



## brian john (Mar 11, 2007)

More reading material

http://www.metercenter.com/cgi-bin/...972&command=link--Note_Choosing_a_Megger.html

http://www.metercenter.com/biddle/A%20Stitch%20In%20Time.pdf

http://www.aemc.com/techinfo/techworkbooks/megohmmeters/tech_megohm.pdf

http://www.metercenter.com/cgi-bin/...=link--Note_Advanced_Insulation_Analysis.html


----------



## gilbequick (Oct 6, 2007)

Thanks Brian, that's some good reading material.

I got my megger a few days ago and am having trouble figuring out the results. I'm working with NM and am using 1000v to test and that will test at a max of 2000M ohms. This is on new wire fresh off the roll.

Untouched, it will test at 1 (infinity). If I smash the wire by hitting down on a hard surface with my linesman pliers it'll still test at 1. I can actually break open a part of the insulation on one of the conductors and measure from the conductor with the hole in the insulation to the ground (which is bare) I still get 1. It seems that the only way I could get a poor reading was by directly shorting it out. I can get those same results with any standard multimeter with a continuity test.

What am I doing wrong? I would think certainly it would test poorly with the hole in the insulation. Is a megger really only good for "old" wire?

I'm off to reading more "A Stitch in time".


----------



## brian john (Mar 11, 2007)

Paper and #2 pencil


----------



## gilbequick (Oct 6, 2007)

After watching your slideshow I did that a couple of times! I don't think my pencil is #2 though, but I got some sparkage. The thing with the lead I used was when the megger would arc, at the point it arced it wouldn't do it anymore...like it burned all of that material up and wouldn't do it again. 

Even with a #2 pencil, what readings should I get?


----------



## brian john (Mar 11, 2007)

Thinner paper.....


----------



## gilbequick (Oct 6, 2007)

But what is the paper supposed to tell me?


----------



## brian john (Mar 11, 2007)

You can set up resistors of varying resistance from a dead short to any of a variety of resistance.


----------



## gilbequick (Oct 6, 2007)

So you think my readings are valid and not possibly a faulty unit? This being my first experiences with a megger I just want to make sure that I'm using it properly.


----------



## brian john (Mar 11, 2007)

Ultimate test

1. Leads separated reads full scale infinity.

2. Leads connected "0" zero dead short.

3. Paper with lead dead short, sparks arcs and goes to full scale or you 
start a fire, whicj burns the dead short open


----------



## Pierre Belarge (Feb 3, 2007)

*Longer runs*

As Brian has mentioned, longer installations of cable or conductors will take longer to reach capacitance-saturation.
What is fun, is as you learn to use your equipment, you will instinctively notice which runs you are testing are longer runs.


Again a repeat of what has been presented. When you do get a reading of a dead short, the first thought should be that there is a load on the circuit being tested...and hopefully you did not damage it.
When I am performing a megger test, I am not the one who is preparing the job for testing. That itself can take quite some time, as the electronic devices and loads have to be disconnected before testing..such as removing lamps from fixtures, electronic switchs, bell transformers, etc...
What I do is walk the job to take a look before I start.

I keep a clipboard and pad with a table that I designed for recording my findings. This is a lot quicker, neater and more professional. I can download it from my computer whenever I need more. This way I can document whatever I find. In newer buildings it is usually not too bad. In older buildings, you never know what you are going to find.

Also, the electrician can start putting back the circuits you have been testing, leaving the ones you may have issues with for them to find after you are done.

Without practice, you will never get as good as Brian, so practice makes Brian


----------



## brian john (Mar 11, 2007)

> Again a repeat of what has been presented. When you do get a reading of a dead short, the first thought should be that there is a load on the circuit being tested...


When there is a dead short ALWAYS look for connected loads and KWHR sub-meters. I have seen jobs where the contractors were going to replace feeders as they read a dead short on conductors that were previously energized. I was summoned by the owner as the cost was through the roof, they had every load off but sub meters on each floor tapped ahead of the panel main.


Also when testing a circuit where there may be loads I always start with the lowest voltage. One of my meggers has a 100 VDC, but a 250 VDC should be fine. Remember that a 120 VAC load sees 169 volts (and -169 volts)at the peak of the sine wave, so 250 VDC normally is not damaging, but 1000 VDC could be. 

I do a lot of testing as part of my job I have learned to always question results espically when they seem strange. Good when they should be bad and vice a versa.


----------

