OK, I'm getting a megger

22012 Views 41 Replies 12 Participants Last post by brian john, Dec 17, 2007

gilbequick

Discussion Starter · Dec 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!

1 - 20 of 42 Posts

itsunclebill

· #2 · Dec 6, 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.

See less See more

TOOL_5150

· #3 · Dec 6, 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?

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!

\

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

· #4 · Dec 6, 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

· #5 · Dec 6, 2007 (Edited)

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

· #6 · Dec 6, 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

· #7 · Dec 6, 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

· #8 · Dec 7, 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

· #9 · Dec 7, 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

· #10 · Dec 7, 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

· #11 · Dec 7, 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

· #12 · Dec 7, 2007 (Edited)

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.

See less See more

HighWirey

· #13 · Dec 7, 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

See less See more

gilbequick

Discussion Starter · #14 · Dec 8, 2007 (Edited)

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

· #15 · Dec 8, 2007 (Edited)

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

· #16 · Dec 8, 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

Discussion Starter · #17 · Dec 9, 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

· #18 · Dec 9, 2007

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

EEP!

:laughing:

~Matt

ElectricianJeff

· #19 · Dec 9, 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

· #20 · Dec 9, 2007 (Edited)

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

TermAbbreviationValue (Scientific)Value (Normal)
TeraT1 x 10121,000,000,000,000
GigaG1 x 1091,000,000,000
MegaM1 x 1061,000,000

See less See more

1 - 20 of 42 Posts

This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.