# megger question



## goose134 (Nov 12, 2007)

I'm at a hospital job and we were meggering some 480 volt feeders today. Something I had to convince my boss to do, but that's another story. We are using the Ideal digital megger and I had it set to 500v. All the cables ring true until we get to one A phase of a parallel feed. All the other cables were reading 2000 M ohms and then we got around 300 M ohms to ground.

Right away I'm thinking we've got a nick near a connector or something like that. I am then told to switch it to 1000v and do a three minute test on it and report the drop in reading. Now the thing reads true and didn't drop a lick. Am I missing something? Why would it read true at a higher voltage and longer test? We have not energized it and do not plan to do so for a while, so any input would be greatly appreciated since a 600 A blow up is not on the schedule.


----------



## Zog (Apr 15, 2009)

goose134 said:


> I'm at a hospital job and we were meggering some 480 volt feeders today. Something I had to convince my boss to do, but that's another story. We are using the Ideal digital megger and I had it set to 500v. All the cables ring true until we get to one A phase of a parallel feed. All the other cables were reading 2000 M ohms and then we got around 300 M ohms to ground.
> 
> Right away I'm thinking we've got a nick near a connector or something like that. I am then told to switch it to 1000v and do a three minute test on it and report the drop in reading. Now the thing reads true and didn't drop a lick. Am I missing something? Why would it read true at a higher voltage and longer test? We have not energized it and do not plan to do so for a while, so any input would be greatly appreciated since a 600 A blow up is not on the schedule.


First off the minimum test voltage for 600V rated cable is 1000VDC, so you should have done that anyways. 

What does a Megger measure?? Resistance? No, not really. A megger (OR any ohmeter for that matter) as seen below, is actually measuring current. The meter applies a test voltage and measures the current flow through the circuit and calculates resistance which is displayed on the meter in Ohms. If the applied voltage is not high enough,(Or the restiance is too high) the amount of current passed through the resistance is below the sensitivity range of the meter. 

Megohm meters are connected to a component which is removed from the circuit as illustrated in the drawing. Ro is an adjustable resistor whose purpose is to zero the ohm meter and correct for battery aging. It is also a current-limiting resistor which includes the meter resistance Rm. Zeroing the megohm meter is accomplished by shorting the ohm meter terminals and adjusting Ro to give full-scale deflection; this is done automatically by modern digital megohmmeters.

When the unknown resistance Rx is connected across the megohm meter terminals, the current is measured by calculating the total series resistance. Using the following equation:

*I= V/ (RO+RX)*

A digital megohmmeter will automatically do this calculation for you while an analog meter reading 0 ohms will cause full meter deflection. The meter face says ohms, but it is actually indicating amps, therefore full deflection (All the way to the right) will indicate 0 ohms while a reading of near 0 amps will cause little reflection of the meter and indicate near infinite resistance. (There is also no such thing as infinite resistance but we will save that argument for future discussion) 

For digital displays, if the applied voltage of the circuit under test is not high enough (Or resistance is too high) to cause enough current flow through the circuit for the meter to sense, the display will read “O.L”. Contrary to popular belief, this does not mean overload, it means “Outside Limits”, in other words, the measured current flow is below the sensitivity capabilities of the meter.

So back to your question, when you applied double the voltage through the same reistance, double the current flowed through the circuit, causing double the voltage drop across Rx, everything cancels (Almost). The reistance of your cables insulation is the same as it was before. 

Either way the minimum spec for a cable like this is 100M (At 20 degrees C) so you are all good.


----------



## nick (Feb 14, 2008)

Well Goose i agree with Zog your fine , most electricians see a 350 meg reading as bad if the megg ohm meter doesnt give them a full reading they think its bad . You dont need to get a high reading every time .

We actually have seen people pull out a run for no other reason then they did not know how to read a megg reading . Heres some basic thoughts from my times megging you get lets say 350 megg on a conductor test lets say A phase to ground but B & C phases are up to 800 meg A phase is fine .

Clean up the ends of the conductor under test or in most cases turn it on under a load and turn it back off the next day and test it again now its up higher its better because the temp and the voltage has burned steamed or boiled the water mositure out of the jacket high voltage helps . 

Water ,moisture , dampness, wire soap & pulling soap when it gets under the jacket of that insulation effects your readings . The time of day effects your readings morning low readings .
Higher readings in the afternoon testing .

We use a hair dryer sometimes on the ends of conductors and magically its a good reading .


If i get 100 meg or 50 meg ill pass it load it up and shes fine .
If it holds for one minute its good !

We only megg at 1000volts for 600 volt wire !

Heres how we test a bad conductor and fix it one thats shorted to ground or another conductor . We already know which one is bad by color code or phase .

We pull them all out roll each up on a spool take the spool over and un roll it into a 6 inch pvc conduit slightly bent and its filled with water .

Hook the megger test up to the conductor thats hanging out on the side middle of that spool dont forget to do that ! 
Drop a number no.10 bare wire in the water then clip on your megger test lead pull wire test stop! pull wire test stop ! 
Youll find a pin hole stuff you cant see with your human eyes . 

Once we locate the spot we clean and dry it with rubbing Alcohol then wipe it with mineral oil, dabb it with scotch coat , shrink tube over a 5 foot area . It has saved us once or twice on a week end or when the times we cant wait for a replacement on a damaged or long run of 750 copper but just needed to get it done . lots of fun 

http://i611.photobucket.com/albums/tt195/stringking/Mitsubishi025.jpg

We have a rule if its a new conductor and the cut is 1 inch long down to the copper its sent back to the supply house ! 

Rule two if we have a nick or cut and the wire is hard to get and its a week away we fix and factory pays us to repair it . Take care


----------



## goose134 (Nov 12, 2007)

Thanks for the info guys. I feel kind of slow for using the 500v settings. I suppose I thought that a higher voltage would somehow be harmful to the insulation of the cable. I mean I know it's only for a moment, but it was something that entered my head. Now it makes more sense.

I'm sure you are both right in saying the cables are fine. I think I'll wander over to southwire. com and see if they have the tolerances for their cables. See if I can run the numbers for myself. Thanks again.


----------



## nick (Feb 14, 2008)

Well Goose actually when we pull in high voltage cable or conductors and we hi pot test we use the old reynolds sandwich plastic it can hold up to 12000 volts with just one rap each rap gets to the next level of voltage we use it while testing high voltage cables .

We cover the terminations the lug or load break connector during test to keep the air from conducting during the test .

Ive tested up to 75000volts with just sandwich rap !

A 600 volt AC rated wire can hold up to 15000 volts AC how do i know this i play with high voltage as a hobbie .

That factory wire is rated for a lot more than the factory rating thats given to us its pretty tuff .

Meaning what they tell or we read about .


So at 1000 volts your fine . Take care


----------



## nrp3 (Jan 24, 2009)

I have been watching these posts with interest. I don't have a megger yet. I bought the little book from megger on the use of it and still don't have a grasp of it. When things get a little better money wise, I think I will buy a good basic one and try it out on some scraps before I try it in the field. I think like most test equipment, it requires some experience in use before you get good with it.

How are the leads attached? I am assuming in this case that nothing is connected at the ends, no loads?


----------



## 480sparky (Sep 20, 2007)

nrp3 said:


> ..........How are the leads attached?


Just like any meter you have. Here's a screen shot from the instruction manual for a Fluke 1507:







 


nrp3 said:


> I am assuming in this case that nothing is connected at the ends, no loads?


Correct. Otherwise, the output voltage of the megger can burn things up.


----------



## nrp3 (Jan 24, 2009)

So, is one lead on the insulation and the other on the conductor, or both leads on two different conductors? Forgive the seemingly dumb questions, just trying to broaden my skillset.


----------



## 480sparky (Sep 20, 2007)

nrp3 said:


> So, is one lead on the insulation and the other on the conductor, or both leads on two different conductors? Forgive the seemingly dumb questions, just trying to broaden my skillset.


The illustration is overly simplified. You can check one conductor to ground, or conductor to conductor.


----------



## nick (Feb 14, 2008)

Well is this a little clearer??http://i611.photobucket.com/albums/tt195/stringking/P1010125_01.jpg


----------



## nrp3 (Jan 24, 2009)

Yes, that helps. In the case of a new installation like that, you are checking each wire against each other and the metal tub/conduit its fed from/in?


----------



## Zog (Apr 15, 2009)

nrp3 said:


> I bought the little book from megger on the use of it and still don't have a grasp of it.


You mean "A stich in time"?? You know that is free right?


----------



## 480sparky (Sep 20, 2007)

Zog said:


> You mean "A stich in time"?? You know that is free right?


 
Stitch in Time.


----------



## nick (Feb 14, 2008)

nrp3 said:


> Yes, that helps. In the case of a new installation like that, you are checking each wire against each other and the metal tub/conduit its fed from/in?



Well if your look at the test leads they are hooked up to the conductors only 

When you test put the test leads on AB BC CA AN BN CN A GRD B GRD C GRD not to the actual encloser or equipment case ground but just the grounding conductor itself . Do like Zog has shown go to that web page and it will explain better iam getting old and i need to take a nap . Take care


----------



## nrp3 (Jan 24, 2009)

I know someone had a link to it online here, but sometimes I find it easier to read at night out of a real book. I have to compete with the wife (working on masters degree) and kids for computer usage. I find its easier and quicker sometimes to look at products in the bulky catalogs than online especially when checking the specs.

What if theres no grounding conductor in the pipe? An older install for instance.


----------



## nick (Feb 14, 2008)

nrp3 said:


> I know someone had a link to it online here, but sometimes I find it easier to read at night out of a real book. I have to compete with the wife (working on masters degree) and kids for computer usage. I find its easier and quicker sometimes to look at products in the bulky catalogs than online especially when checking the specs.
> 
> What if theres no grounding conductor in the pipe? An older install for instance.



Well if there is no grounding conductor in a pipe then yes hit the metal case or grounding bar in switchboard or panel . 

Its not much more than a simple test you apply high voltage which will pass thur a hole or cut or nick in the insulation to ground .


Our company has a rule we pull grounds in pipe !


----------



## nrp3 (Jan 24, 2009)

Thanks for the photos and commentary. Makes much more sense. I put the grounds in what little pipe I do. I am more likely to have to figure out whats wrong in one without the ground.


----------



## nick (Feb 14, 2008)

Well if someone asks a question ill try and answer what little i know .Take care


----------



## Mr. Sparkle (Jan 27, 2009)

I got a megger off the bay pretty cheap to school myself with. I have not gotten around to it, but eventually I will.

Anyone have a experimental test I can try to see if the megger I got works properly? It is pretty much brand new but it is a brand I never heard of, I'll post a link to it in a minute.....

Here it is, I got it for $40.00 on ebay.


----------



## MDShunk (Jan 7, 2007)

Mr. Sparkle said:


> Anyone have a experimental test I can try to see if the megger I got works properly?.


Attach one lead to your bottom lip and stuff the other one up your left nostril. Press the test button. :laughing:


----------



## brian john (Mar 11, 2007)

Mr. Sparkle said:


> I got a megger off the bay pretty cheap to school myself with. I have not gotten around to it, but eventually I will.
> 
> Anyone have a experimental test I can try to see if the megger I got works properly? It is pretty much brand new but it is a brand I never heard of, I'll post a link to it in a minute.....
> 
> Here it is, I got it for $40.00 on ebay.


Take a piece of paper scribble with pencil and then megger it.



Click on the image.


----------



## Mr. Sparkle (Jan 27, 2009)

MDShunk said:


> Attach one lead to your bottom lip and stuff the other one up your left nostril. Press the test button. :laughing:


That is a great way to take care of my nosehair issue....hmmm



brian john said:


> Take a piece of paper scribble with pencil and then megger it.
> 
> 
> 
> Click on the image.


Oh that is awesome, this I will do.

Are those your pics?


----------



## brian john (Mar 11, 2007)

Yes I made a series of pictures and did a slide show with PhotoBucket.

Softer lead will help as will thinner paper, you may not get it first time, too many scribbles or not enough make a large area of scribble where you plan to clamp.


----------



## Zog (Apr 15, 2009)

brian john said:


> Take a piece of paper scribble with pencil and then megger it.
> 
> 
> 
> Click on the image.


You have gotten a lot of miles on that pne Brian, I like Marks idea too.


----------



## John Valdes (May 17, 2007)

Mr. Sparkle said:


> Anyone have a experimental test I can try to see if the megger I got works properly? It is pretty much brand new but it is a brand I never heard of, I'll post a link to it in a minute.....


Yes, Set the megger to any voltage and hold the test leads apart (not touching anything) and push the button. This will be the best reading the megger can produce. Then clip each test lead to each other and press the button. This is a dead short. This will give you an idea how the megger works. Reading values whether good or bad sometimes depends on the person testing. Some companies/contractors and electricians SOP's have values to refer to. There is no magic number, unless you assign one.


----------



## brian john (Mar 11, 2007)

I have a study completed by the federal government with monkeys they stuck one probe in the rectum and one in the mouth to see the effects of electricity on our nearest relatives in the animal world.


----------



## aksparky (Dec 1, 2007)

I got a Flike 1507 not too long ago and the first test I did was what John suggested.
Then I tried an experiment test where I stuck the stripped ends of a couple wires in a dry sponge and megged, then added a lil water and megged again.
Then I got the wires close together in the sponge and did it again.
Was trying to simulate what kind of readings one might get in an underground run that has moisture problems.
I was talking to the boss about it recently and he said he had a few old meggers stored under a shelf collecting dust that he bought at a going out of buisness sale and has never used.
I took them out and they are the hand crank type in leather cases, I believe one was a biddle megger and looked brand new.
I have never used the crank type but am looking forward to comparing the readings from that to the 1507 I have.


----------

