# Megger



## brian john (Mar 11, 2007)

I posted this before here I THINK, but if you are going to megger busway you should have an understanding of whats involved.

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


----------



## brian john (Mar 11, 2007)

Pictures later leaving for the beach....


----------



## rlc3854 (Dec 30, 2007)

Brian, thank you for the information. I for one appreciate the knowledge you and John from VT past along on this forum. Although a lot of us are firmirial with the processes and perform the steps as outlined in another post about bus duct not all of us get to perform the tests. The information that you provide gives the rest of us reasons to ponder the installation prior to and after energizing.


----------



## 5volts (Jan 11, 2008)

Brian, i would like to add. You should read 1000 ohms per volt. A 480 Volt Wye connected motor would requre at least 480,000 ohms. So a reading of .5 of a meg or higher would be o.k. One point VFDs are very sensitive, You be the judge!


----------



## randomkiller (Sep 28, 2007)

Other than basic megger, volt, and amperage reading I don't really get to do much testing for troubleshooting reasons. I test various CTs, transducers and resistors as parts of an installed control system for calibration purposes. But, I kind of envy you Brian, you get to play with some nice (expensive) toys.


----------



## nick (Feb 14, 2008)

*megger*

well Brian John , i waited to see what response your post would receive on megger use. we use this tool everyday and i must say your post was well written i appreciate the time you took to put all that in one post nice reading and i was not bored at all ,i have but a few years in the trade but lots of time using a megger i found most electricians do not know how to read the readings ,since ive been in the commercial and industrial field now. one problem is new feeder pulls ,you get low readings when you first check ,after they dry up for one day you get better readings ive seen electricians pull out a feeder justs pulled in for no reason but they could not understand that soap or a damp conductor after a pull needed to be clean and dry , lots of money wasted. even gear one day low readings and put a heater in that room for one day and wow its ok high readings ,but the most common mistake electricians make is they think the megger must always read infinite 999 on the digital displays. i tell them no but they think iam nuts , if they get, 100 300 or 400 meg ohms its good not perfect but it will come up more after you put current or a load on that feeder and it drys up a bit .same with the gear ,also ive seen electricians say the gear is bad after they megg because they do not know to pull out the control fuses or turn off the control breaker in the switchboard main section and are reading thur the control transformer or ect ect , when i was a navy em i was told 1 megg ohm and 1000 volts dc was normally fine to turn on in most conditions not all cases but most .thanks for the post we should have more of these brain .good job ! like most of your post i read .:thumbsup:


----------



## Wireless (Jan 22, 2007)

Brian, we never got the pictures! how was the beach?


----------



## TGGT (Oct 28, 2012)

I'm resurrecting this thread @brian john. I'm about to test dozens of newly pulled cables through short runs of underground pvc conduit from a xfrmer pad to 2x ats' that feed 2x 4000amp switch gear. The testing procedure they sent me merely suggested 500v or 1000v as industry standards.

I've been provided an Ideal insulation tester and it goes up to 1000v. I was told by my supervisor that they only need to record a minimum of 250megohms. So far, testing at 1000v the readings usually start around 1300megohms and will climb up to 22gigohms if I wait long enough. If I test with 500v, the megohms quickly max out to 550 megohms.

Damage to the cables is not likely, but we test and record before any terminations.

If all the minimum they want to see is 250gigohms should I even bother testing to 1000v when it's a 480v system? Is 60 second testing time important when we're not troubleshooting a known issue?


----------



## SWDweller (Dec 9, 2020)

TGGT said:


> I'm resurrecting this thread @brian john. I'm about to test dozens of newly pulled cables through short runs of underground pvc conduit from a xfrmer pad to 2x ats' that feed 2x 4000amp switch gear. The testing procedure they sent me merely suggested 500v or 1000v as industry standards.
> 
> I've been provided an Ideal insulation tester and it goes up to 1000v. I was told by my supervisor that they only need to record a minimum of 250megohms. So far, testing at 1000v the readings usually start around 1300megohms and will climb up to 22gigohms if I wait long enough. If I test with 500v, the megohms quickly max out to 550 megohms.
> 
> ...


I used to hold 2 minutes at 5k or 10kv looking for a Gig ohm or more for the surface conductors at the mine. These were either 7200vac or 4.16k vac. We made all of the cables the same so intermixing them was never an issue. Get a haul truck (~400 tons loaded) driving over the cables then fixing them and then having as many as 6 splices in a 2500' cable gets weary. We vulcanized all of the splices due to the "water" in the mine. 
Water was defined as a dilute sulfuric acid and secrete sauce mixture. Yes it was a leaching mine.
I was told by a square d engineer that anything over 1 meg ohm at 1000v was fine. I always liked to see it a bit higher than that.
I never had the time to test before we assembled the products.

Watch out meggering large frame breakers, >400 amp lots of electronics in them now days, would not do good to fry the protection device.


----------



## TGGT (Oct 28, 2012)

SWDweller said:


> I used to hold 2 minutes at 5k or 10kv looking for a Gig ohm or more for the surface conductors at the mine. These were either 7200vac or 4.16k vac. We made all of the cables the same so intermixing them was never an issue. Get a haul truck (~400 tons loaded) driving over the cables then fixing them and then having as many as 6 splices in a 2500' cable gets weary. We vulcanized all of the splices due to the "water" in the mine.
> Water was defined as a dilute sulfuric acid and secrete sauce mixture. Yes it was a leaching mine.
> I was told by a square d engineer that anything over 1 meg ohm at 1000v was fine. I always liked to see it a bit higher than that.
> I never had the time to test before we assembled the products.
> ...


None of the conductors have been terminated yet, they are free on both ends, and one side it taped off with the other exposed for testing. I won't hide the fact I'm looking for permission to not test for 60 seconds if it's not really necessary. I have a couple hundred conductors to test out in the cold so I want to be efficient.


----------



## micromind (Aug 11, 2007)

TGGT said:


> None of the conductors have been terminated yet, they are free on both ends, and one side it taped off with the other exposed for testing. I won't hide the fact I'm looking for permission to not test for 60 seconds if it's not really necessary. I have a couple hundred conductors to test out in the cold so I want to be efficient.


I don't know if it was the right thing to do or not but I've meggered more than one conductor at a time.........


----------



## 460 Delta (May 9, 2018)

micromind said:


> I don't know if it was the right thing to do or not but I've meggered more than one conductor at a time.........


micro’s got a great point, you don’t meg each individual winding in a motor, but really all in actuality. This isn’t really different to me if you’re looking for a pass/fail. Put as many as you can in a 750 Kearney snugly and crank away. If it’s good, this group passes, if not, halve and test, and halve and test.


----------



## SWDweller (Dec 9, 2020)

TGGT said:


> None of the conductors have been terminated yet, they are free on both ends, and one side it taped off with the other exposed for testing. I won't hide the fact I'm looking for permission to not test for 60 seconds if it's not really necessary. I have a couple hundred conductors to test out in the cold so I want to be efficient.


The 60 seconds hold is more important on long runs. I was doing over 2000' at the mine.
If your under 500 feet and it is not humid, 30 seconds should be good as long as you watch the needle like a hawk. Any movement to my eyes would prolong the test. 
A good test tek will know his equipment and have a good "feel" for what he should see. Every machine is capable of energizing so much cable so fast. Biggger longer cable takes longer than smaller, shorter cable.


----------



## CMP (Oct 30, 2019)

A stitch in time, Megger's complete guide to electrical insulation testing.


----------



## splatz (May 23, 2015)

The closest thing I could find to an "industry standard" for testing is 

ANSI/NETA ATS-2009
AMERICAN NATIONAL STANDARD STANDARD FOR ACCEPTANCE TESTING SPECIFICATIONS 
for Electrical Power Equipment and Systems 

(There's more recent versions, this is just the one I could find a free copy.) They have tables with values to use if the manufacturer of the equipment doesn't provide you with the specifics. 










There are other tables for transformer windings and motor windings. For some reason they specify the time (one minute) for the others, but not this one. 

Unless you can find something showing less than one minute is "standard" I thikn you're stuck testing for one minute. 

The best test would probably be to jumper all the conductors and ground, and remove one at a time and test between that one and the bunch. I would think that would give you your best chance of catching a nick or etc.


----------



## paulengr (Oct 8, 2017)

One minute is in the standards.

This does two things. The first thing is to wait long enough to charge up the system capacitance. The line is or should be insulated from ground, forming a capacitor. This contains no useful diagnostic information so we just want to charge it to get it out of the way. This is usually over in a few seconds to tens of seconds.

The second is so that if we both run the test we get consistent results.

PI does a 10 minute and 1 minute test and takes the ratio. After the system capacitance is charged individual molecules in the insulation slowly align themselves to the field, increasing resistance further. PI uses this like a yard stick. Typicallly PI is 2-6. If it’s low you have a failure. High indicates test problems or cracked insulation.

IEED is kicking around making the Megger test graphical since that tells you more than an absolute number.


----------

