# megging a motor wired up



## successisforme

Ok....i have been told by some that you can not megg a cable that is wired up to a motor.....and some have said that you can only meg from conductor to ground only. well, i wish some one that is 100 percent sure of what they are talking about to answer this.......i recently installed a MCC with both VFD and regular wiring from a bucket.....some of the motors are wired and some are not......i had a couple cables that someone else installed and was bad......well anyway, the question is...... i wanna know some details of how and what it is that i can do with a megger on both situations i have.......i cant say that i am very knowledgable on the use of meggers and would like to be educated about them and what i need to know to be able to use a megger in the future without any doubts. thanks


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## mollydog

success, the megger is used to measure the integrity of insulation on mostly motor windings. we normally megged A-B, B-C, C-A, first open the motor disconnect. if your megging a few branch circuit or MCC feeders, a standard ohmeter should be fine.


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## AussieApprentice

I am no expert but, be careful around any electronics such as VFD's. I usually disconnect them.

I do not test any cable active to neutral if there are appliances attached.

As for motors, if it has a neutral connection such as in a single phase or star connected 3 phase, and the neutral is grounded then obviously you will get a low reading.


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## successisforme

*megger testing*

hello....thanks for the reply.......i dont think i got the question completely answered......im kinda looking for a "teacher" type response......i only understand about 50% of what meggers are used for and what they CAN do......i know its to test to see if the wires are bad.......just wanna know the DIFFERENT ways you can use a megger......for years i thought you couldnt check a cable if it was wired up........two days ago. a guy that i know is a good electrician megged some wires which was wired up in the pecker heads and i always thought you couldnt do that......i wanna know the ends and outs of how this can be done.


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## TheRick

A megger is used to test the integrity of insulation. If you are testing a motor circuit wired to a MCC bucket or a VFD;

Open the breaker in the bucket or isolated the VFD.
Always test your Megger first with leads together and leads apart.
Clamp onto the equipment ground with one lead and one of the T-leads with the other. This test should give you a very high resistance reading (should be 1K ohm per volt to motor).
Repeat for each lead.
If you get a low reading to ground on one or more lead, you have a problem...but is it in the motor or the conductors going to the motor?
Next repeat this process on the motor leads, disconnected in the peckerhead. Be sure to follow all LO/TO rules!
If this test gives you a low resistance you have a grounded motor...if not repeat test on the T-leads going back to the MCC.
Hope that helps!


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## MDShunk

mollydog said:


> we normally megged A-B, B-C, C-A, first open the motor disconnect.


That will tell you absolutely nothing, and you stand a pretty good chance of messing up the motor if you do it that way. 

You need to check between each phase conductor and the equipment ground when you're doing an insulation resistance check with a megger.


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## TheRick

MDShunk said:


> That will tell you absolutely nothing, and you stand a pretty good chance of messing up the motor if you do it that way.
> 
> You need to check between each phase conductor and the equipment ground when you're doing an insulation resistance check with a megger.


 
Meggering between the phases will tell you absolutely nothing if you are trying to identify a ground fault...but it will most definitely identify an open winding.

I am not sure how it can "mess up the motor" though. :blink:


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## Mountain Electrician

I always check for an open winding by megging phase to phase. I've checked literally hundreds and hundreds of motors this way, and never hurt one.


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## 5volts

1. Personally I never use a megger to take readings from Phase to Phase on a motor.

(I use a ohm meter such as a simpson 260.):thumbsup:

I don't thinkl you will hurt a winding if you chose to use a megger between windings, because the winding should be able to handle the megger I do see electricians doing it but personally I see it as a waste of time.

2. I use a megger to take readings from phase to ground. This checks for breakdown of insulation of motor windings and feeds. The rule of thumb is 1000ohms per volt.

I always take my first megger reading after I lift the T leads from the VFD and with feeds still connected to the motor. This will Check the feeds and the motor windings. If one reads grounded I break open the pecker head Isolate the motor from feeds and then take the reading again.

(For the test I use a Amprobe Analog Megger.)

If you are meggering your motor feeds and they are disconnected at the motor peckerhead, make sure they are also disconnected from the VFD. If not you can back feed the VFD and blow it up.

3. You can always megger out your conductors seperate from the motor. Disconnected from both ends making sure they are not grounded or shorted from phase to phase. When conducting this test I tend to also sometimes tie all three phases together and take a ohm reading at the other end and look for continuity between all three phases.

4. Remember a motor can megger out to be good read balanced and still be bad.


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## ce2two

Using a megger to check a motor winding relative to ground A TO GR. B to gr. etc pretty basic ,motor failures are not always electrical ,bearings fail also lack of p.maintenance...we check loops that are placed in the ground for detection on traffic signals , loops on freeways ,on-ramps used for counting cars:jester: i've used meggers for years hate the digital ,love the old crank style:thumbsup:


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## John Valdes

Just to add to the discussion. Never meg a circuit with a VFD connected to it. Never meg any electronic devices that are connected to the circuit as a whole.

Megging phase to phase can expose a short that may not be grounded. I meg phase to phase and then phase to ground. Cover all the bases.


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## CFL

I wish I could put my 2 cents in but I'm actually trying to learn here. There are too many conflicting answers. Some of you are posting misinformation. There's no reason everyone shouldn't be on the same page.


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## 5volts

Came across a problem today where a 480 volt 30 HP motor was kicking out on a over current Fault. Controlling it was A PowerFlex 700 VFD.

This strange thing was at Idle speed which was 17 Hertz the drive and motor would run fine. As soon as the drive ramped up to 60 Hertz The drive would Fault out immediately. After Verifying I had 480volts supply to the VFD, I lifted the T leads off a contactor that is wired on the load side of the VFD. With All three phases still attached to the 30 HP motor I meggered the T leads to ground And I read a direct short. 

Then I proceeded to the Motor peckerhead and broke loose the connections. I then took A megger reading on all the feeds making sure they were not grounded and verifying they were not shorted from phase to phase. I then read the motor and it read a direct short. I even read it with a Analog simpson meter at the Rx10,000 scale From phase to ground and the motor read shorted.

Before meggering When the motor was running fine At Idle speed I took A voltage reading at the load side of the contactor and I read about 127 volts from phase to phase. 

To conclude the insulation on the motor winding insulation was starting to break down and at 17 hertz which was 127 volts, not enough to "jump" or cause a overload, or the VFD to Fault. The megger 500 scale test found the faulty motor. 

A couple of guys before I got to the call were about to change the VFD out they assumed the drive was the problem because the motor seemed to be running fine only a problem when the drive rammped up.


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## InControl

p_logix said:


> A couple of guys before I got to the call were about to change the VFD out they assumed the drive was the problem because the motor seemed to be running fine only a problem when the drive rammped up.


Unfortunately, that seems to be the norm these days. Most of the guys I work with just swap parts until the machine works. They don't seem to care about understanding the whole process of the machine. 

Good catch :thumbsup:


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## TheRick

> A couple of guys before I got to the call were about to change the VFD out they assumed the drive was the problem because the motor seemed to be running fine only a problem when the drive rammped up.





InControl said:


> Unfortunately, that seems to be the norm these days. Most of the guys I work with just swap parts until the machine works. They don't seem to care about understanding the whole process of the machine.
> 
> Good catch :thumbsup:


 
That kind of stuff drives me nuts...the "shotgun troubleshooting" of just replacing parts until the thing starts working 

I prefer to spend a little extra time up front to TROUBLESHOOT and find the actual problem, then you can repair/replace only what actually needs it. 

The "shotgunner" may get lucky once in a while, but the true troubleshooter will be right, and will get you up and running quicker, more consistently.


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## waddah

*inductive kick using megger*

yes true you can use the megger to test motor winding endsand outs and we call this test (the inductive kick) where in if you have athree phase motor with six leads out of the winding and you do not know thethe ends and outs of the winding you can use the inductive kick method instead of opening the motor and following up the winding


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## ampman

i downloaded from meggers website a publication called "a stich in time" a very helpfull (and long) read on meggers it also shows going from a conductor with one lead to the insulation on the same conductor with the other lead to test insulation


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## Kelem

Could you post a link for me please Ampman?

Thanks


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## Raj

successisforme said:


> Ok....i have been told by some that you can not megg a cable that is wired up to a motor.....and some have said that you can only meg from conductor to ground only. well, i wish some one that is 100 percent sure of what they are talking about to answer this.......i recently installed a MCC with both VFD and regular wiring from a bucket.....some of the motors are wired and some are not......i had a couple cables that someone else installed and was bad......well anyway, the question is...... i wanna know some details of how and what it is that i can do with a megger on both situations i have.......i cant say that i am very knowledgable on the use of meggers and would like to be educated about them and what i need to know to be able to use a megger in the future without any doubts. thanks


 
Hi Success,

Ok, disconnect the motor cables from VFDS's, and from motor terminals.
Now carry out megger test of the cables, phase to earth and phase to phase - should have open circuit on all. If not trace fault on cable for defects.
Then proceed to megger test motor, sometimes our senses makes work more easier. As you approach the motor terminal box if there is any abnormalities with the motor it will show up such as burnt smell of windings, any cables black/burnt, terminals loose, broken shattered pieces etc.If not then megger is your best friend.
Usually motors are Delta connected or even star, just make a note and label the winding leads connected to the teminals. Now disconnect and separate all the leads and carry out megger test.Properly secure your earth lead to the body and test megger first. Then carry out insulation test with leads to earth. Reading should be as high as possible - open circuit. If not the tested lead is shorting to earth.
Then proceed to carry out phase to phase insulation test. If there are 6 leads, 3 sets of test will show short circuit or continuity thus a set of windings. Other 3 should result in open circuit if not then phase to phase short. Delta & star configurations should be understood when phase to phase test is performed.

DO NOT MEGGER ANY CABLES CONNECTED TO ANY ELECTRONICS, STATIC, UNDER-VOLTAGE RATED COMPONENTS.
Meggers produce high level of energy as specified in selection panel which can cause equipment failure, component damage, and even at time injuries & electric shock.

Also note the prime purpose of meggers is to determine the cable, winding installed insulation resistance or in otherwords integrity.It does not been that after megger test results found ok, the motor or installation will perform to the best.There are many other factors involved to down grade the performance as one says in motors, the bearing conditions, lubrication performance, rotor alignment, base mount and alignment, coupling to drive, envrionment, type of load applied, cable size, protection used and many more.

Hope this will show some light.


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## ampman

kelem ,you must go to meggers website and log in then go to publications


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## danickstr

i think he means megger.com


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## AWKrueger

Can anyone clarify what the "peckerhead" is on a motor?


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## Bob Badger

AWKrueger said:


> Can anyone clarify what the "peckerhead" is on a motor?


The junction box mounted on larger motors.


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## captkirk

CFL said:


> I wish I could put my 2 cents in but I'm actually trying to learn here. There are too many conflicting answers. Some of you are posting misinformation. There's no reason everyone shouldn't be on the same page.


 I agree. I try to learn what I can about megging but there are way to many opinions on the subject here. And im not sure which one is correct. I would love to take a comprehensive course on the subject. FWIW fellas, I would pretty much trust what Marc or Bob say.


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## Lone Crapshooter

If a motor is made up for a voltage and it is in service and was just shut down for maintenance or has just tripped on OL there is no need to meg all 3 phases. The reason being that all 3 phases are tied together in the motor. 

To prove this take a operable motor made up for a voltage and apply a megger to any one to any one of the 3 "T" leads and ground (motor frame) . Take a voltmeter and set it up to measure the output of the megger now place the voltmeter on aether of the other 2 "T" leads and the motor frame and you will see the output of the megger.
This proves that the motor is connected inside.

In troubleshooting motors you need a ohmmeter to check the winding resistance phase to phase . This is called a balance test. Now in general the actual winding resistance is not important what is important is is that all 3 resistances reading are the same . I can tell you this that the larger the motor (HP) the lower the winding Resistance. Last week I was troubleshooting a 75HP motor the phase to phase resistance was .4 ohms 
It is not uncommon for motors to have winding resistances of less than 1 ohm phase to phase.

The reason for that is that AC Induction motors windings have impedance characteristics that is the opposition to the flow of of AC current that dc cannot produce . If you would pass a single phase AC current through the windings phase to phase and monitor the volts and amps 
and do a simple Ohms law calculation (R=E/I) you would see a larger impedance values 
To properly state the equation Z=E/I Z being impedance.

LC

Think Listen Solve


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## Zog

captkirk said:


> I agree. I try to learn what I can about megging but there are way to many opinions on the subject here. And im not sure which one is correct. I would love to take a comprehensive course on the subject. FWIW fellas, I would pretty much trust what Marc or Bob say.


I used to teach a 16 hour course on insulation testing, there is a lot of stuff to cover for different applications. AVO offers some good courses.


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## John Valdes

captkirk said:


> I agree. I try to learn what I can about megging but there are way to many opinions on the subject here. And im not sure which one is correct. I would love to take a comprehensive course on the subject. FWIW fellas, I would pretty much trust what Marc or Bob say.


It's not as easy as we might think, or we just naturally have to make things as difficult as we can.. Megger readings are interpreted, not a value written in stone. 
Take a group of good technicians and megger a motor. Everyone will interpret the readings differently. All should know what a dead short is and all should have an idea of what is considered acceptable and what is unacceptable. 
This does cause confusion.
Some companies and some contractors have minimum acceptable values. This would be called their SOP.
Just because a motor reads 5 megohms to ground does not mean the motor will not run. But it also shows insulation breakdown. So what is right and what is a wrong reading? There is no right or wrong reading. The values are interpreted by the technician and the application. 

So maybe in your factory the magic value is 10 megohms or more for acceptability. But in my factory or company 5 megohms may be required as minimum to pass muster.

So what would you consider acceptable and what do you consider unacceptable? Only you can make this determination. In other words just because one uses 5 meg and another uses 10 meg, it makes no difference, unless it will not operate. 

Equipment manufacturers can sometimes help, but ultimately the electrician/technician makes the call.


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## Zog

Or you can follow recognized standards recommendations like IEEE, ANSI, NEMA, or NETA.


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## RIVETER

Zog said:


> I used to teach a 16 hour course on insulation testing, there is a lot of stuff to cover for different applications. AVO offers some good courses.


I guess I am going to have to admit that I am GREEN on this. What are the pitfalls of meggering out motor windings. I can see that you will have continuity but how would it damage a motor, unless somehow you had way too much generated voltage?


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## Zog

RIVETER said:


> I guess I am going to have to admit that I am GREEN on this. What are the pitfalls of meggering out motor windings. I can see that you will have continuity but how would it damage a motor, unless somehow you had way too much generated voltage?


It (Meaning phase to phase ) likely will not do any damage but it is a worthless test that only shows the person doing it they have no idea what the right test equipment should be used. a ohmeter won't tell you much either, the right equipment is a winding resistance test set and you are looking for +/-5% variance between windings. 

Your phase to ground test should be done for 10 minutes, the displayed IR values tempature corrected to 40C and DAR and PI calculated.


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## RIVETER

Zog said:


> It (Meaning phase to phase ) likely will not do any damage but it is a worthless test that only shows the person doing it they have no idea what the right test equipment should be used. a ohmeter won't tell you much either, the right equipment is a winding resistance test set and you are looking for +/-5% variance between windings.
> 
> Your phase to ground test should be done for 10 minutes, the displayed IR values tempature corrected to 40C and DAR and PI calculated.


OH! I knew that. I was only asking for all the other guys out there who didn't know.


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## John Valdes

Zog said:


> Or you can follow recognized standards recommendations like IEEE, ANSI, NEMA, or NETA.


Yes, I have worked in facilities that used these values as their SOP.



RIVETER said:


> I guess I am going to have to admit that I am GREEN on this. What are the pitfalls of meggering out motor windings. I can see that you will have continuity but how would it damage a motor, unless somehow you had way too much generated voltage?


It's really not so bad on the motor as the winding insulation value should exceed the test voltage.



Zog said:


> It (Meaning phase to phase ) likely will not do any damage but it is a worthless test that only shows the person doing it they have no idea what the right test equipment should be used. a ohmeter won't tell you much either, the right equipment is a winding resistance test set and you are looking for +/-5% variance between windings.
> 
> Your phase to ground test should be done for 10 minutes, the displayed IR values tempature corrected to 40C and DAR and PI calculated.


I must disagree with the first statement. Megging phase to phase on occasion has shown me a phase to phase fault that could not be found with field equipment. I see very few facilities or contractors that have the test equipment you describe. You would need to bring in another party (motor shop) to take advantage of surge testers and high pots. They have some fantastic motor testing equipment available, but most in the field do not have access to this equipment and would not know how to use it if they had it.

In the field an ohmmeter (multimeter) may be all you have. If the motor is small, the ohmmeter will work better than expected, as there will be readings to compare against.


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## Zog

John Valdes said:


> I must disagree with the first statement. Megging phase to phase on occasion has shown me a phase to phase fault that could not be found with field equipment.


We are discussing a motor here, all 3 phases are connected so how do you get a phase to phase fault? 



John Valdes said:


> I see very few facilities or contractors that have the test equipment you describe


That is why there are certified testing companies, that is all they do and they are certified to do it. They have the training, experience, and equipment to do the tests and interpret the results the right way.


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## Electric_Light

p_logix said:


> take a ohm reading at the other end and look for continuity between all three phases.


Ohm reading is fine for testing for open winding, but the DC resistance of large motor windings is so small that its impossible to discern good winding from those with partially shorted turns, which should be tested using an inductance meter. 

All three windings should have similar inductance. If one reads significantly low, then that one has a partial short, which will probably show as phase current imbalance in operation.


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## garrfish

successisforme said:


> Ok....i have been told by some that you can not megg a cable that is wired up to a motor.....and some have said that you can only meg from conductor to ground only. well, i wish some one that is 100 percent sure of what they are talking about to answer this.......i recently installed a MCC with both VFD and regular wiring from a bucket.....some of the motors are wired and some are not......i had a couple cables that someone else installed and was bad......well anyway, the question is...... i wanna know some details of how and what it is that i can do with a megger on both situations i have.......i cant say that i am very knowledgable on the use of meggers and would like to be educated about them and what i need to know to be able to use a megger in the future without any doubts. thanks


O K most important is disconnect the VSD from the motor leads and megg to the motor or field disconnect if the disconnect is flagged you are reading the wires I assume this is 480 volt 3 phase that will tell you if your wiring is good you can go phase to phase and phase to ground If the wiring is good and the field disconnect is flagged it should read as if your leads were not together if they read like the leads are together you have a problem some where


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## robbert

TheRick said:


> A megger is used to test the integrity of insulation. If you are testing a motor circuit wired to a MCC bucket or a VFD;
> 
> Open the breaker in the bucket or isolated the VFD.
> Always test your Megger first with leads together and leads apart.
> Clamp onto the equipment ground with one lead and one of the T-leads with the other. This test should give you a very high resistance reading (should be 1K ohm per volt to motor).
> Repeat for each lead.
> If you get a low reading to ground on one or more lead, you have a problem...but is it in the motor or the conductors going to the motor?
> Next repeat this process on the motor leads, disconnected in the peckerhead. Be sure to follow all LO/TO rules!
> If this test gives you a low resistance you have a grounded motor...if not repeat test on the T-leads going back to the MCC.
> Hope that helps!


 motors were disconnected sitting on the floor when I megged them. they had been wet


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## KennyW

I don't get why you'd megger across phases. It wouldn't tell you anything an ohm meter wouldn't and I agree it could be hard on the motor. You're connecting 500-5kvdc straight across a low resistance winding.

I disconnect motor cables at the VFD and megger all three phases together against ground all the time. It only makes sense, you're checking insulation resistance of the motor/cable as a system in one shot. If it's a DOL starter instead of a VFD then you just open the disconnect and again test on the load side terminals inside the bucket.

Of course, if it fails then you have to lift the motor leads at the motor JB to isolate of the fault is the cable or the motor, but this way you only disconnect the motor leads if a test fails.

The only case where I can see that you *have* to check it all separately is if it's a large motor and a very large cable run, there could be enough capacitance in the system to make the test difficult. Also depends on the megger being used.


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## robbert

KennyW said:


> I don't get why you'd megger across phases. It wouldn't tell you anything an ohm meter wouldn't and I agree it could be hard on the motor. You're connecting 500-5kvdc straight across a low resistance winding.
> 
> I disconnect motor cables at the VFD and megger all three phases together against ground all the time. It only makes sense, you're checking insulation resistance of the motor/cable as a system in one shot. If it's a DOL starter instead of a VFD then you just open the disconnect and again test on the load side terminals inside the bucket.
> 
> Of course, if it fails then you have to lift the motor leads at the motor JB to isolate of the fault is the cable or the motor, but this way you only disconnect the motor leads if a test fails.
> 
> The only case where I can see that you *have* to check it all separately is if it's a large motor and a very large cable run, there could be enough capacitance in the system to make the test difficult. Also depends on the megger being used.


its a 10hp 460v ac motor. No vfd being used. I shot 500v in each phase with megger


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## gnuuser

TheRick said:


> That kind of stuff drives me nuts...the "shotgun troubleshooting" of just replacing parts until the thing starts working
> 
> I prefer to spend a little extra time up front to TROUBLESHOOT and find the actual problem, then you can repair/replace only what actually needs it.
> 
> The "shotgunner" may get lucky once in a while, but the true troubleshooter will be right, and will get you up and running quicker, more consistently.


how true and when in an industrial environment you have management deciding on the training and they focus on the shotgun approach because its quicker but not always correct.
and it really cranks me up when they try to tell me how to do my job when they have no idea themselves


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## Bbsound

Huh? 4 years later?



Huh?


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## AllClear

I have a 500 volt hand crank meter, I usually start by megging at the starter, looking for shorts and opens in the motor and wiring at the same time, then if i get a bad reading i'll open up the motor and test it individually.
Hardest part is teaching new guys that the motor windings are supposed to read short circuit.
I've never thought twice about applying 500 volts DC to a 480 volt motor that is wired with 600 volt wire. Also Don't Confuse Megging with a Hi Pot Test.


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## Big John

Remember that during the AC cycle the equipment is seeing 1.414 times the RMS voltage. So if it's rated for 600VAC, it's actually seeing a peak of 850V. 

That's why we test at 1kVDC, it's a very slight over-potential, but not by as much as it seems.


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## sparky970

Does anyone ever look at the dates of threads?


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## markore

sparky970 said:


> Does anyone ever look at the dates of threads?


Naw, sometimes old threads are the best!

What's with the bias against old threads/stickies anyway?

I'd rather REZ PLZ than have a million duplicate threads on the same thing.


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## magmash

Read this, you will find what you need to know about meggering in this PDF :thumbsup: 



http://www.filedropper.com/megger


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## jcdpk

*Meggering*

*Commonly used DC test voltages for AC equipment are:*
* AC equipment rating DC test voltage*
*Up to 100 VAC 100 and 250 VDC*
*440 to 550 VAC 500 and 1000 VDC*
*2400 VAC  1000 to 2500 VDC*
*4160 VAC and above 1000 to 5000 VDC (or higher)*
So, what is “good” insulation? Since we know that insulation has a high resistance to current flow, “good” insulation must be able to provide a high resistance to current flow and be able to maintain that high resistance over a long period of time. In order to evaluate the quality of the insulation certain standard tests have been developed which provide a reliable indicator to determine what comprises “good” insulation.
There are two tests that the production technician can easily perform using a battery powered megger like the one shown in Figure 29. The first test is the short time or spot reading test and the second test is the one minute test.
*The spot reading test*
In the spot reading test you simply connect the “earth” lead of the megger to a good ground and the “line” lead to the conductor and operate the megger for a short time, say for *30 seconds or so*. If the apparatus you are testing has a very small capacitance, such as a short run of cable, the spot reading is all that is necessary. However most equipment (like electric motors and long runs of electrical cable) is capacitive, so the very first spot reading can be only a rough guide as to how good or how bad the insulation is.
Bear in mind that the temperature and humidity will affect the readings and electrical circuits do not have to read infinity (perfect insulation on the megger scale) for the circuit to be serviceable.  The *NEMA* standard for *minimal insulation resistance is*: 1 megohm per rated KV plus 1 megohm. What that means is that if you have a 1000 volt circuit, you should have a _minimum_ of 2 megohms to ground for the circuit to be considered safe to energize and operate.  If you have a *4000 volt* circuit, you need (*5 megohms**)* and if you have a *460 volt* circuit, you should have *(1-1/2 megohms)*, and so on. *Insulation that is in good* condition will normally have *40 - 50 megohms*, or more, to ground.
*The one minute reading test*
The other test is the *one minute* reading. This method is fairly independent of the influence of temperature. It is based on the current absorption of good insulation compared to the *current absorption of moist or contaminated insulation*. A characteristic of good insulation is that it will show a *continual increase in resistance* (which means less leakage current is flowing) over a period of time. The initial test current (called the absorption current) is absorbed by the capacitance of the equipment being tested and then after that, any current flowing is the leakage through the insulation. If the insulation contains much moisture or other contaminants, the absorption current is masked by a high leakage current which stays at a fairly constant value, keeping the resistance low.
*The value of the one minute test* is that it can give you a better idea as to the condition of the insulation and alert you to a problem even when the spot reading indicates that everything is OK.
For example, let’s say a spot reading on a *460 volt* induction motor was 10 megohms, which at first glance is well above the minimum requirement. Now lets assume that the one minute test showed the resistance quickly climbing to 10 megohms and then from there, holding *steady* for the rest of the *60 seconds*. This means that there may be *dirt* or *moisture* on the windings. On the other hand, if the *reading gradually increases* between the 30 and 60 second time interval, then you can be reasonably certain that the windings are in good condition.
*The comparison of the 30 second reading to the 60 second reading is called the dielectric absorption ratio (or D.A.R.).* The way the ratio is calculated is to divide the *60* second reading by the *30 *second reading. Using that method, the chart in Figure 30 will give you an idea of how to determine if the insulation is good.
Insulation​ condition​ 60/30 second​ ratio​ Poor​ Less than 1​ Questionable​ 1.0 - 1.25​ Good​ 1.4 – 1.6​ Excellent​ Above 1.6***​
**In some cases, with motors, values approximately 20% higher than shown here indicate a dry, brittle winding which will fail under shock conditions or during starts. For preventive maintenance, the motor winding should be cleaned, treated and dried to restore winding flexibility.*
Figure  30 Dielectric absorption ratio chart​ There is another megger test called the “*Ten minute test*” which is similar to the one minute test. Because this test requires ten minutes to perform, it is better accomplished by line operated (120 volt) equipment. Essentially the test is performed for ten minutes, the ten minute reading is *divided by* the one minute reading and the resulting ratio is called the *Polarization Index.* This test is mostly used on *larger equipment* that has *large capacitance* and requires longer time to stabilize the absorption test current. The conclusions drawn from the test are the same as those drawn from the dielectric absorption ratio *but* the actual ratio values of the polarization index are not the same as for the dielectric absorption chart in Figure 30.
Using the some of the symptoms listed in the “Problem” column of the troubleshooting chart, let’s go through the process that should be followed to perform the tests and develop the solution
*1.1.1 Breaker trips free when motor start is attempted.*

If the circuit breaker trips free (or trips during normal motor operation) it should* not* be re-closed and another motor start attempted before testing the *motor* and the *power cables* for an insulation failure. If there is a problem, additional starts will just cause further damage.
The symptom of the circuit breaker tripping is *always *caused by overcurrent; either a *phase to phase short circuit* or a *phase to ground short circuit.*


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## Zog

Thanks for the big ugly copy and paste, half of that info is incorrect or outdated.


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