Low megger readings
Low megger readings
(OP)
Has anyone ever heard of anything like this?
We have recently implemented a megger PM in our plant. We were taking a megger reading on a motor feeder the other week and found that on two of the phases the reading is indicating 0 megahomes.
The cable that we were testing was a 600V 500MCM cable with a distance of about 500ft. We disconneted to cable both at the motor and at the starter iteself. The cables are being tested at 1000V for a duration of at least one minute. The cables go direcly from the starter to the motor itself with no junction in between
I would think with a reading of "0" on any of the phases we would surely have a L-G fault or an L-L fault and trip a OCPD. However this motor continues to run and does not show any signs of a fault. We have taken both current readings and voltage to ground readings with this motor operating and do not see anything strange.
Has anyone ever seen or heard anything similar to this?






RE: Low megger readings
If there are transformers connected between lines and neutral or ground, the resistance actually is close to zero ohms.
Motors, cables and distribution panels should be meggered disconnected from each other. And, never megger a drive, thyristor or VFD. Even if it is rated at 1 kV (which very few are).
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Low megger readings
We have disconnected the cables at both ends, and are only isolating and testing the cable itself.
There are no meters transformers etc...
RE: Low megger readings
RE: Low megger readings
RE: Low megger readings
Yes we are connecting one megger lead to the end of cable and the other to ground.
We have tried several different meggers with same results.
RE: Low megger readings
What does an ordinary ohmmeter tell you?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Low megger readings
Our system is not supposed to be an ungrounded system, but rather it is a solidly grounded system.
If for some reason the system became ungrounded and indded there was an earth fault like you mentioned then shouldn't I read 480V with a L-G voltage reading on the phases that read 0 megaohms. Taking a voltage reading at the starter end of these cables while the motor is operating we have 277V L-G on all three phases.
RE: Low megger readings
RE: Low megger readings
I did have a look at your earlier posts. There, you are telling us about a system without any neutral. For instance the post where you ask how the 30 degrees phase shift between L-L and L-G comes about. There are other posts where this system without (grounded) neutral is mentioned.
Either you have an ungrounded system or you have a corner grounded system. In a corner grounded system, one of the phases have zero ohms to ground. Would that be the case in your plant?
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Low megger readings
Gunnar
Our utility transformer has a 4.16kV wye secondary which is Low Resistance Grounded @ 6ohms. This is a 3-wire system with no neutral carried from the transformer.
The 4.16kV system is stepped down at several unit substations throughout the plant which are 4.16kV Delta - 480V wye. The secondary wye connections of these transformers are solidly grounded therefore the 480V system which I am refering to is solidly grounded. This 480V system is also a 3-wire system with no nuetral carried from the transformer.
However this system is a newly installed system (new construction) for which no ground testing has been completed (ground continuity, earth resistance) so it is possible that although this sytem is supposed to be solidly grounded it may inadvertantly not be.
Is it possible that this system may inadvertantly not be solidly gronded and may be ungrounded, and therefore although I have a L-G fault it is not showing up?
As dpc said (correcting my mistake from earlier) if I had a L-G fault on any phase then I should have a L-G voltage reading of 0V. I am reading 277V on all phases at the starter. Also if more than on phase had a megger reading of 0 wouldn't this be a L-L fault?
RE: Low megger readings
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Low megger readings
I'll check with the electricans for I believe one of the times he measured with an ohmeter but dont recal the value. What should the values on the ohmeter be above to indicate that the cable is not grounded? I'm assuming that Ideally it should read open.
RE: Low megger readings
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Low megger readings
I spoke with the electricians who tested this cable and they have informed me that indeed they did use an ohmeter to measure between the cable and ground. The readings that they found when doing so were about 100ohms.
RE: Low megger readings
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If it is broken, fix it. If it isn't broken, I'll soon fix that.
RE: Low megger readings
The motor is an 100hp motor and with it running we are measuring anywhere from 55-60A. The is no signifigant current imbalance in any of the phases.
RE: Low megger readings
Keith Cress
kcress - http://www.flaminsystems.com
RE: Low megger readings
The cables under test are pretty much isolated from any other energized cables. The cable under test to the motor is isolated in a conduit for the entire run.
The only place where it is remotely close to other cables is possibly in the MCC trough, but only for a ft or so since the starter is at the bottom of the MCC
RE: Low megger readings
Is most the conduit available? Big wire thru conduit can get skinned pretty easily. As TurbineGen points out you are talking some heat dissipation and likely at one place. Get a thermal gun and walk the conduit after the motor has been running a while. You should certainly see a thermal anomaly. If you do you are on borrowed time and not much I suspect.
You could always run your hand down the conduit, but in present circumstances I'd not want my hand on the problem.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Low megger readings
Keith
Most of the conduit is avaliable for only a small portion is underground. I think that is a great idea to walk it down with a thermal gun to look for a heat spot. I will have someone walk this conduit down top to bottom to see if we see anything.
If we have a measured resistance of 100ohm and a current of 2.7A then it looks like we are still reading 277V L-G because of the voltage drop across this 100ohm resistance. If this fault has less of a resistance such as absolute 0 then we would measure 0V btwn L-G because there would be no voltage drop across the resistance?
I'm also guessing that the dissipated heat is a result of the current going through this 100ohm resistance. At first I was confused but now I'm clear that this heat is exactly a result of this resistance. There is not this kind of heat dissipated inside the cable with the cable operating normally because there is very littl resistance in the cable and thus very little voltage drop. I understand that there is heat dissipated in the cable as a result of the cable resistance but this value is very small.
RE: Low megger readings
Unfortunately when going to trace through the conduit with a temp gun we realized that alot of the conduit it turns is inaccessable due to non-existance of platforms and height requirements. We may try to get a lift to get up to one particular fitting that could have possibly caused damage during the pull in order to try to shoot the temp there.
This was a great tip however for future troubleshooting!
RE: Low megger readings
I find temp guns to eminately useful.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Low megger readings
RE: Low megger readings
You need a clamp-on current transducer with a wide claw to do this measurement.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Low megger readings
If your conduit doesn't touch earth points in a bunch of places you can even clamp around the conduit to see the earth current returning.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Low megger readings
Ok I see basically use the clamp on meter as a zero sequence CT. I'll have to look around to see if I can find a large enough claw or ring to encompass all three of the cables. Im assuming that whatever I use should be sensitive to pick up this very small amount of current.
Great Idea, I'm going to see what I can come up with for this measurement.
RE: Low megger readings
RE: Low megger readings
RE: Low megger readings
RE: Low megger readings
You took the words out of my mouth.
Bill
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Jimmy Carter