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Low megger readings

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

Zero megohms could easily be 100 kohms. A typical resistance in many metering devices. Are there any protective or metering devices connected?

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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Low megger readings

(OP)

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

Check your test leads to make sure they are not the problem.  Run a megger test of the leads only.  

 

RE: Low megger readings

Just to be clear, you're connecting one of the megger leads to one end of the cable and the other to ground?

RE: Low megger readings

(OP)

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

Looks like you have got an earth fault then. If your system is ungrounded, then nothing needs to trip. But there should be some isolation supervision system that ought to indicate something.

What does an ordinary ohmmeter tell you?  

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Low megger readings

(OP)

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

If there is 0 ohms between two points, you'll measure 0 volts between them.   

RE: Low megger readings

Rockman,

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
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Low megger readings

(OP)

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

Hmm.. Difficult to say anything with any certainty. I think that you should take a few measurements with an ohmmeter (a megger has a problem showing low ohms - they read close to zero) and see how your system is connected to ground and other things. Or simply inspect it. That shouldn't be too difficult if the system is newly installed.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Low megger readings

(OP)

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

Yes. That's right.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Low megger readings

(OP)

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

Rockman, am I reading you post correctly?  100 ohms L-G two phases?  At 277V L-G, thats 2.7 Amps - over 700 Watts in each phase to ground.  Certainly something should be getting overheated. What are your current readings when the motor is running?  Is there an imbalance?
 

------------------------------------------------------------------------
If it is broken, fix it.  If it isn't broken, I'll soon fix that.

RE: Low megger readings

(OP)

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

Is this near any other cables that could be energized during the testing?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Low megger readings

(OP)

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

Then no jiggered meter reading.

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

(OP)

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

(OP)

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

There are highly discrete temp guns you could also use.  These are guns that see only a small spot at large distances.  They typically come with what looks likes a rifle scope,(with cross hairs), for pinpointing the reading location.

I find temp guns to eminately useful.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Low megger readings

(OP)
If the resistance we are reading to ground is indeed about 100ohm corrolating to a current of 2.7A, is there a way to see if this current exists? Possibly with some kind of distortion or something on a scope?  

RE: Low megger readings

Yes. If you measure the sum of all currents in the three phases, you will read zero if no current is lost to ground. And, of course, the leakage current if there is one.

You need a clamp-on current transducer with a wide claw to do this measurement.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Low megger readings

Wide claw to get all those wires thru at the same time.

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

(OP)

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

Ground the non-tested phases. With all cables isolated, you should have a minimum of 1 Mohm resistance to ground. If not then either you have a faulted cable or the cables are not isolated.  

RE: Low megger readings

rocakman- There are flexible split CT's for rent or purchase that could wrap around all the conductors.  They are usually sold for load studies.  I don't recall who makes them.  Several are shown in  instrument catalogs. (Mithcell Instruement).

RE: Low megger readings

Or just unhook the cables at the motor end and insulate them.  Then energize the starter and measure if there is any current flowing at all.  It should be 0 Amps in each conductor if everything is ok.  If there is a L-G leakage path you will read your 2.7A on one of the cables.

RE: Low megger readings

coalmonkey;
You took the words out of my mouth.
 

Bill
--------------------
"Why not the best?"
Jimmy Carter

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