Low megger readings 1

[rockman7892]

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?

 

[rockman7892]

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.

 

[rockman7892]

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!

 

[itsmoked]

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

 

[rockman7892]

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?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[itsmoked]

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

 

[rockman7892]

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.

 

[Ceast]

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.

 

[rcw retired EE]

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).

 

[coalmonkey]

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.

 

[waross]

coalmonkey;
You took the words out of my mouth.


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