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?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[rockman7892]

We have disconnected the cables at both ends, and are only isolating and testing the cable itself.

There are no meters transformers etc...

 

[dpc]

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

 

[scottf]

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

 

[rockman7892]

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.

 

[Skogsgurra]

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

http://www.gke.org

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

 

[rockman7892]

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.

 

[dpc]

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

 

[Skogsgurra]

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

http://www.gke.org

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

 

[rockman7892]

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?

 

[Skogsgurra]

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

http://www.gke.org

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

 

[rockman7892]

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.

 

[Skogsgurra]

Yes. That's right.

Gunnar Englund

http://www.gke.org

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

 

[rockman7892]

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.

 

[TurbineGen]

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?


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

 

[rockman7892]

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.

 

[itsmoked]

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

Keith Cress
kcress -

http://www.flaminsystems.com

 

[rockman7892]

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

 

[itsmoked]

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