Interesting Megger Readings

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mull982

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During a down day here at our plant we went to several existing motors to take megger readings as part of a motor trending PM plan we have just started.

On one particular 150hp 480V motor we noticded that the megger was giving us a reading of 0 on each phase to ground. We then went ahead and disconnected the feeders from the motor and megged the motor again. This time we got the expected high resistance satisfactory readings to ground.

We then proceeded to disconned the motor feeders from the starter and take megger readings on the cables. We found that it was actually the cables that were giving us the bad megger readings to begin with for each cable was reading 0ohm to ground.

With each cable reading 0 ohm to ground how are we not tripping any breakers or buring anything up with a ground fault? Since all three of these phases are reading 0 to ground I would expect this to be a bolted fault condition and trip something or see a problem somewhere but this motor continues to run.

We have gotten readings with several different meggers and they are the same. The feeder distance is probalby a couple hundred feed and are 500MCM cables. Has anyone ever seen this before?
 
Carbon Tracking

Carbon Tracking

Check the contactor for carbon tracking...

This can caused voltage to bleed through a open contactor on one or more phases and strange megger readings.
 
Also check if your meggering thru a transformer. You many need to lift leads back at the bucket depending on how its wired. I feel that your reading thru something else and cables are probally ok.
 
bth0mas20 said:
Also check if your meggering thru a transformer. You many need to lift leads back at the bucket depending on how its wired. I feel that your reading thru something else and cables are probally ok.
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The electrician who performed the readings informed me that both ends of the cable were disconnected. The cables were all disconnected from the contactor at the starter bucket, and disconnected from the motor as well. Each cable was seperate and isolated when giving these readings
 
mull982 said:
The electrician who performed the readings informed me that both ends of the cable were disconnected. The cables were all disconnected from the contactor at the starter bucket, and disconnected from the motor as well. Each cable was seperate and isolated when giving these readings
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Considering the readings you posted and the fact it works I have to think the electrician made a mistake.
 
The only way I can see that both ends are disconnected and the motor ran fine without tripping is that someone drove Icepicks in each of the feeder cables to ground them.

Are you sure You disconnected the right end on both sides....not from a different motor by mistake.
 
bth0mas20 said:
Are you sure You disconnected the right end on both sides....not from a different motor by mistake.
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I have a very experienced electrician performing these readings and he is assuring me that the feeders are disconnected entirely at both ends and that he is megging (at 1000V) each individual cable seperately and is positive he is on the same cables at the top and bottom.
 
mull982 said:
With each cable reading 0 ohm to ground how are we not tripping any breakers or buring anything up with a ground fault? Since all three of these phases are reading 0 to ground I would expect this to be a bolted fault condition and trip something or see a problem somewhere but this motor continues to run.

We have gotten readings with several different meggers and they are the same. The feeder distance is probalby a couple hundred feed and are 500MCM cables. Has anyone ever seen this before?
Click to expand...

Either your breaker is not working or your electrician is not testing these cables correctly, it is one or the other.
 
zog said:
Either your breaker is not working or your electrician is not testing these cables correctly, it is one or the other.
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Even if the breaker was not working properly or if we had the instantaneous settings too high we would still be able to measure the fault current with a clamp meter correct?

Depending on the avaliable fault current in this location even if we did not trip the motor breaker I would expect to trip a feeder breaker upstream either on a ground fault or L-L fault.

The electrician did say that after measuring the L-G resistance with a regular fluke meter, he read about 100ohm.
 
mull982 said:
Even if the breaker was not working properly or if we had the instantaneous settings too high we would still be able to measure the fault current with a clamp meter correct?
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Correct, unless it is a 3 pahse cable, then they could cancel each other out unless you seperated 1 phase. Did you take any measurements?

mull982 said:
Depending on the avaliable fault current in this location even if we did not trip the motor breaker I would expect to trip a feeder breaker upstream either on a ground fault or L-L fault.
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Most likely, but not 100% sure it would.

mull982 said:
The electrician did say that after measuring the L-G resistance with a regular fluke meter, he read about 100ohm.
Click to expand...

Is that for all 3 to ground?

What type of breaker is feeding this circuit?
 
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mull982 said:
We then proceeded to disconned the motor feeders from the starter and take megger readings on the cables. We found that it was actually the cables that were giving us the bad megger readings to begin with for each cable was reading 0ohm to ground.
Click to expand...

What was the physical set up for the test? How were connections made?

mull982 said:
The feeder distance is probalby a couple hundred feed and are 500MCM cables. Has anyone ever seen this before?
Click to expand...

Any chance a part of this cable is under water?
 
With unexpected readings I

Always question my self and review everything I am doing.

Second question my test equipment and if possible retest and/or use a second piece of test equipment.

3rd I question the equipment I am testing.
 
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