Megger Results 600V 3-Phase in PVC conduit

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MrJLH

Senior Member
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CO
Was out trouble shooting a motor that tripped on a startup today. Meggered the wires with the motor disconnected and disconnected at the starter. This is a 3-phase feeder in a underground PVC conduit.

The results are as follows:

1000V 60 seconds

A-B: 1.4 Gig ohms
B-C: 1.61 Gig ohms
A-C: 1.62 Gig ohms

A-G: 631 Meg ohms
B-G: 623 Meg ohms
C-G: 1.6 Gig ohms

The A-G and B-G does not seem right to me, but NETA states the minimum requirement is 100 Meg ohms. Does any one have any insights to these readings?
Thanks
 
MrJLH said:
Was out trouble shooting a motor that tripped on a startup today. Meggered the wires with the motor disconnected and disconnected at the starter. This is a 3-phase feeder in a underground PVC conduit.

The results are as follows:

1000V 60 seconds

A-B: 1.4 Gig ohms
B-C: 1.61 Gig ohms
A-C: 1.62 Gig ohms

A-G: 631 Meg ohms
B-G: 623 Meg ohms
C-G: 1.6 Gig ohms

The A-G and B-G does not seem right to me, but NETA states the minimum requirement is 100 Meg ohms. Does any one have any insights to these readings?
Thanks
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Have seen readings that go from low 10 to 20 meg then others at 2.2 gigiohms while using a 1,000 Volt megger. Readings similar to yours was never a problem for me on 480 volt circuits. Seems that VFD'S demand at least 10 megohm to ground reading ( 20 megohms even better ) so you do not get earth fault shutdowns. Some literature state to leave megger on a circuit for 10 mintues. Have to write down first reading then at 5 & 10 mintues. My large megger that puts out 500,1000, 2500 & 5000 volts has a twist lock on pushbutton to keep megger on. Pain on my smaller megger holding in the button for 10 minutes. Were you able to take any ampere readings before it tripping out? If possible I always liked to remove the coupling to make sure the liad on motor was not causing trip. Had mechanics tell me that their pump or other liads were not a problem until I showed them that I could not turn motor shaft on a 20 HP motor with a 3' pipe wrench. Is there a nearby motor that you can use its wires to test you motor?
 
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