MCC Low Megger Reading

[bkhadori]

We were taking megger readings (phase to ground and phase to phase) for an MCC before first-time energization, and the results that we're getting were below 1.0 gega ohms. All the other MCC's on site registered greater than 2.2 gega ohms for the same test. We noticed that once we start pulling out buckets one by one the megger reading started to increase. Eventually, the megger reading was greater than 2.2 gega ohms when about half of the buckets were pulled out. Do you know what could be causing this current leakage and how we can fix it? Thanks

 

[tommom]

Just a question: are all the fuses pulled from the PT's?

 

[bkhadori]

We have a voltmeter on the horizontal MCC bus that is fused. Other than that we have no other PT's in the MCC. Does that answer your question?

 

[gepman]

Are the readings the same for phase to ground as they are for phase to phase? There could be some kind of circuit in your buckets that obtains power from two phases which has an extremely high but additive parallel resistance (they also may be toast by now). Is there anything different about the type of starters or other devices that you have in this MCC versus the other MCCs?

 

[bkhadori]

gepman, the phase to ground reading are pretty much the same as the phase to phase (minor difference). The MCC's contains a mix of E3 Plus type overload starters and fuse disconnects ... same as the other MCC's we have.

 

[gepman]

Is the reading different with the fused disconnects open than with the fused disconnects closed? Do you have more buckets in this MCC than the other MCCs?

I don't see anything in the E3Plus manual that would indicate that it would be a problem but it is a self powered solid state device.

If the readings change with the disconnect position then you know that the problem is after the disconnect. You could remove an E3Plus and check the resistance between phase leads.

If that doesn't narrow it down or solve your problem then I would contact A-B.

 

[Zogzog]

Whats the voltage rating of the MCC's and the Megger test voltage? 1.0 G isnt bad with several buckets in the circuit.

 

[gepman]

After thinking about it some more I am inclined to agree with Zogzog that the value isn't bad. The relative values between the different MCC's tested (depending on the type of megger test) should be somewhat proportional to the size of the MCC or the number of buckets. It would be expected that the this latest MCC would have a lower reading if it had twice as many buckets. Were the environmental conditions the same for each test especially humidity and dust? Was this a 30 or 60 second spot test or was it a 10 minute test?

If they were similar sized MCCs in similar environments then I would contact A-B about their recommended test procedure and minimum test results.

 

[Gianoli]

I am going to assume we are talking about a 480/600 volt class MCC. Insulation resistance values this high are of little concern. Accurate resistance measurements of several hundred Mega ohms and higher are very dependant on a number of conditions. Environmental conditions can play a big role. Is this section of MCC a little dustier? Is it a little warmer? Is it a little more humid in this location?

Come back when you have resistance values in the 1Mega ohm range and we can talk. Accurately measuring resistance values in the Giga ohm range requires precision equipment, controlled environmental conditions and a skilled technician.

 

[bkhadori]

Thank you for all your responses. The MCC is 600V, 800A bus rating, 3 wire, 3 phase, and it is 7 section long. The MCC is located in an electrical room around 10 degrees C (50 F). We have MCC's that are 12 sections long and they all meggered aboved 2.2 G-ohm. When the MCC's are meggered all disconnects are in the open position.

We initially thought that there could be some moisture in the MCC, so we directed warm air from a unit heater inside the MCC bus and left the starter buckets open. The megger reading was taken and it was even lower than before, around 300 M-ohm (which doesn't make sense to me). We kept the heat running for 24 hours and the reading stayed the same. Then we removed the heat, waited few hours, and the reading went up to 700 M-ohm.

I hope this helps. Thanks again!

 

[Zogzog]

Did you tempature correct your readings after using the heater? If not, then thats why they appeared to go down and back up after the heater was removed.

 

[bkhadori]

Zogzog, no I didn't correct @ different temps. But should the resistance go up when the temp inceases?

 

[unclebob]

With 600 V equipment, I don't bother as long as the values are over 600 MOhm.