25kV Cable Megger Test

[lkdiaZ]

I posted this yesterday in the general forum but I thought I would try in the engineering section as well to get some perspective.

We installed a 1500LF pull of (3) 1/0 25kV Aluminum conductor with copper concentric neutrals at 240mils insulation. Subsequent to the pull we meggered at 5kV and found 2 phases at approx 20G-Ohms, whereas the 3rd phase was reading only 8G-Ohms. Was wondering if anyone had any experience with this and whether the 8G-ohms is just too low. We will probably hi pot the cable without terminations to see what it does. Just looking for some feedback. I know there's no definitive answer here.

 

[JoeStillman]

One rule of thumb is 1 MΩ per thousand volts, so 25 MΩ is enough - orders of magnitude less than 8GΩ. Al you can really say is yo know which cable is going to fail first :/.

 

[petersonra]

My niece lives in the USVI.

I have no idea why brand new cable would meg out so different after the pull. Is it all off the same spool? Did you meg it before pulling it?

 

[jtinge]

Was this installed in underground duct bank or in conduit? Was the 1500 LF of cable installed in one continuous length or several sections spliced together, say at 500 LF intervals in manholes? If installed in several sections spliced together, could have a workmanship issue with a splice.

 

[paulengr]

Minimum with NETA ATS is 100 Megaohms. You are so far off the chart it’s not even close. Did you do a graphical version? Watch how it polarizes. If it shoots up quickly then levels off it may indicate something or it stair steps where if it gradually keeps increasing it looks good.

 

[paulengr]

One rule of thumb is 1 MΩ per thousand volts, so 25 MΩ is enough - orders of magnitude less than 8GΩ. Al you can really say is yo know which cable is going to fail first :/.

Pre 1970. Not applicable on polymeric insulation.


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[MrJLH]

Not sure or not but is this shielded cable? Why not just VLF test the cable? You should refer to IEEE 400.1,400.2,400.3