Megging 5000 volt lines

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stardust

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Megged some 2 year old 5000 volt lines today and got good readings phase to ground, below are phase to phase readings @ 5000 volts 10 min test.

A to B 793 M ohm
B to C 15.9 G ohm
A to C 269 M ohm

First time since construction of building we have been able to Megger.

Anyone think we should be concerned? All numbers look good to me, I am just concerned about the differences.
Thanks for any input in advance.
 
stardust said:
Megged some 2 year old 5000 volt lines today and got good readings phase to ground, below are phase to phase readings @ 5000 volts 10 min test.

A to B 793 M ohm
B to C 15.9 G ohm
A to C 269 M ohm

First time since construction of building we have been able to Megger.

Anyone think we should be concerned? All numbers look good to me, I am just concerned about the differences.
Thanks for any input in advance.
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If the lines were all opened at both ends, the fact that B-C is so much greater than the sum of A-B and A-C suggests to me that the megger voltage is not high enough to be breaking down a marginal insulation region when the two resistances are in series.
I would be concerned about the difference, but without a day zero measurement to compare, I am not sure what to say.
 
What do you mean by "lines"? Cable? What type? Typically a megger test is worthless on MV cable. A VLF or Tan Delta or even PD test is preferred, and required per NETA specs.
 
It was MV cable, not sure of the type. I know it was rated 5000v and was 1/0. The merger was on 5000 volts. I did not know that a megger was not a good test for MV cables. An older electrician ( now passed) showed me how to do it. He always megged and had the lines replaced when readings started trending lower quickly.
 
What type of megger are you using? Zog would be correct. Get the right equipment and the right methods in place.
 
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