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yousafali15
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Join Date: Sep 2010
Location: Saudi Arabia
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Medium Voltage Cable Insulation Test (Megger)

04/04/2011 9:02 AM

Dear Members what happens to the insulation of the underground cable after it is pulled. we tested the 8.7/15 KV cable for 5KV DC before pulling the result is 30 G Ohms but after pulling it is now 600 G Ohms,, what can be the possible reason for this difference , also if it comes out to be in (k ohms) after pulling..
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ronron
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Join Date: Jun 2008
Location: beverly hills
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#1

Re: Medium Voltage Cable Insulation Test (Megger)

04/04/2011 10:05 AM

Hi,

As long the insulation test result is within the acceptable requirement, no issue.

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Tornado
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Join Date: May 2009
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#2

Re: Medium Voltage Cable Insulation Test (Megger)

04/04/2011 10:52 AM

Goodness, what did you do--break the wire?

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yousafali15
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Join Date: Sep 2010
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#3
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Re: Medium Voltage Cable Insulation Test (Megger)

04/04/2011 11:03 AM

30 G to 600 G means ???????????

20 G to 200 k means ???????????
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PeterT
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#4
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Re: Medium Voltage Cable Insulation Test (Megger)

04/04/2011 2:03 PM

What were you measuring with your megger - conductor to shield? When you are getting such high insulation resistance values, they represent fractions of microamps leakage current (30G ohms = 0.167 uA at 5kVDC). Even a little difference in humidity in the air or on the surface of the insulation can make a lot of difference. So 30G on the reel to 600G when pulled into place probably means that the air's relative humidity was lower at the second test, or you cleaned & dried off the end of the cable better.

The second, from 20G to 200k, means that you probably contaminated the bare end of the cable, so that you now are getting 25 milliamps of leakage - Way too much! It is vital to control environmental issues when working with HV cables. A little moisture trapped when the cable is terminated can lead to (often catastrophic!) failure later. You need to clean and dry the cable ends before you go any further.

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yousafali15
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Join Date: Sep 2010
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#5
In reply to #4

Re: Medium Voltage Cable Insulation Test (Megger)

04/04/2011 2:20 PM

Thanks i got the point .
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Fredski
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#6
In reply to #4

Re: Medium Voltage Cable Insulation Test (Megger)

04/05/2011 8:43 AM

I agree, my first thought was quality of connection. He also mentioned it was now buried. Unless it's buried as a coil I have to assume it's run a decent length. I wonder how long the test leads are on his megger for the second test?
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PeterT
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#7
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Re: Medium Voltage Cable Insulation Test (Megger)

04/05/2011 9:12 AM

But you don't megger across the length of the cable, only at the end, between conductor & shield or conductor & ground, or else from phase to phase conductors. Don't need long leads for that.

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