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#67966 - 07/25/06 10:10 PM Insulation Capatence  
hypress  Offline
Member
Joined: Jul 2004
Posts: 129
Charleston WV
At work we heve 3 new meggers and they have a capatence test feature. This is not a capacitor test function but a insulation capatence. How can I relate insulation capatence to insulation quality. I know Doble testing measures capatence but it is done with AC.
THANKS


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#67967 - 07/25/06 10:21 PM Re: Insulation Capatence  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
Hmm,
Now there is one for the books.
What brand/model meggers are they?
Are you sure you aren't referring to Insulation Resistance?.
Reason I say that, is because capacitance is the function of Insulation Resistance.

[This message has been edited by Trumpy (edited 07-25-2006).]


#67968 - 07/25/06 10:44 PM Re: Insulation Capatence  
hypress  Offline
Member
Joined: Jul 2004
Posts: 129
Charleston WV
2 Megger BM21 and one Vangard and I am not sure of it's model number. I can say that all can output 5KV


#67969 - 07/25/06 11:01 PM Re: Insulation Capatence  
hypress  Offline
Member
Joined: Jul 2004
Posts: 129
Charleston WV
The Meggers will gige you eather megohms or uF. the Vangard when it prints out it shows both megohms AND uF


#67970 - 07/26/06 02:21 AM Re: Insulation Capatence  
Trumpy  Offline


Member
Joined: Jul 2002
Posts: 8,223
SI,New Zealand
All along the "cable under test" there will be small capacitances, caused by the cable insulation, between cores, the longer the cable, the higher the capacitance.
This is why it is necessary to discharge the charge built up by meggering a certain cable.
One question, what type and voltage cables are you testing?.


#67971 - 07/26/06 06:08 AM Re: Insulation Capatence  
hypress  Offline
Member
Joined: Jul 2004
Posts: 129
Charleston WV
We megger 600v building wire, 600v & 2400v motors,600v & 2400v draw out circuit breakers.Our 13.2KV equipment and underground is hipoted.


#67972 - 07/26/06 08:51 AM Re: Insulation Capatence  
Zog  Offline
Member
Joined: Sep 2005
Posts: 120
Charlotte, NC
Capacitive charging current is one of the 3 leakage currents you are measuring with IR testing, it tends to decay pretty quickly as the cable is charged, we typically look at the capacitive element by calculating a DAR (Dielectric Absorbsion Ratio) which is determined by dividing the 60 second reading by the 30 second reading. Different ratios apply to different types of equipment but you should always have a DAR of at least 1.0 or more (>1.6 is excellent). >8.0 DAR however would usually means the insulation has become embrittled.


MV/HV Testing Specialist, "BKRMAN"

#67973 - 07/26/06 12:19 PM Re: Insulation Capatence  
Helectric  Offline
Member
Joined: Jan 2005
Posts: 90
Brandon, SD
How often do you hi-pot your cables and what other equipment do you perform hi-pot testing on?

I'm just curious as this is "destructive" testing and many peaple don't use this method except when commissioning new equipment.


#67974 - 07/26/06 12:54 PM Re: Insulation Capatence  
Zog  Offline
Member
Joined: Sep 2005
Posts: 120
Charlotte, NC
While I agree that DC hipot testing should be considered destructive, newer hipot methods are not and are acceptable for routine testing, VLF seems to be the leading canadite, but there is also PD ans Tan delta tests that are as effective and non destructive.


MV/HV Testing Specialist, "BKRMAN"

#67975 - 07/26/06 09:54 PM Re: Insulation Capatence  
hypress  Offline
Member
Joined: Jul 2004
Posts: 129
Charleston WV
Back again. We hipot our cables including the 600V cables as part of our comissioning process. If we have a in servic failur we will hipot those cables.


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