Acceptable Leakage current value at dielectric Withstand test 1

[shkim2000]

6/10kV cables are installed for the power network at 6.6kW. After completing cable installation, Dielectric high Voltage Withstand Test is to be applied to cables with meggering. Test gives us to confirm the insulation of cable by recording leakage current during test. the detected leakage current should be compared with criteria stipulated in IEC, or IEEE, or other international standard. Then I can determine if test is O.K.
Can I have acceptable value(or calculation Formula) in accordance with international standard by using Dc tester and Ac tester ?

 

[dewone]

Contact the cable manufacture for the amount of time per lengh, test voltage, and allowable leakage current.

 

[benlanz]

What type of insulation does your cable have? IEEE cable test standards no longer support the use of a DC test for extruded, shielded power cable. An acceptable value does not exist. Other than a short condition a dielectric insulation test tells you next to nothing. The only test that can assure reliability on a new installation is to repeat the manufacturers' PD test in the field.






Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[Zogzog]

This is not from any spec or anything but anything >100 microamps leakage would be a reason for concern.

I agree with Ben, a DC hipot test is viewed a a destructive test by the industry these days. An alternate means of AC testing is prefered such as PD testing.

Ben, why not VLF or Tan Delta? Has the 400 group settled on VLF only now?

 

[benlanz]

Zogzog,

DC high potential testing is not considered destructive to new cable installations but, according to IEEE 400, it does not fulfill acceptance test criteria in that even massive defects will not show appreciable leakage current and will be missed by the test.

Very low frequency (VLF) is only an AC voltage source. VLF is not a test or a diagnostic. A VLF high potential withstand is the best 'HIPOT' on the market. However, it is only slightly better than a DC withstand. A HIPOT by definition can never assure reliability. In order to assure reliability you must measure the response of the stress you apply!

Tangent Delta (TD) and all global condition assessment (GCA) types of tests to date are not intended to test new cable systems. There are virtually no measurable losses in a new cable system. Even massive defects in new systems will go undetected with GCA.

The only test, according to IEEE 400, which can prove that the cable system is in excellent condition is to repeat the manufacturer's PD test in the field. (an off-line power frequency PD test with 5pC sensitivity)

As with all engineering decisions there is a risk benefit story that must be considered.

DC HIPOT is the cheapest but is blind to defects in extruded cable.

VLF HIPOT is more costly, it can actually fail some defects that DC can not detect, but you can unknowingly do more harm than good.

Off-line PD is the most costly but, it can actually profile the entire system and compare the results of each component with manufacturer's standards (IEEE, AEIC, ICEA) and predict future performance. Generally if the cost test failures and future outages are added to the price of a VLF or DC HIPOT withstand, there is a significant cost benefit to using an off-line PD test.








Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants

 

[Zogzog]

Thansk Ben, you get a star!

We do PD, TD, and VLF testing. We use TD for maintenance testing only and we like it because the test set is small and lightweight. The biggest problem we have had with PD testing is explaining the results to the customer.