What is the minimum test duration for a VLF acceptance test on MV cable?

[wrh9278]

What is the minimum test duration for a VLF acceptance test on MV cable? Another tech told me an hour is a good test but that seems like a really long time, is 15min acceptable?

[madMAX]

IEEE 400.2 recommends cable be VLF tested at 2.5 – 3 times its normal line-to-ground voltage for at least 30 minutes, longer at frequencies lower than 0.1Hz.

[Rojasaj46]

Regarding the VLF test times:

⎯ The recommended minimum testing time for a simple withstand test on aged cable circuits is 30 min at 0.1 Hz (Goodwin, Oetjen, and Peschel [B13]). If a circuit is considered as important, e.g., feeder circuits, then consideration should be given to extending the testing time to 60 min at 0.1 Hz (Hampton, et al. [B19].

⎯ The recommended minimum testing time for an installation and/or acceptance withstand test on new cable circuits is 60 min at 0.1 Hz.

⎯ A test time within the range 15–30 min may be considered if the monitored characteristic remains stable for at least 15 min and no failure occurs. It should be noted that the recommended test time for a withstand test is 30 min.

[wrh9278]

Thanks for the responses. Is it acceptable to gang test with VLF or should each cable be tested individually?

[Rojasaj46]

I havent read anything against it, but remember when you gang test them you will:

1. Have higher readings which you can't use for a baseline.

2. Will in the end have to disconnect and test one by one if there are problems.

3. Taking a short cut to save time but possibly cost your company money in the long run when they have to prove it took X amount of time for X amount of $. (The client is paying to have each cable tested one at a time. It may seem long and tedious, but in the long run, it is the correct way to perform the test with the most accurate results.)

[madMAX]

Does this mean you can terminate a 60 minute acceptance test after 30 minutes if your capacitance and leakage current is stable for at least 15 minutes?

⎯ The recommended minimum testing time for an installation and/or acceptance withstand test on
new cable circuits is 60 min at 0.1 Hz.
⎯ A test time within the range 15–30 min may be considered if the monitored characteristic remains
stable for at least 15 min and no failure occurs. It should be noted that the recommended test
time for a withstand test is 30 min.

[Rojasaj46]

Madmax that is directly out of the IEEE 400.2 the 2013 edition. From what I gather, yes your assumption would be correct. Before you start changing your testing standards get it approved through your management. The section that I found that on is page 10.

[joaogemal]

Hi all!
A VLF test is a simple pass/fail, unless you have a means to, together with your VLF, test the partial discharge. The timing is all about percent, this is, when testing a bad cable with a VLF, 50% of them fail on the first 1/2 hour, very close if not there, 100% fail in an hour of test.

[benlanz]

The statements like "50% of issues fail in 30 minutes and 100% in 60 minutes" are quite common folklore in the industry but they are not founded in good science. VLF test times are arbitrary and have nothing to do with defect growth rate. Since a VLF test can not fail the vast majority of defects (components not meeting the minimum performance requirements of the cable and accessory manufacturers' QC standards) it is not reasonable to recommend a test times over the operating voltage.

Based on a database of over 131 million feet of cable assessment experience and performance over the last 20 years, I would recommend that you minimize your VLF test to less than the operating voltage and for a minute or less just to confirm there are no shorts in the cable system. If you truly want to confirm cable system reliability, you will need to consider a completely different approach.


Ben Lanz
Past Chair IEEE 400