We find there are often plenty of questions from readers about the differences between VLF, AC and DC hipot testing methods. And of course, plenty of arguments surround whether DC testing should be performed or completely replaced by VLF testing.
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It is clear from past experience that DC testing has garnered criticism by those who feel it is ineffective at detecting flaws in the quality of electrical insulation and could possibly have detrimental effects as well. With that in mind, we thought it good to address questions about VLF high potential testing to provide clarity to the issue.
What exactly is VLF?
VLF Hipot stands for Very Low Frequency, High Potential testing. This frequency is usually at 0.1 hertz or lower. VLF testing is AC high potential testing with a much lower output frequency. VLF can be used for field testing of cables and rotating machinery that wasn&#;t previously practical using AC tests at power system frequency, due to size and power requirements of the equipment.
Where is VLF used?
When testing highly capacitive loads, VLF has a much lower power requirement than equipment that operates at power system frequency. For the most part, much of this testing is performed on solid dielectric cabling and rotating machinery. While many utilities do not use VLF cable testing, this method does allow users to inspect installation quality for damage due to a variety of influences over time, including high voltage (thumping) and poor workmanship. It is especially good for new installations or repairs of cable prior to re-energizing.
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Editor&#;s Note: This post was originally published on June, and has been updated freshness, accuracy and comprehensiveness.
It depends on ones point of view.
Elevated voltages (above normal) on a weak cable can cause cable failure at one of those weak points. AC high voltage testing at elevated voltages (at 50/60Hz or VLF testing) can cause weak elements in a cable to fail. &#;It has nothing to do with the frequency of the applied voltage waveform and everything to do with the pressure - that is the voltage, applied.
As long as one follows the correct standards for withstand testing and the cable fails this test, then depending on your interpretation of word "destructive", you have exposed and detected a severe weakness in the cable, that would very likely have failed in normal service. The big difference is that a failure under test has very limited current and therefore power injected into the fault, while a unplanned failure under service, can cause catastrophic and collateral damage to other devices in the area of the failure and customer interruptions etc.
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&#;Note that based on extensive research performed by a large university, they found that about 3% of cables may fail a withstand test if one applies the correct withstand voltages, so don't think that every cable you perform a withstand test will fail. Only those that have very severe defects will run to failure.
Also, learn more about VLF and Hipot testing here.
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