Proper cable installation comes with its own set of challenges. But if your field technicians are finding that medium-voltage cables are failing common DC hipot (high potential) or VLF (very low frequency) cable testing, it might not be the cables that are at fault.
In a recent article in NETA World, Mose Ramieh, vice president of business development at CBS Field Services, explained the challenges of conducting accurate cable tests and also how to be efficient and confident with the results.
No matter whether you’re working with typical cold shrink or heat shrink terminations or the increasingly popular load break and dead break terminations, proper isolation and test apparatus setup are crucial.
What looks like failed cables can in fact be an improper test setup. In cases where cables are terminated with load-break elbows on each end, insulate the load breaks with the appropriate dead-end insulated stand-off plugs and create a test block that properly insulates the cable termination system. Depending on the equipment rating, the cable testing tool kit should include 15, 25, and possibly 35 KV insulated stand-offs to plug into one end of the cable that is terminated with load breaks.
Junction bar used to withstand set of cable
Dead break T-bodies have similar requirements to those of load-break elbows but will require at least two insulated plugs to insulate the non-test end. Having six plugs in your test kit will minimize needing to move them from cable to cable.
Standard terminations, including heat and cold shrink terminations, require less specialty equipment to properly insulate them. For DC hipot testing, for example, a short piece of PVC pipe works well to shield standard cable terminations.
Despite this relative ease, it is not uncommon to encounter cable test setups that lack proper shielding. A qualified evaluator should be able to spot setup problems from the test results. Large jumps in leakage current are a strong indicator of improper shielding of the cable end to control the corona that begins to occur. Cables that exhibit flat leakage current measurements are another possible indicator of improper test setup.
Many of the same criteria hold true for VLF cable testing, but there are exceptions. For best results in VLF testing, corona balls — metal spheres that significantly reduce air ionization (corona) and thereby improve overall readings — are recommended.
IEEE 400.2–2013, IEEE Guide for Field Testing of Shielded Power Cable Systems Using Very Low Frequency (VLF)(less than 1 Hz) is an important guide to get to know.
Our NETA-certified technicians provide electrical testing and maintenance services that ensure efficient testing and results you can have confidence in for your medium-voltage cables and beyond. For more information, to discuss your requirements, or to request an estimate or quotation, contact us.
Read the full NETA World article here.
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