During construction projects, electrical contractors are often required to re-use existing medium and high voltage cables. Prior to re-use of existing in-service cables, it is important to properly test its insulation integrity to ensure satisfactory long-term performance. Consultation with both a master electrician and a professional engineer will determine the most appropriate test method, maintain cable integrity during testing, and achieve better analysis of the test results.
Testing of cables for insulation resistance is accomplished using a standard megohmmeter to determine the ohmic resistance across its insulation. However, this test is performed using a DC voltage that is lower than the nominal voltage rating of the cable. In using a lower test voltage, the cable’s insulation material will not be adequately stressed, which could result in a “false pass” for a cable that would otherwise be deemed unacceptable.
DC high-potential (hipot) testing involves applying a high DC voltage that is above the cable’s nominal voltage rating and measuring the leakage current across its insulation. DC hipot testing can provide better results pertaining to the insulation integrity, yet this test can be destructive to the cable. With extended usage, in-service cables can form tiny tree-like water-filled cavities within the cable’s insulation. DC hipot testing causes space charging effects within these cavities that can result in insulation breakdown.
AC hipot testing of cables is similar to DC hipot testing, with the exception that a high AC voltage is applied at nominal 50 Hz or 60 Hz frequencies. Unlike its counterpart, AC hipot testing does not cause space charge within insulation cavities. However, in terms of practicality for field testing of in-service cables, AC hipot test units are not feasible due to their large size and high equipment cost.
Very Low Frequency (VLF) testing is the newest test method that is more suitable for field testing of in-service cables. Under this method, cable testing is performed using a high AC voltage, but at a frequency range between 0.01 and 1.0 Hz. The VLF method provides the benefits of AC hipot testing, while being more practical for field testing of in-service cables. It is to be noted that VLF testing parameters are very specific and customized for each tested cable segment. The test procedure needs to be closely coordinated between a master electrician and a professional engineer. This includes determination of capacitive cable reactance, test duration, test voltage, test frequency, and appropriate test waveform. If improperly performed, VLF testing will yield inconclusive test results and may result in damage to cable insulation.
For further information on VLF Testing, feel free to call me at (240) 582-3900 — Priyan De Silva, Helios Electric LLC.