A journeyperson is performing an insulation resistance test on a brand new 15 kV medium-voltage cable connecting a transformer to a switchgear. The cable is approximately 100 metres (328 feet) long. After applying the test voltage for 10 minutes and recording the value, they note the insulation resistance reading is unusually low, even after considering temperature correction. What additional test is most critical to perform before energizing the cable?

Question: A journeyperson is performing an insulation resistance test on a brand new 15 kV medium-voltage cable connecting a transformer to a switchgear. The cable is approximately 100 metres (328 feet) long. After applying the test voltage for 10 minutes and recording the value, they note the insulation resistance reading is unusually low, even after considering temperature correction. What additional test is most critical to perform before energizing the cable?

Answer options:

• A power factor (dissipation factor) test
• A loop impedance test
• A voltage drop test under load ✅ A high potential (hipot) test

Correct answer: A high potential (hipot) test

Explanation: Given an unusually low insulation resistance reading on a new medium-voltage cable, a High Potential (Hipot) test is critical. A hipot test applies a DC or AC voltage significantly higher than the operating voltage for a specific duration to stress the insulation, revealing potential defects or weaknesses that might lead to failure in service. While a power factor test could reveal general insulation degradation, a hipot test is a more direct way to find specific defects that could be causing the low megger reading, allowing for repair or replacement before an in-service failure.