Voltage‑Withstand and Partial‑Discharge Testing

for Medium‑ to High‑Voltage (MV–HV) distribution systems—especially underground cables

Test Objectives

1. Verify insulation strength—ensure the cable can withstand high voltage without breakdown.
2. Detect internal defects—such as voids, air pockets, contamination, or manufacturing damage.
3. Confirm overall insulation integrity before installation or energization.

Test Principles

1. AC Withstand Test

The cable is subjected to an over‑voltage AC stress (50/60 Hz) significantly above its service rating:

• Apply 3.5 × U₀ for 5 minutes
  (U₀ = phase‑to‑earth rated voltage)
• Example: a 12/20 kV cable is tested at 3.5 × 12 kV = 42 kV.

2. Partial Discharge (PD) Test

• Raise the voltage to 2.0 × U₀, then reduce to 1.73 × U₀.
• A PD detector records discharge activity inside the insulation, expressed in pC (pico‑coulombs).

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Acceptance Criteria

1. AC Withstand:  Pass if no breakdown occurs during the 5‑minute test.
2. Partial Discharge:  Measured PD must not exceed 10 pC.