Insulation Resistance Testing for Cable Acceptance Tests

Why Cable Acceptance Testing Matters

Insulation resistance (IR) testing is the first line of defense in cable commissioning. Before a new XLPE or PVC cable is energized, an acceptance IR test verifies that conductor-to-ground and conductor-to-conductor insulation meets minimum thresholds set by IEEE 400, IEC 60502, or local utility specifications. Skipping this step can mean an in-service failure within months, with downtime costs that dwarf the test itself.

Selecting the Right Test Voltage

Test voltage must match cable rated voltage. Common ratings:

• Cables rated up to 1 kV: test at 500 V or 1 kV DC
• Cables rated 1–6.6 kV: test at 2.5 kV DC
• Cables rated above 6.6 kV: test at 5 kV or 10 kV DC

EK megohmmeters in the EK-3000 series provide selectable outputs from 250 V to 10 kV with current limiting to protect cable insulation during the test.

Polarization Index (PI) and DAR

A single resistance reading is rarely enough. Two diagnostic ratios refine the picture:

• Polarization Index (PI) = R(10 min) / R(1 min). PI > 4 = excellent, 2–4 = good, 1–2 = questionable, < 1 = unacceptable.
• Dielectric Absorption Ratio (DAR) = R(60 s) / R(30 s). DAR > 1.4 indicates dry, clean insulation.

Field Procedure

1. Disconnect the cable from all equipment at both ends and ground for 5 minutes to drain residual charge.
2. Connect tester guard to cable shield, line lead to conductor, earth lead to local ground rod.
3. Apply selected DC voltage and log readings at 30 s, 60 s, and 10 min.
4. After test, short the conductor to ground for at least 4× the test time before reconnecting.

Pass Criteria and Documentation

Minimum acceptable IR per IEEE 400 is typically 100 MΩ per kV of test voltage, corrected to 20 °C. Always record temperature, humidity, cable length, and applied voltage alongside resistance values for year-over-year trending.