Medium-voltage cables (6 to 33 kV) are the main arteries of energy distribution in industry and substations, and a cable fault can mean hours of lost production. Depending on whether the cable is new or has been in service for years, a set of tests is used to evaluate insulation health.
Why test?
- Detect installation and transport damage (excessive bending, impact, moisture ingress)
- Detect defects in joints and terminations, the most common failure points
- Assess XLPE insulation ageing and water treeing
- Plan preventive maintenance rather than emergency repair
1. Insulation resistance (megger) and polarisation index
DC voltage (typically 2.5, 5 or 10 kV) is applied and insulation resistance measured between each phase and sheath/earth and between phases. The polarisation index (PI) is the ratio of resistance at 10 minutes to that at 1 minute; values below 1 indicate contamination or moisture and values above 2 are usually satisfactory. The test is quick, inexpensive and safe but does not necessarily reveal a localised defect.
2. Very-low-frequency high-voltage test (VLF 0.1 Hz)
High DC testing is not recommended on aged XLPE cables because it leaves space charge in the insulation and can cause failure. VLF testing with a 0.1 Hz cosine-rectangular or sinusoidal wave needs less power and lighter equipment, and is used for acceptance and maintenance testing per IEEE 400.2. Voltage and duration (typically 15 to 60 minutes) follow the standard tables, the cable manufacturer's instructions and IEC 60502-2. Pass criterion: no insulation breakdown during the test.
3. Partial discharge (PD) measurement
Partial discharges indicate voids, contamination or damage in the insulation, and PD can occur months before complete failure. Measurement is performed with VLF or power-frequency voltage (offline) or during service (online). Output: magnitude (picocoulombs), inception voltage (PDIV) and extinction voltage (PDEV), and source location by reflectometry (TDR).
4. Tan delta (tan δ)
Measures the dielectric loss factor and shows the overall ageing of the insulation (especially water treeing). It is usually performed at VLF and at several voltage levels (for example 0.5, 1 and 1.5 U0). A rising tan δ with voltage (tip-up) and a large value indicate a deteriorating cable.
5. Sheath test
DC voltage (for example 5 to 10 kV per IEC 60229) is applied between screen/armour and earth to verify the cable's outer covering (PVC/PE). A damaged sheath allows moisture in and corrodes the screen.
6. Other tests
- Continuity of copper and screen, and conductor resistance
- Phase identification and matching
- Fault location with TDR, surge and arc reflection (ARM) methods
Suggested sequence
- Visual inspection of terminations and joints, and cleaning
- Megger and PI (before the high-voltage test)
- Sheath test
- VLF (with PD and tan δ where required)
- Final megger and comparison with the initial result
- Discharge and earth, then record the report
Safety
- Complete isolation, verification of dead, temporary earths and full discharge of stored charge after the test
- Safe distance and barriers at both cable ends, with a responsible person at each end
- Radio or telephone link between the two ends
- Disconnect surge arresters and VTs connected to the cable
- Follow the operator's procedure and work permit
Report
Includes cable details (manufacturer, voltage class, cross-section, length, year), ambient temperature and humidity, voltages and results of each test, equipment and calibration, comparison with previous results and a technical recommendation.
With VLF, megger and fault-diagnosis equipment, ENP performs cable acceptance and maintenance testing for industrial clients and substation contractors.