Cable Testing and Diagnostics
Cable Testing and Diagnostics covers insulation resistance, very-low-frequency testing, partial discharge, tan delta, and time-domain reflectometry. The exam focuses on selecting a test that does not damage extruded cable and interpreting locating data.
Core rules
- VLF testing is commonly used for medium-voltage extruded cables at 0.1 Hz; it reduces charging current compared with 50/60 Hz.
- DC hipot is generally avoided on XLPE cable because space charge can remain and cause post-test failure.
- Partial discharge locates voids, delamination, and tracking; PD testing can be offline or online.
- Tan delta measures dielectric losses; a rising trend with voltage or time indicates aging or moisture.
- TDR sends a pulse and times the reflection. Distance = velocity × time/2.
Worked example
A TDR pulse travels at 0.67c and the reflection returns in 2.0 µs. Distance = (0.67 × 3 × 10^8 m/s × 2.0 × 10^-6 s)/2 = 201 m. This is the fault distance from the test end. If the cable route has splices, compare the reflection signature with the route length; a splice can produce a reflection that is not a fault.
Traps
Applying DC hipot to aged XLPE; not calibrating the TDR velocity factor; testing without isolating and grounding; interpreting PD in noisy switchyards without filtering; assuming a passing IR test rules out water trees.