Cable Testing and Diagnostics Flashcards

The bridge method used for locating cable faults compares the resistance of:

Answer: The faulted conductor to a looped good conductor of the same type and size

The Murray loop test (bridge method) loops the faulted conductor with a good conductor of equal resistance. A Wheatstone bridge comparison of the two halves of the loop gives the fault distance based on resistance proportionality.

Surge generator (thumper) fault location techniques use high-energy DC pulses to:

Answer: Vaporize moisture in the fault channel, creating an audible thump detectable by listening near the fault location above-ground

The thumper (surge generator) repeatedly discharges high-energy capacitor banks through the cable fault, causing arc discharge at the fault point that creates a sharp acoustic thump heard through the ground surface.

Cable insulation resistance decreases with increasing temperature because:

Answer: Increased thermal energy gives charge carriers more mobility, increasing ionic and electronic conduction through the insulation

Insulation resistance is inversely related to temperature: as temperature rises, charge carrier mobility increases, reducing the resistivity of the insulation by approximately half for every 10 degrees C rise (varies by material).

The purpose of a pre-location step before cable fault pinpointing is to:

Answer: Determine the approximate fault distance using TDR or bridge methods to narrow the search area for acoustic pinpointing

Pre-location narrows the fault position to within approximately plus or minus 10 to 15 feet using TDR, bridge, or other distance measurement methods, so the field crew can focus acoustic pinpointing in a small area rather than walking the entire cable route.

What does a tan delta tip-up result indicate during cable diagnostic testing?

Answer: Tan delta increases significantly with increasing test voltage, indicating water treeing or other nonlinear insulation defects

Tip-up (delta tan delta) is the difference between tan delta at high voltage and low voltage. For healthy insulation, tan delta is essentially flat (voltage-independent). A significant increase at higher voltages indicates water trees or other field-dependent loss mechanisms.

During cable splicing in the field, the most critical step for preventing splice failures is:

Answer: Following the splice kit manufacturer's instructions exactly, including the critical dimensions for each layer of the cable construction

Pre-molded and heat-shrink splice kits are engineered for specific cable dimensions and constructions. Deviation from specified dimensions (especially insulation outside diameter) causes field enhancement, voids, or inadequate stress cone positioning — the primary causes of splice failure.