NETA Cable Testing and Diagnostics 2 — Questions and Answers
Question 1: The bridge method used for locating cable faults compares the resistance of:
- The faulty cable to a calibrated resistance standard
- The faulted conductor to a looped good conductor of the same type and size (Correct answer)
- The primary cable to its concentric neutral for ratio calculation
- Two adjacent cable phases to identify the faulted phase
Correct 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.
The Murray loop test connects the far end of the faulted conductor to a parallel good conductor (same size and material) to form a loop. A Wheatstone bridge is formed with adjustable resistors and the loop halves. At bridge balance, the fraction at balance directly gives the fault location ratio. This test requires a ground fault and a parallel good conductor. Accuracy depends on uniform conductor resistance and temperature. Modern fault locators automate this calculation but use the same underlying principle.
Question 2: Surge generator (thumper) fault location techniques use high-energy DC pulses to:
- Burn away the fault resistance so it can be precisely located by TDR afterward
- Vaporize moisture in the fault channel, creating an audible thump detectable by listening near the fault location above-ground (Correct answer)
- Pre-stress the cable insulation to cause weak points to fail cleanly
- Provide precise impedance measurement at the fault location
Correct 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.
The surge generator (thumper) method is used for pinpointing cables after the fault has been approximately located by TDR or bridge methods. A high-voltage capacitor bank (typically 1 to 10 kJ at 2 to 10 kV) is repeatedly discharged through the cable. When the voltage reaches the breakdown threshold at the fault, a high-current arc occurs. This arc creates a miniature mechanical explosion detectable as a thump at the fault location. A technician walks above the cable route with headphones and a ground microphone, listening for the thumping sound that maximizes directly above the fault.
Question 3: Cable insulation resistance decreases with increasing temperature because:
- Higher temperatures cause physical expansion of the dielectric, reducing its thickness
- Increased thermal energy gives charge carriers more mobility, increasing ionic and electronic conduction through the insulation (Correct answer)
- High temperatures cause breakdown of the chemical bonds in the polymer insulation
- Thermal expansion of the conductor reduces the conductor-to-shield clearance
Correct 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).
Insulation resistance of polymeric and paper-oil insulation decreases significantly with temperature, approximately halving for every 10 degrees C rise. This means a megohmmeter measurement at 40 degrees C may read 50% of the same cable's value at 30 degrees C. IEEE 43 and NETA testing standards require that IR measurements be corrected to a standard temperature using correction factors. Comparing measurements at different times requires temperature normalization. This relationship also means insulation that appears acceptable when cool may exhibit excessive leakage at operating temperature.
Question 4: The purpose of a pre-location step before cable fault pinpointing is to:
- Verify the fault is within the cable under test and not in connected equipment
- Determine the approximate fault distance using TDR or bridge methods to narrow the search area for acoustic pinpointing (Correct answer)
- Pre-heat the fault location to ensure reliable arc discharge during thumping
- Verify the fault type (open, short, ground) before selecting the correct test method
Correct 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.
Cable fault location is typically a two-step process. Pre-location determines the approximate fault distance (plus or minus 3 to 5% accuracy) using TDR, Murray loop test, impulse current method, or arc reflection method. This tells the crew approximately where to go. Pinpointing (using a thumper with acoustic or electromagnetic detector) precisely locates the fault within plus or minus 1 foot. Without pre-location, acoustic pinpointing of a 5000-foot cable route could take days. Specialized cable fault locators integrate pre-location and pinpointing functions with GPS logging.
Question 5: What does a tan delta tip-up result indicate during cable diagnostic testing?
- The tan delta is higher at low voltage than at high voltage
- Tan delta increases significantly with increasing test voltage, indicating water treeing or other nonlinear insulation defects (Correct answer)
- The cable tip was damaged during installation causing increased losses
- The test voltage must be increased to get a valid reading
Correct 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.
Water trees are dendritic channels of water-filled micro-voids in XLPE insulation that grow over decades. They create voltage-dependent dielectric losses. The tip-up calculation: delta tan delta = tan delta at 2*U0 minus tan delta at 0.5*U0. Per IEEE 400.4: delta tan delta less than 0.1 times 10^-3 is excellent; 0.1 to 0.5 times 10^-3 is good; 0.5 to 1.0 times 10^-3 is marginal; greater than 1.0 times 10^-3 indicates heavy water treeing requiring close monitoring or replacement. The voltage-dependent characteristic distinguishes water tree degradation from uniform aging, which shows flat tan delta.
Question 6: During cable splicing in the field, the most critical step for preventing splice failures is:
- Selecting the highest-rated splice kit available regardless of cable type
- Following the splice kit manufacturer's instructions exactly, including the critical dimensions for each layer of the cable construction (Correct answer)
- Applying as much insulating tape as possible to ensure adequate insulation
- Testing the splice with a hi-pot immediately after installation before burial
Correct 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.
Cable splice failures are a leading cause of cable system outages. Pre-molded cold-shrink and heat-shrink splice kits are factory-engineered to exact cable dimensions. The internal geometry is calculated to control the electric field distribution. If the installer uses the wrong kit, measures incorrectly, or deviates from step-by-step instructions, the result is localized field enhancement causing ionization and eventual insulation failure. NETA field testing after splicing (tan delta, partial discharge, or hi-pot) helps verify splice quality before the cable is returned to service.
NETA Cable Testing and Diagnostics 2
The bridge method used for locating cable faults compares the resistance of: