Typical Failure Analysis: Tracing Problem Origins from Test Data

In the world of power distribution, cable accessories represent the most vulnerable points in any electrical network. When failures occur, they rarely happen without warning—instead, they leave behind a trail of detectable evidence in their test data. Modern failure analysis has evolved into an exacting forensic science, where specialists act as electrical detectives, interpreting subtle clues in test results to pinpoint root causes and prevent future occurrences. This systematic approach transforms catastrophic failures into valuable learning opportunities for improving product design, installation practices, and maintenance strategies.

The Failure Analysis Methodology: A Systematic Approach
Effective failure analysis follows a rigorous investigative protocol that begins the moment an anomaly is detected. The process typically unfolds in four distinct phases: evidence collection through comprehensive on-site documentation, laboratory-based physical and chemical examination, data correlation that matches test results with observed damage patterns, and ultimately root cause identification. This methodical approach ensures no critical evidence is overlooked and establishes clear chains of causality from initial symptom to fundamental cause. The investigation team must maintain complete objectivity, considering all possible failure mechanisms rather than jumping to premature conclusions based on superficial evidence.

Common Failure Patterns and Their Diagnostic Signatures

Through decades of systematic analysis, several distinct failure patterns have emerged with characteristic diagnostic signatures:

Partial Discharge-Induced Failure

This insidious failure mechanism begins with microscopic electrical discharges within insulation voids or at interfaces. The diagnostic trail starts with elevated partial discharge (PD) readings in periodic maintenance tests, often showing increasing magnitude over time. As the condition worsards, tan delta (dissipation factor) measurements show progressive increases, indicating deteriorating insulation quality. The final failure stage reveals distinctive "electrical treeing" patterns—branching carbonized channels that propagate through the insulation material. These trees typically originate from points of high electrical stress, such as improperly installed stress cones or contaminants within the insulation system.

Moisture Ingress and Water Treeing
The presence of moisture creates a dramatically different failure signature. Insulation resistance measurements show a steady decline during routine testing, while time-domain reflectometry (TDR) may indicate impedance changes along the cable length. In advanced stages, power factor tests reveal significantly elevated values. The smoking gun emerges during physical examination: dendritic water trees growing from points of moisture concentration, particularly in XLPE insulation. These trees appear as feathery patterns under microscopic examination and typically originate from damaged outer sheaths, compromised seals, or during installation in humid conditions.

Interface and Tracking Failures
Poor interfacial contact creates unique diagnostic patterns. Thermal imaging during load cycling reveals localized hot spots at the stress cone-cable interface, while dielectric spectroscopy shows characteristic frequency-dependent responses. The ultimate failure manifests as surface tracking—carbonized pathways across insulation surfaces—along with evidence of corona damage at the semiconductor termination points. These failures frequently stem from improper preparation of cable surfaces, contamination during installation, or use of incompatible silicone greases.

Thermal Degradation and Overload
Excessive current or poor heat dissipation leaves unmistakable evidence. Regular thermal monitoring shows progressively higher operating temperatures, while dissolved gas analysis (DGA) in fluid-filled systems detects specific hydrocarbon breakdown products. The end result shows distinctive insulation carbonization patterns, with different thermal gradients visible in cross-section analysis. Contributing factors include undersized conductors, overload conditions, restricted ventilation, or incorrect installation in conduits with inadequate heat dissipation properties.

The Diagnostic Toolkit: Interpreting Test Results
Modern failure analysis employs an array of sophisticated diagnostic tools, each providing crucial pieces of the investigative puzzle:

Partial Discharge Mapping
Advanced PD testing goes beyond simple magnitude measurement to include phase-resolved partial discharge (PRPD) patterns. Different failure mechanisms produce distinctive PD "fingerprints"—void discharges show symmetric patterns across voltage phases, while surface discharges appear as asymmetric patterns concentrated at specific phase angles. PD mapping also locates discharge sources within the accessory, helping distinguish between internal insulation defects and interface problems.

Dielectric Response Analysis
Frequency domain spectroscopy (FDS) and polarization/depolarization current (PDC) measurements provide deep insights into insulation condition. These techniques help differentiate between moisture absorption (showing characteristic frequency shifts) and thermal aging (manifesting as changes in conductivity curves). The real power emerges when comparing results over time, establishing degradation trajectories that predict remaining service life.

Thermal Analysis and Imaging
Infrared thermography has evolved from simple spot measurements to sophisticated thermal mapping with differential analysis. Modern systems compare accessory temperatures against reference cables and environmental conditions, automatically flagging anomalies as small as 2°C above expected values. Thermal patterns also help distinguish between internal heating (showing uniform temperature elevation) and contact resistance issues (displaying localized hot spots).

Advanced Chemical and Material Analysis
When physical evidence is available, laboratory techniques provide definitive answers. Fourier-Transform Infrared Spectroscopy (FTIR) identifies chemical changes in polymer insulation, such as oxidation products or cross-linking degradation. Scanning Electron Microscopy (SEM) with Energy Dispersive X-Ray Spectroscopy (EDS) reveals microscopic defects and identifies contaminant elements, while Thermomechanical Analysis (TMA) detects changes in material properties that precede visible damage.