Multi-scale investigation of overcurrent failure in PVC and XLPE wires, part I: Experimental evaluation of distinct pyrolysis and ignition behaviors

Understanding wire insulation failure under overcurrent conditions is crucial for fire protection in low-voltage systems. This study reveals fundamental differences in overcurrent-induced failure pathways between single-core PVC and XLPE insulated copper wires through fault simulation experiments, TG-FTIR-GC/MS, and FESEM. Under identical overcurrent, XLPE wires spontaneously ignite after pyrolytic gases escape from their open ends in the form of turbulent jets, whereas PVC wires release gases through surface micropores and rely on a fault arc to trigger ignition. Thermochemical characterization reveals that XLPE undergoes single-step, non-charring pyrolysis (411-508 °C), releasing a concentrated burst of highly flammable hydrocarbons (61.59% aromatics with unsaturated side chains) within a narrow temperature window (∼450-500 °C). These gases, ejected as jets from the wire ends, undergo rapid mixing with ambient air through entrainment during buoyant rise. The fuel concentration remains above the lower flammability limit while the gas temperature is still sufficiently high for spontaneous ignition. In contrast, PVC exhibits dual-stage degradation (184-341 °C and 425-700 °C), emitting non-combustible HCl for gas-phase inhibition alongside limited flammable hydrocarbons (57.81% aromatics at 452 °C) that must diffuse through a dense char layer-a condensed-phase physical barrier-before reaching the surface. Quantitative comparisons show that XLPE wires exhibit significantly longer combustion durations (30-60 s, increasing with current) and greater initial flame heights (up to 14.7 cm at 240 A), whereas PVC wires sustain shorter burning times (∼10 s) and lower flame heights (6.3-8.5 cm) across the tested ignition conditions. Microscopic observations confirm that XLPE leaves negligible solid residue, while PVC develops a porous char layer that encapsulates the conductor and suppresses volatile escape. These experimental findings-from macroscopic failure phenomena to chemical signatures and morphological evolution-provide a comprehensive basis for understanding why the two materials follow distinct ignition paths. Based on these observations, material-specific protection strategies are proposed: arc detection for PVC circuits and temperature monitoring for XLPE circuits.