From Thermal Ablation to Mechanical Fracture: Lightning-Caused Breakage of Insulated Conductors Under Thermal–Mechanical Coupling of Power-Frequency Follow Current Arcs

Arc erosion induced by lightning-triggered power-frequency follow currents is a primary cause of insulated conductor breakage. However, significant thermal–mechanical differences remain between laboratory low-current tests and field high-current faults, and existing models fail to quantitatively characterize the transition of damage mechanisms under different currents. This study compares the fracture morphologies of two types of broken samples using scanning electron microscopy. The laboratory low-current samples exhibit an oxygen content as high as 41.22% at the fracture surface, accompanied by a reef-like progressive melting morphology, indicating thermal erosion dominance. In contrast, the field high-current samples display extremely low oxygen content, numerous cracks and pores, and pronounced mechanical fracture features. To quantitatively reveal these mechanisms, a magnetohydrodynamic arc ablation model coupled with material phase change was established using a JKLYJ-70 insulated conductor, and simulations were conducted for a typical fault current of 5 kA. Results indicate a maximum arc temperature of 12,974 K. The axial temperature distribution exhibits a “higher-at-ends, lower-in-middle” profile, while the radial temperature distribution approximates a Gaussian distribution. The peak electrodynamic force on the conductor surface reaches 3951 N, significantly exceeding other force components. Theoretical predictions indicate that the breakage time under pure thermal effects is 14.8 s, which is shortened to 2.8 s after incorporating the electrodynamic force, representing a reduction of 81.1%. Further calculations identify a theoretically predicted critical current threshold of approximately 1.45 kA. Above this threshold, the electrodynamic force is dominant, and mechanical fracture occurs prior to melting. Below this threshold, thermal accumulation and melting erosion are dominant, requiring the conductor to reach its melting point before breakage. This threshold reveals the critical condition for the transition from thermal-dominated to force-dominated breakage under thermal–mechanical coupling, providing a general modeling approach for high-temperature arc–conductor interaction analysis and a theoretical basis for differentiated protection design in electrical engineering.