A fire case in the copper powder workshop of the Chongqing Huahao Smelting Limited Company is discussed, and the 10 kV highvoltage transmission lines near the copper powder workshop are examined by the analysis of mechanical properties and residual magnetic field tests. In combination with the thunder weather analysis, it is concluded that the fire was caused by the lightning strike on the 10 kV highvoltage transmission lines. The findings provide a reference for determining whether the transmission line fault was caused by lightning strikes in the case of not installing a wave recorder and lightning counter.