## 1 Introduction  
Transmission lines are exposed to various forms of combustible materials such as forests, structures, and plains. As such, if a fire occurs on the ground below a transmission line, the transmission line may be exposed to high temperatures, leading to its physical damage such as a decrease in its tensile strength [1]. In addition, electrical damage may occur between the transmission line and the ground or between several transmission lines. Table 1 details the annual number of forest fires and the number of transmission line breakdowns caused by them. An annual average of 3 transmission line breakdowns occurs due to forest fires. Moreover, in 2000, the entire 345 kV East Sea substation temporarily suspended operations due to a forest fire that occurred in the Donghae and Samcheok districts of Gangwon-do [2,3]. In 2014, the electrical facility technology standards were revised and structures on the ground beneath transmission lines of 400 kV or more were allowed, thereby creating an environment in which various combustible materials such as forests and structures could exist on the ground beneath transmission lines in South Korea. If a fire occurs on the ground beneath a DC transmission line, a line short-circuit between phases or a short-circuit between the transmission line and the land may occur due to the flames and combustion products. This may lead to widespread blackouts due to the reduced reliability of electrical grids and breakdowns of high-voltage transmission lines [4]. [PERSON] investigated the combustion of gasoline, lumber, sugarcane leaves, and ash in the air. The dielectric strength of air during the burning of each material was compared to the dielectric strength of normal air, and it was found that dielectric strength was reduced by 49% for gasoline, 27% for lumber, 37% for sugarcane leaves, and 42% for ash. This finding confirmed that the dielectric strength of air varied according to the combustible materials [5]. [PERSON] measured the dielectric strength of air gaps by measuring the amount of conductive particles created during forest fires. The experiments were conducted assuming that the fires occurred in fir tree branches, bamboo, or straw. It was found that as the length of the conductive particles increased, their effect on the dielectric strength of air also increased. In particular, a difference of up to 40% was noted in the presence of conductive particles. Therefore, to study the reduction in dielectric strength caused by flames, it is necessary to select materials that generate fewer conductive particles [6]. Assuming a forest fire, [PERSON] used 24 cm \(\times\) 24 cm lumber with a length of 6 m to create wood cribs of 23.3 kg, 46.6 kg, and 69.9 kg, and used them as fire sources. Voltage levels of 110 kV, 220 kV, or 500 kV were applied. The experimental results indicated that fire duration, flame height, and fire load were influential and that flame height was the most influential variable [7]. [PERSON] et al. analyzed CCTV footage in which transmission lines had short-circuited due to forest fires. They performed direct experiments to analyze the dielectric strength of air during similar forest fires. The experiments were performed at a voltage of 250 kV, and the fire source was a wood crib formed by stacking 2 cm \(\times\) 3 cm \(\times\) 45 cm wood blocks. KCI alkali metal salt was added to the fire source to study the effect of its presence and absence in combustible materials. In addition, the space between poles was measured at 50% and 100% flame presence then compared. The dielectric strength of air was lower for the wood fire to which alkali salt had been added than for the normal wood fire, and dielectric strength was lower when more flames were present in the space between poles. The dielectric strength of the wood fire to which KCI was added was reduced by 30% compared to a normal wood fire. This can be attributed to the effect of thermal ionization on dielectric strength [8]. [PERSON] et al.