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This study proposes a new method for estimating lightning outage rate of catenary based on an improved electro-geometric model (EGM), considering the effects of ground obliquity and earthing span. A new method is presented by establishing an improved EGM of catenary based on striking distance theory. The model is based on Eriksson's equation for attractive radius of lightning on overhead horizontal conductors and the equation for the lower striking distance to ground proposed by IEEE WG. Combined with the probability density function of lightning current, the attract width of lightning on catenary is solved, and it has a non-linear relationship to the line height for different ground obliquities. By confirming the attract width of catenary, probabilities of lightning on suspension of different overhead return wire (OHRW) heights are then worked out. Taking lightning withstand level of strikes to OHRW and Rusck's-induced voltage formulation into consideration, the formula for calculating the lightning outage rate of catenary is proposed. By calculating a practical catenary above a flat open ground, the presented result shows that the proposed formula is in good agreement with the operating data. This study also depicts the variation in lightning outage rates changing with ground obliquity and earthing span, so the two effects in the outage rate calculation can be more obvious.