Managing overvoltages caused by lightning strikes is a crucial aspect in maintaining the reliability of the electric power system. This study examines the impact of lightning arrester (LA) failure on the operational stability of the PT ITS 150kV substation using a quantitative scientific approach. The methods used include a literature review to formulate a theoretical framework, field observations to inventory the condition of the existing LA, interviews with technicians and engineers at PT PLN ULTG Citra Raya, and resistive leakage current measurements on the LA using a Leakage Current Monitor (LCM). Primary data from the leakage current tests were analyzed descriptively— including calculating the percentage leakage current, the average, and the range of leakage current variations between phases and locations—and modeled through power system transient simulations to assess the system's response to a LA failure. The results showed that the percentage of LA leakage currents ranged from 2.00% to 7.33%, still far below the acceptable threshold (<50%). Among the three test locations (Bay Cengkareng Baru, Tangerang Baru, and Bay Konsumen), the T phase in Bay Konsumen had the highest leakage current value (7.33%), which, while safe, indicates the potential for accumulated degradation of the insulation material. Transient simulation analysis revealed that the failure of a single LA unit can increase the transient voltage level by 15– 20% at the furthest point on the busbar, potentially triggering a flashover event in critical equipment if not promptly addressed. Based on these findings, it is recommended, conduct periodic LA inspections and replacements every two years, strengthen online leakage current monitoring methods for early detection of degradation, prioritize evaluation of environmental conditions (humidity, air pollutants) in Bay Konsumen, and update the protection coordination scheme by taking into account the transient simulation results. These findings are expectedvii to serve as a reference for PLN and academics in improving maintenance policies, mitigation strategies, and overvoltage protection design in the future.