Abstract: ［Objective］ Analysis is carried out targeting the power outage issues occurring during the operation of urban rail transit substations, the key components causing power failure are identified, and the safety hazards leading to substation power outages caused by these critical components are addressed. ［Method］ By analyzing the causes of power outage in substations, it is pointed out that the voltage relays are the key components leading to substation power failure. The internal circuit working principle of the low voltage detection relay in a 400 V distribution box is examined, and parameters of the internal circuit problematic parts are calculated. The reasons for abnormal operation of key components are argued based on the calculation results. Based on the cause analysis, an internal circuit optimization scheme is proposed. The optimized internal circuit is then analyzed and tested to verify the reliability of the voltage relay. ［Result & Conclusion］ After optimizing the voltage relay internal circuit, maloperation caused by secondary detection circuit failures inside the substation 400 V switchgear cabinet is effectively avoided. The detection and logic judgment circuits of the voltage relay should be designed with electrical isolation, aiming to enhance reliability of the detection circuit in various operating scenarios, ensuring the adaptability of detection components to different working conditions.