Aiming at the operation of SS4 improved electric locomotive, protection systems such as auxiliary grounding, zero voltage, main circuit grounding and traction motor overcurrent are realized. After the main circuit breaker trips, due to virtual connection or other reasons, the display screen of the main and auxiliary stations does not display. However, the main circuit breaker was forced to close manually, and after the camera was started, the traction handle was lifted, so that the whole locomotive had no flow and the circuit improvement was put forward. Keywords: main circuit breaker; Fault display screen; Auxiliary grounding; Zero pressure; The main circuit is grounded; Traction motor overcurrent; Electronic cabinet; When the reformed SS4 electric locomotive was used in Shenshuo Railway, there were many times when the main circuit breaker tripped without display, and the driver forced the main circuit breaker to close. After starting the split camera, the traction handle is lifted, and the whole locomotive does not flow. However, the fault display screen of the main and secondary stations is not displayed, which makes the flight attendants unable to judge the fault location. For example, 0524# locomotive is engaged in traction task between Shenmu North and Shenchi South, and the trip main circuit breaker is not displayed during operation. If the driver adopts the above method, the whole vehicle has no flow, and finally the machine is broken. Grounding after the return section is used as a supplement, and it is normal after removing the grounding point. Another example is that 0654# locomotive failed to show that the main circuit breaker frequently tripped during operation, resulting in the slow operation of the section, and the turn-back section was detected as a zero-voltage transformer fault. The above two examples all show that when the lighting display circuit of SS4 modified electric locomotive is virtually connected or opened, the fault location cannot be correctly displayed, which brings inconvenience and misleading to the emergency handling of the crew, and the circuit needs to be improved. The fault cannot be displayed correctly because there is an auxiliary circuit grounding protection circuit between the auxiliary winding X6 of the transformer and the ground.