Before raising the bow, SS4 electric locomotive must lock the high-voltage room doors of two locomotives. Taking the front bow as an example, in order to ensure the locking of the high pressure chamber door when lifting the bow, SS4 locomotive uses 5 15KF (pressure switch) to detect the interlocking locking of the non-operating door when lifting the bow. If the door interlock is not locked, and there is no pressurized air in the door interlock, 5 15KF cannot be closed, and its contact will open the bow lifting circuit, so the front bow will not be lifted. If the interlock of the control section door is not closed, even if the electro-pneumatic valve 1YV for pantograph lifting is energized, the pantograph cannot be lifted because there is no pressurized air.

To put it simply: SS4 locomotive raises the front bow by monitoring the rear door interlock (5 15KF) with the circuit and monitoring the front door interlock with the wind path, which is the same as raising the rear bow. Only after the door interlock is locked in place can the bow be raised. Of course, in field use, due to quality problems, the high-pressure chamber door will open when arching. As long as the driver takes safety measures when dealing with faults, personal injury and death can be avoided.