Abstract: A 1000MW thermal power unit encountered an abnormal increase in the current of the shaft seal heater fan during operation. The equipment was inspected on site and relevant parameters were analyzed. The reason for the abnormal increase in fan current of the shaft seal heater was ultimately determined to be the damage to the water seal of the U-shaped drainage pipe at the fan outlet, resulting in an increase in load during fan operation. The U-shaped drainage pipe has been modified to address this issue, and the problem has been thoroughly resolved, which can be used as a reference for other power plants.