Abstract: To find out the causes of the impressed current cathodic protection failure in valve chambers and adjacent pipes, as well as the resulting long-term lack of protection in Sichuan-to-East long-distance natural gas transmission pipeline, a simple physical model of partial cathodic protection in valve chambers was established based on a case study of RTU valve chamber. Through the method of model deduction in combination with field test verification, it can be concluded that the insulation failure, which is caused by conduction between pipeline and grounding grid, should be the direct cause of lack of protection. During the process of finding the causes of pipeline insulation failure and positioning, many problems, existing in design, construction, operation and maintenance of cathodic protection and lightning protection grounding system for long-distance pipelines, were exposed. Countermeasures, such as circuit isolation, grounding electrode modification by sacrificial anode, regional hot spot protection by sacrificial anode, and regional cathodic protection by impressed current, have been proposed for the rectification of pipeline insulation failure and restoration of protection.