An accident occurred in a scenario where a secondary CT disconnection of the protective screen caused overcurrent protection action. During the CT open circuit failure, a special phenomenon of spikes in the AC volume of the protected recording report is found. According to the analysis, the main reason for this phenomenon is that a high open circuit voltage is generated at the CT open circuit point, causing instantaneous breakdown of the open circuit point, and resulting in the protection device collecting a high amplitude spike. If the weekly wave continues to show a spike for a period of time, it will cause overcurrent protection. The simulated field test reproduces the fault condition at the scene and concludes that the higher the open circuit voltage and the more the CT open gap increases, the more likely the spike will occur. However, the previous CT disconnection criterion does not allow for this feature. This paper proposes a new method to identify CT disconnection using spike characteristics in AC current, and it is used as a supplement to the original CT disconnection criterion of the protection device. This work is supported by Science and Technology Project of the Headquarter of State Grid Corporation of China (No. SGBJDK00KJJS900088).