Abstract: In practice, cable length is mainly determined based on engineering experience. If the control cable is too long, protection malfunction may be happened due to distributed capacitance. In order to clarify the influence of cable distributed capacitance on a given length and summarize demonstration methods for cable length selection, taking an 800 m 4-core control cable as the research object, a distributed capacitance model of cable was established, and two testing methods for distributed capacitance were introduced. MATLAB simulation was also used to verify the feasibility of previous measurement methods. An electrical circuit model for control cables and relays based on calculated capacitance values was establish, and the impact of capacitance on the first-order and second-order voltage response process was calculated to demonstrate the possibility of relay action. Grounding short circuit faults were simulated by existing protective devices, the impact of AC entering DC was analyzed, and the effect of cable voltage drop was also calculated. In summary, the feasibility of using ultra long control cables for relay protection could be presented.