Abstract: Aiming at the problem of failing to operate of the existing ultra-high voltage direct current（UHVDC） grounding electrode line impedance supervision（ELIS for short） system, the fault impedance expression under different short-circuit fault conditions of the grounding electrode line is theoretically derived, and the correctness of the theoretical derivation is verified as well. Based on the fault impedance expression, taking the effects of different transition resistances, fault distances and grounding line lengths into account, the law of impedance differences between fault impedance and the normal operation impedance of the grounding electrode line is studied. The adaptability of ELIS system is also analyzed under different fault conditions, and the mechanism of failing to operate of ELIS system is explained. The research results show that the existing ELIS system has a protection dead zone, and it is difficult to correctly detect the faults with high resistance and long distance. In particular, while the length of the grounding electrode line is an integer multiple of the half wavelength for the injected signal in aerial mode, and the single-line fault distance is an odd multiple of the quarter-wavelength for the injected signal in aerial mode, the ELIS system will fail to operate.