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C5-PFK has attracted much attention as one of the most promising SF6 substitutes for medium voltage (MV) power equipment because of its high insulation strength and low global warming potential (GWP). In this paper, the dielectric properties of C5-PFK-Air and C5-PFK-CO2 mixtures are experimentally investigated, to explore the most effective solution for C5-PFK mixtures for application in MV power equipment. The basic physical properties of C5-PFK and its mixtures are calculated and compared with those of C4-PFN based gases and SF6-N-2 mixtures. The GWP and saturated vapor pressure for gas mixtures are calculated, and a relative value R-GWP is defined to evaluate the greenhouse effect at specific pressure. The 50% breakdown voltages of C5-PFK-Air and C5-PFK-CO2 mixtures under lightning impulse voltage are determined experimentally. The mixture ratio and filling pressure are determined taking into account of the ambient temperature and pressure for MV apparatus. The possibility of practical applications for these mixtures are analyzed by combining the saturated vapor pressure characteristics and the breakdown electric field strength. The synergy effects of the C5-PFK-CO2 and SF6-N-2 mixtures are greater than those of the C5-PFK-Air and C4-PFN-CO2 mixtures. The C5-PFK-Air and C5-PFK-CO2 mixtures have higher dielectric strength than SF6 at 0.1 MPa when the C5-PFK is about 28% under the temperature limitation of -5 degrees C. The pressure increments of 0.014 and 0.066 MPa are required for C5-PFK-CO2 and C5-PFK-Air mixtures, respectively to reach the dielectric strength of pure SF6 at 0.1 MPa under the temperature limitation of -25 degrees C.