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An insulation system for the 66 kV class high-Tc superconducting fault current limiter (FCL) coil has been developed. The breakdown characteristics were investigated for three kinds of model coils in 77 K liquid nitrogen and 65 K sub-cooled liquid nitrogen at atmospheric pressure. For the model coils, the breakdown tests at AC voltage and lightning impulse voltage in 77 K liquid nitrogen were carried out. The breakdown voltage in 65 K sub-cooled liquid nitrogen confirmed that the breakdown voltage is almost equivalent to 77 K liquid nitrogen. To verify this insulating capability, performance tests with lightning impulse voltage of positive and negative 350 kV were carried out in the 8-layer coil. A power lead that was insulated by ethylene propylene rubber (EPR) without filler was constructed based on the results of the R&D. The insulation systems of these power leads were evaluated for crack resistance and electrical insulation properties. No cracks occurred after seven trials of thermal cycling. The power leads withstood AC 140 kVrms for 1 min and the lightning impulse of positive and negative 350 kV three times each.