In this paper, we propose an algorithm with high-reliability for A-Type leakage current detection by applying a Fourier transform. In this study, Fourier transforms were used to improve the reliability of distinguishing between A-type leakage currents and normal currents due to ZCT saturation. The components of each current waveform were analyzed and verified in the frequency domain, and an algorithm using the content of the third harmonic compared to the fundamental was applied. By applying the proposed algorithm, high-reliability were secured compared to the operation of the existing analog A-Type earth leakage circuit breaker.