The M54124L is a semiconductor integrated circuit for use in the amplifier section of earth-leakage circuit breakers. It consists of a differential amplifier, one-shot circuit, output circuit and voltage regulator. It is connected to the secondary side of the zero-current transformer, ZCT, and detects leakage current in both inputs of the differential amplifier. Signals amplified by the differential amplifier are integrated by an external capacitor, and applied to the input pin of a one-shot multivibrator circuit having time-delay characteristics that are suitable for high-speed earth-leakage circuit breakers (such as specified in JIS C 8371). The one-shot multivibrator circuit normally maintains a low output. When the input current (earthleakage) exceeds a specified level, a one-shot high pulse is output to turn on an externally connected thyristor.