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An optically coupled and optically powered hybrid current measurement system for use in high voltage power systems is described. The transmitter, which uses pulse frequency modulation, is designed for minimum power operation of 2.5 mW. The signal to noise ratio is 54 dB corresponding to a resolution of 1 A. System accuracy is determined by the temperature sensitivity of the transmitter which is approximately 400 p.p.m. degrees C-1.