Abstract: Shunt means supply a diode bridge with an alternating current proportional to the AC to be measured. The bridge has at least two electroluminescent diodes which emit two alternating streams of half-wave light pulses which are supplied to two fibre optic systems. These convey the light signals to a safe distance from the high tension line where two light receptors receive the respective streams of half-wave light pulses and apply electrical signals proportional thereto to opposite inputs of a differential amplifier. The output of the amplifier is a readily measurable AC. Balancing means may be provided to compensate for any difference in the overall transmission paths for the two half-wave pulse streams. A resistive load may be provided adjacent the line to sink the current flowing through the diode bridge.