apparatus is disclosed for monitoring an a . c . distribution line comprising a novel split transformer core which may be inserted remotely about a distribution feeder line without de - energizing of that line , an encoder for producing a train of voltage signals whose duration are characteristic of the magnitude of the a . c . current flowing through the line and for producing a train of light pulses from the voltage levels which characterize the magnitude of the a . c . current flowing through the line , a decoder which is opticlaly linked to the encoder for measuring the time intervals of the light pulses and for converting the time intervals into a numerical value representing the current flowing through the line .