Abstract: The currents flowing through high voltage conductors in a network or switchyard are monitored by magnetic field sensors in the form of air-cored epoxy-filled coils which are electrostatically shielded by discontinuous shields. The magnetic field sensors are located a safe working distance away from the conductors being monitored to remain substantially at earth potential and are orientated to maximize the magnetic field influence from the conductors being monitored. The value of the magnetic field detected by the sensors are sampled instantaneously and the sampled value is processed, having regard to calibration and cross linking effects of the network being monitored to calculate the instantaneous current flowing through the network.