A ring-shaped magnetic circuit is provided for a current sensor of an apparatus for supplying an instrument, wherein two lines supplying the instrument and a neutral electrode or the instrument pass through the magnetic circuit. The coatings of the two lines comprise sufficient thickness with regard to their dielectric strength to the voltage load of lines. The thickness is limited so that an insulation body fits between the two lines consisting of a nonpolar insulation material with high insulation capabilities, similar to the coatings, such as a plastic with a low loss angle. The insulation body reduces partial discharges in the area of the current sensor, which would otherwise result in electromagnetic emissions and thus distortions of the signals created by the current sensor.