Electrical current sensor

A current sensor for measuring a time-varying electrical current (Im) in a portion of primary conductor (1, 1′, 1″), the sensor including a support or board and a sensor conductor in the form of conductive traces thereon or therein comprising first and second ends (3, 4, 803, 804, 803′, 804′, 903, 904) for connection to a signal processing circuit, a first conductor portion extending from the first end and having a plurality of windings (5, 205, 305, 405, 605, 705, 705′, 1005) to form a first coil portion extending from a first coil portion extremity to a second coil portion extremity, and a second conductor portion returning from the second coil portion extremity to a position proximate the first coil portion extremity and to the second end, centres of the windings defining a median coil plane substantially parallel to the magnetic field (Hm) generated by the time-varying current (Im) to be measured, the second conductor portion being arranged such that the surface area of the one or more projected interference field surfaces (As) is less than ten times the measurement surface area (Am) of a winding times the number of windings, wherein the first and second conductor portions cross over each other to define a plurality of interference field surfaces, as seen projected on the median plane, enclosed by a plurality of clockwise and anti-clockwise interference field current circulating portions (17, 22, 23, 222, 223, 322, 323, 422, 423, 522, 523, 622, 623, 922, 923, 1022, 1023) having surface areas and arranged such that the voltage induced by a magnetic interference field component (Hs) perpendicular to the median plane in the clockwise interference field circulating portions is substantially cancelled by the voltage induced in the anti-clockwise interference field circulating portions.