Current sensor with differential magnetic field transducer

Abstract

A current sensor (2) comprising a differential magnetic field transducer (4) having at least two magnetic field sensor portions (14), configured for measuring a primary current flowing in a primary conductor positioned adjacent and in proximity to the differential magnetic field transducer (4), and a housing (10) in which the differential magnetic field transducer is lodged, characterized in that the current sensor further comprises a magnetic shield (8) having lateral branches (16a) joined together at one end thereof to a bridging branch (16b) such that the magnetic shield (8) has a general U-shape. The magnetic shield surrounds a primary conductor passage (5) through which the primary conductor (1) is installed in use, the differential magnetic field transducer positioned adjacent and in proximity to the bridging branch (16b), between the bridging branch (16b) and primary conductor passage (5).

Description

[0001] CURRENT SENSOR WITH DIFFERENTIAL MAGNETIC FIELD TRANSDUCER

[0002] The present invention relates to a current sensor with a differential magnetic field transducer for measuring a current flowing in a primary conductor.

[0003] To measure the current flowing through a conductor most current sensors rely on measuring the magnetic field induced by the current. While many solutions exist, the choice for the right solution is generally driven by its accuracy, cost, size or integration level. Conventional magnetic-based solutions currently used in automotive and industrial applications for sensing current in ranges from a few hundred to a few thousand Amperes include magnetic core based transducers and coreless transducers: i) Core-based sensors: A ferromagnetic core surrounds the conductor to integrate the magnetic flux around it and concentrates it on small gap where a single field sensor is located. The magnetic field sensed is proportional to the current flowing through the conductor and is measured in single mode by the sensing element(s). While this principle is widely used and offers numerous advantages such as high signal to noise ratio and high immunity against external fields, one of the important drawbacks arise from magnetic hysteresis and saturation / linearity which leads, in certain applications, to higher costs, size and weight compared to coreless sensors;

[0004] Hi) Coreless differential field sensing / gradient field sensors: These sensors directly measure the differential field induced around the conductor by the current flow. Compared to core-based systems, these sensors do not face saturation or magnetic hysteresis issues, and are more compact. They however have lower signal-to-noise ratios and higher sensitivity to mechanical placement tolerances and variations. While differential sensing allows to reject common mode fields, and thus provide a certain immunity against external fields, such systems are still sensitive to external field gradients and crosstalk from adjacent conductors (in multiphase conductor systems) which limits integration with close proximity between the conductors.

[0005] In view of the foregoing, it is an object of the invention to provide a current sensor that can measure large current amplitudes yet is compact, accurate and has a low sensitivity to external fields.

[0006] It is advantageous to provide a current sensor that is easy to integrate in multi-conductor systems, such as three phase conductor systems, for instance for inverters. It is advantageous to provide a current sensor that is economical to manufacture and install.

[0007] It is advantageous to provide a current sensor that is reliable and robust.

[0008] Objects of the invention have been achieved by providing a current sensor according to claim 1.

[0009] Dependent claims set out various advantageous features of embodiments of the invention.

[0010] Disclosed herein is a current sensor comprising a differential magnetic field transducer having at least two magnetic field sensor portions, configured for measuring a primary current flowing in a primary conductor positioned adjacent and in proximity to the differential magnetic field transducer, and a housing in which the differential magnetic field transducer is lodged. The current sensor further comprises a magnetic shield having lateral branches joined together at one end thereof to a bridging branch such that the magnetic shield has a general U-shape, the magnetic shield surrounding a primary conductor passage through which the primary conductor extends, the differential magnetic field transducer positioned adjacent and in proximity to the bridging branch, between the bridging branch and primary conductor passage.

[0011] In an advantageous embodiment, the current sensor further comprises a circuit board, the differential magnetic field transducer being mounted on and connected to the circuit board.

[0012] In an advantageous embodiment, the circuit board is positioned adjacent the bridging branch of the magnetic shield, either against the bridging branch or separated therefrom by an insulating layer.

[0013] In an advantageous embodiment, the differential magnetic field transducer comprises an integrated circuit chip, the magnetic field sensing portions being formed in the integrated circuit chip on a side facing the primary conductor passage.

[0014] In an advantageous embodiment, a length of the lateral branches of the magnetic shield is greater than the sum of the thickness of the differential magnetic field transducer and the circuit board and a thickness of the primary conductor configured for mounting in the primary conductor passage, such that the free ends of the lateral branches extend beyond the primary conductor which is fully positioned within and overlapped by the lateral branches.

[0015] In an advantageous embodiment, a section of the primary conductor is integrated with and forms part of the current sensor.

[0016] In an advantageous embodiment, the magnetic shield has a length in a direction of extension of the primary conductor that is greater than a length of the differential magnetic field transducer so as to completely overlap the differential magnetic field transducer.

[0017] In an advantageous embodiment, the primary conductor has a measuring section over which the differential magnetic field transducer is mounted that has a reduced cross-section area compared to adjoining sections of the primary conductor.

[0018] Also disclosed herein is a multi-conductor current sensor system for measuring currents in a plurality of primary conductors, comprising a plurality of current sensors, each for measuring a primary current flowing in a different one of the plurality of primary conductors, wherein at least one of said plurality of current sensors is a magnetically shielded current sensor according to any preceding embodiment.

[0019] In an advantageous embodiment, at least one of said plurality of current sensors is not a magnetically shielded current sensor.

[0020] In an advantageous embodiment, each current sensor comprises a differential magnetic field transducer.

[0021] In an advantageous embodiment, the multi-conductor current sensor system comprises at least one said current sensor with magnetic shield positioned around one of the primary conductors and at least one primary conductor coupled to a current sensor without a magnetic shield.

[0022] In an advantageous embodiment, the multi-conductor current sensor system has three primary conductors, the current sensor with magnetic shield being coupled to a centre positioned primary conductor of the three primary conductors.

[0023] In an advantageous embodiment, the current sensors are lodged in a common housing having a plurality of primary conductor passages, one for each primary conductor.

[0024] In an advantageous embodiment, the multi-conductor current sensor system further comprises the primary conductors integrated into the housing.

[0025] Further advantageous features of the invention will be apparent from the following detailed description of embodiments of the invention and the accompanying illustrations.

[0026] Brief description of the figures

[0027] Figure 1a is a simplified schematic perspective view of a current sensor according to an embodiment of the invention;

[0028] Figure 1 b is a schematic cross-sectional view of the embodiment of figure 1a;

[0029] Figure 1c is a view similar to figure 1b with a circuit board and housing removed, to illustrate the effect of a magnetic field generated by a primary current on the magnetic field transducer;

[0030] Figure 2 is a simplified schematic perspective view of a laminated magnetic shield of a current sensor according to an embodiment of the invention;

[0031] Figure 3a is a simplified schematic cross-sectional view of first and second primary conductors with a current sensor according to the prior art;

[0032] Figure 3b is a simplified schematic cross-sectional view of first and second primary conductors with a current sensor according to an embodiment of the invention;

[0033] Figure 4a is a plot of the percentage of cross-talk versus the distance between primary conductors according to the prior art and according to an embodiment of the invention for comparison;

[0034] Figure 4b is a plot of the percentage of cross-talk versus the height of lateral branches of a magnetic shield of a current sensor according to an embodiment of the invention;

[0035] Figure 5a is a simplified schematic cross-sectional view of a three-phase primary conductor system with coreless magnetic field transducers for current sensing according to an example of a prior art configuration;

[0036] Figure 5b is a simplified schematic cross-sectional view of a three-phase primary conductor system with a current sensor according to an embodiment of the invention;

[0037] Figure 6a is a simplified schematic cross-sectional view of another embodiment of a three-phase current sensor according to an embodiment of the invention; Figure 6b is a view similar to figure 6a of variant with primary conductor bars integrated within the current sensor;

[0038] Figure 6c is a top view of the embodiment of figure 6b;

[0039] Figure 7a is a simplified schematic perspective view, similar to figure 1a but of another embodiment of a current sensor according to the invention;

[0040] Figure 7b is a simplified schematic cross-sectional view of the embodiment of figure 7a;

[0041] Figure 8a is a view similar to figure 7a of yet another embodiment of a current sensor according to the invention; and

[0042] Figure 8b is simplified schematic cross-sectional view of the embodiment of figure 8a.

[0043] Referring to the figures, various embodiments of the invention comprise at least one current sensor 2 for measuring the primary current flowing in at least one primary conductor 1 , the current sensor 2 comprising a differential magnetic field transducer 4 having at least first and second magnetic field sensor portions, and at least one magnetic shield 8, and a housing 10 in which the magnetic shield 8 and differential magnetic field transducer 4 are lodged.

[0044] The magnetic shield 8 comprises lateral branches 16a joined together at one end of the lateral branches 16a by a bridging branch 16b such that the magnetic shield has a general "II" shape. The U-shaped magnetic shield 8 is made of a soft magnetic material, in particular a ferromagnetic material that may be formed of a single integral piece of soft magnetic material or that may be made of a stack of laminated soft magnetic material sheets as schematically illustrated in figure 1d. The lateral branches 16a and bridging branch 16b surround a passage 5 through which a primary conductor 1 extends or is configured to extend.

[0045] The primary conductor 1 may for instance be in the form of a perse well-known bus bar that has a rectangular cross-sectional shape with a long edge (width) and a short edge (height), the long edge arranged essentially parallel to the bridging branch 16b of the magnetic shield 8. The long edge of the rectangular cross-section has a width Wp that is less than a width IV between the inside surfaces of the lateral branches 16a such that the primary conductor bar may be inserted into the primary conductor passage 5 that the magnetic shield 8 surrounds. The primary conductor 1 is separated from the magnetic shield 8 by a gap containing an insulating material that may form part of the housing 10.

[0046] Within the scope of the invention, it is however possible to have primary conductors of other cross-sectional profiles, for instance circular, oval, square, polygonal or even irregular cross- sectional shapes.

[0047] The primary conductor 1 may advantageously comprise a measuring section 3 having a reduced cross-sectional area compared to the primary conductor sections adjoining the measuring section, to concentrate the current and induce a magnetic field of greater intensity around the measuring section 3. The measuring section may for instance be provided by one or more indents in the primary conductor as schematically illustrated in figure 1a. The measuring section is preferably centrally located within the primary conductor as illustrated in figure 1a, however it could also be provided asymmetrically with respect to the center line of the bus bar, for instance on one side of the primary conductor bus bar.

[0048] The differential magnetic field transducer 4 is positioned overhead the primary conductor passage 5, and thus overhead the primary conductor 1 mounted in the primary conductor passage. The differential magnetic field transducer 4 is positioned in particular overhead the measuring section 3 of the primary conductor 1 .

[0049] The magnetic field sensor portions 14 are separated by a distance G in order to measure a difference between the magnetic field intensity at the two different positions as per se well- known in the art of magnetic field gradient sensors. As illustrated schematically in figure 1c, the differential magnetic field transducer is positioned substantially symmetrically with respect to the center line of the primary conductor 1 and thus the magnetic field measured at the two magnetic field sensor portions are of opposite direction. The principles of such magnetic field differential measurement for the measurement of a current is per se well-known and does not need to be explained further in detail herein.

[0050] The differential magnetic field transducer 4 may advantageously be in the form of an integrated circuit (IC) chip which may advantageously be a surface mounted on a circuit board 6 comprised in the current sensor 2. Each magnetic field sensing portion may comprise a Hall effect sensor, or a giant magneto-resistive sensor or other forms of magnetic field sensors that are perse well known in the art. The IC chip may for instance be in the form of a CMOS chip although other types of semi-conductor integrated circuit chips with magnetic field sensing portions per se known in the art may be used. The semi-conductor chip does not need to be further described, such semi-conductor chips with magnetic field sensing portions and connection terminals being perse well known in the art.

[0051] The circuit board 6 and differential magnetic field transducer 4 are positioned between the bridging branch 16b of the magnetic shield 8 and the primary conductor passage 5, and therefore between the bridging branch 16b and the primary conductor 1 when it is installed in the primary conductor passage 5. The circuit board 6 and differential magnetic field transducer 4 mounted thereon may advantageously be positioned adjacent and proximate the bridging branch 16b, either in contact thereagainst or separated by a housing layer. The magnetic shield 8, circuit board 6 and differential magnetic field transducer 4 may be assembled in a separately formed insulating housing that encapsulates the magnetic shield and differential magnetic field transducer 4. In a variant, the housing 10 may be overmolded over portions of the circuit board, differential magnetic field transducer and magnetic shield, or entirely encapsulate the differential magnetic field transducer, circuit board and magnetic shield.

[0052] The current sensor comprises a connector 12 or other interconnection means for coupling external power and signal conductors to the power and signal conductors of the circuit board and differential magnetic field transducer.

[0053] The lateral branches 16a have a height H configured to extend the free ends of the lateral branches past the primary conductor that is fully positioned within the primary conductor passage 5 surrounded by the lateral branches 16a and bridging branch 16b. As discussed further on, the extension of the magnetic shield lateral branch 16a past the primary conductor 1 ensures effective shielding of the primary conductor from external fields and in particular when positioned next to other primary conductors in a multi-conductor system such as a two-conductor system or three-phase conductor system as schematically illustrated in figures 5b and 6a to 6c.

[0054] In a single conductor system as schematically illustrated in figures 1a to 1c and figures 7a to 8b, the magnetic shield advantageously prevents external fields from disturbing the measurement signal of the magnetic field induced by the current flowing in the primary conductor and thus allows to increase the signal to noise ratio. This further allows the differential magnetic field transducer to have a high sensitivity without the adverse effects of external magnetic fields on the measurement output. Further, since the magnetic shield is not used to concentrate the magnetic field through an air gap in which a magnetic field sensor is positioned, the problems of linearity, saturation and air gap size in core-based current sensors is avoided. On the other hand, the benefits of a coreless measurement system using a differential magnetic field transducer are achieved. In a multi-conductor system, for instance a three-phase primary conductor system, for instance for use in inverters, where high amplitude currents are present, a current sensor with a magnetic shield may be positioned either around the center primary conductor as illustrated in figures 5b, 6a and 6b, or around the two outer primary conductors (not illustrated), to avoid the effects of cross-talk between the magnetic fields generated by each primary conductor. It may be noted however that it is possible within the scope of the invention to have each primary conductor surrounded by a current sensor with a magnetic shield according to an embodiment of the invention.

[0055] As shown in Figure 1c, while primary current “I” flows through the primary conductor, the magnetic sensing portions 14, noted “SL” and “SR”, will sense the differential magnetic field induced locally, noted “BL” and “BR”, the amplitude of which is directly proportional to the primary current. Although the magnetic shield 8 changes the magnetic field distribution around the primary conductor 1 and blocks external magnetic fields from penetrating the area where the magnetic field sensor portions 14 are located, a magnetic field gradient is still induced locally by the primary conductor and measurable at the sensor portions 14 locations.

[0056] In this configuration, the active field on which the current measurement is based on is the vertical field components (along axis Z) referred to as BL and BR, representing the magnetic field density at sensor location SL and SR respectively; the relationship between the differential field induced at the sensor location is referred to as the Coupling Factor “CF” between the primary conductor to the sensing portions 14 and can be described by equation [1] and [2]:

[0057] CF = (BL - BR) / 1 [1]

[0058] Bdiff = CF x | [2]

[0059] I : current flowing through the primary conductor (Ampere)

[0060] BL and BR : Flux density on axis Z sensed by elements SL and SR respectively (Tesla) CF : magnetic coupling factor of the primary conductor to the sensing magnetic field transducer (Tesla / Ampere)

[0061] Biff : Differential magnetic field sensed by the magnetic field transducer (Tesla)

[0062] When an adjacent current carrying conductor is present another parasitic coupling factor appears, representing the magnetic field coupling between the sensor portions of the magnetic field transducer with respect to the current flow through this adjacent conductor. Assuming a single adjacent conductor, this parasitic coupling can be expressed by the equation [3]:

[0063] CF2 = (BL_2 - BR_2) / 12 [3]

[0064] / 2 : current flowing through the adjacent conductor

[0065] BL_2 and BR_2 : Flux density on axis Z sensed by elements SL and SR respectively when no current flows through the 1stprimary conductor 1a

[0066] CF2 : magnetic coupling factor of the 2ndprimary conductor 1b on the magnetic field transducer in the 1stprimary conductor 1a location (Tesla / Ampere)

[0067] Without proper shielding, the differential field induced on the magnetic field transducer would be impacted by the current flowing through the adjacent conductor as shown by the equation [4] :

[0068] Bdiff = CF1 x H + CF2 x |2 [4]

[0069] Figure 3a shows an example of a conventional coreless over-the-primary conductor sensing configuration, without the magnetic shield, the field induced by the current flow through the adjacent primary conductor is measurable at the sensor location on the first primary conductor 1a, resulting into cross-magnetic coupling leading to measurement errors.

[0070] Crosstalk is usually represented in percentage using the equation [5]:

[0071] Crosstalk% = Parasitic coupling / Main coupling x 100 [5]

[0072] Thanks to the magnetic shield in embodiments of the present invention, the coupling factor of adjacent conductor (CF2) is minimized and becomes negligible, keeping the original relationship [2] between the primary coupling CF1 and the current 11 flowing thought the 1stprimary conductor 1a.

[0073] The magnetic shielding effect is shown in example Figure 3b, not only the shield blocks external field from entering the sensing area, but also contains the field induced around the 1stprimary conductor 1a preventing it to radiate to the second primary conductor 1 b. An example of magnetic simulation results showing magnetic flux density is illustrated in figure 3c.

[0074] Figure 4a shows magnetic simulations results of crosstalk comparison between standard, priorart over-the-primary conductor coreless current sensing configuration and a current sensor with magnetic shield at different adjacent primary conductor distances. In this example the shielded coreless configuration of the current sensor according to embodiments of the invention attenuates crosstalk by a factor of 10 compared to the conventional configuration without magnetic shield.

[0075] The magnetic shield 8 may be made of different ferromagnetic materials (NiSe, SiFe, etc.) and can be a single integral II shaped solid or a laminated II shape (as illustrated in figure 1d) to reduce eddy current generation and optimize high frequency performance. The shield may have various shapes and dimensions (L, H, t, t2 and W) depending on application requirements and mechanical constrains such as primary conductor width, thickness, current range etc.

[0076] Overall, the magnetic shielding performance of the magnetic shield 8 (how much it attenuates crosstalk) is dependent on the material used and dimensions of the magnetic shield 8, but is mostly driven by the height H of the lateral branches 16a with respect to the primary conductor thickness. Figure 4b shows shield height effect on crosstalk on a 3mm thick primary conductor with an adjacent primary conductor at a 20mm center-center distance based on magnetic simulations. As shown in this example, while the shield already provides a significant crosstalk attenuation with limited height (without magnetic shield this configuration shows a crosstalk of ~4%), the magnetic shield height H is an important parameter to attenuate crosstalk and reaches a plateau above a certain value.

[0077] In an embodiment of this invention, a three-phase system comprising adjacent current carrying primary conductors 1a, 1b, 1c, for instance for an inverter application, is illustrated in figure 5b. Figure 5a shows a conventional differential current sensing topology with three differential magnetic field transducers sensing vertical magnetic field components (along Z axis); each magnetic field transducer providing an output proportional to the current flowing through their respective primary conductor. In this configuration, each magnetic field transducer remains sensitive to any (external) field gradient and especially sensitive to the magnetic field induced by current flowing through adjacent primary conductor (so called crosstalk effect).

[0078] While this effect can be minimized by increasing the distance between the conductors or using rotated or S-shape like primary conductor design, it usually comes with a higher cost, more difficult integration, or a mechanically fragile arrangement.

[0079] A three-phase current sensing system with high immunity against crosstalk can be achieved with current sensor system according to and embodiment of the invention, using a single magnetic shield 8, placed around the center primary conductor 1b and associated differential magnetic field transducer 4b. This magnetic shield significantly attenuates the magnetic field radiated by the adjacent primary conductorsla, 1c to the center primary conductor 1 b but also attenuates the radiated field from the center primary conductor 1 b to the adjacent ones 1a, 1c, as shown in example Figure 5b, resulting in high crosstalk immunity using just one magnetic shield 8 in the center.

[0080] This solution offers high crosstalk and external field immunity while achieving a cost effective, light and compact current sensing solution for 3-phase conductor systems.

[0081] In another embodiment as shown in Figure 6a, this principle is integrated into a 3-phase current sensor module offering easy and compact over-the-primary conductors mounting configuration. While prior-art solutions typically require Cores or shields on each phase and surrounding the primary conductors, this 3-phase integrated current sensor module offers easy to mount, lighter and compact solution for 3-phase current sensing, allowing mounting form the top or from the bottom.

[0082] The current sensor system integrates differential magnetic field transducers 4 on each phase, a center magnetic shield 8, passive components, a signal and power connector 12, and can be bonded or clipped to the primary conductors 1a, 1b, 1c.

[0083] Another version of the current sensor system may also directly integrate the primary conductor sections to offer pre-calibrated plug-ang-play current sensing solution as shown in Figures 6b and 6c.

[0084] While these embodiments have only centrally position magnetic shield, it is also possible to provide a magnetic shield over each primary conductor, or over the outer positioned primary conductors, for further immunity to the external field.

[0085] In embodiments, the differential magnetic field transducer 4 may be positioned in a slit orifice extending through and within the primary conductor as shown in Figures 7a and 7b such that the magnetic field sensing portions 14 are arranged in a plane extending through the slit.

[0086] In embodiments, the differential magnetic field transducer 4 may be positioned in an orifice extending through the primary conductor as shown in Figures 8a and 8b such that the magnetic field sensing portions 14 are arranged in a plane parallel to the surface of the primary conductor. The different mounting topologies of the differential magnetic field transducers combined with magnetic shield according to embodiments of the invention offer additional mounting flexibility and advantages depending on application requirements.

[0087] List of references

[0088] Primary conductor 1

[0089] Bus bars 1a, 1b, 1c measuring section 3

[0090] Indents

[0091] Slot 7

[0092] Multi conductor current sensor system 100

[0093] Current sensor 2

[0094] Differential magnetic field transducer 4, 4a, 4b, 4c

[0095] (magnetic field gradient sensor)

[0096] IC chip

[0097] Magnetic field sensor portions 14

[0098] Circuit board 6

[0099] Magnetic shield 8

[0100] II shaped soft magnetic material I ferromagnetic material

[0101] Lateral branches 16a

[0102] Free ends 17

[0103] Bridging branch 16b

[0104] Housing 10

[0105] Connector 12

[0106] Primary conductor passage 5

[0107] W width of space between lateral branches (length of bridging branch)

[0108] H height (length) of lateral branches t thickness of bridging branch t2 thickness of lateral branch

[0109] L length (in primary conductor direction) of magnetic shield

[0110] Wp width of primary conductor (bus bar)

[0111] Wm width of primary conductor measuring section Tp thickness of primary conductor (bus bar)

Claims

Claims1 . A current sensor (2) comprising a differential magnetic field transducer (4) having at least two magnetic field sensor portions (14), configured for measuring a primary current flowing in a primary conductor positioned adjacent and in proximity to the differential magnetic field transducer (4), and a housing (10) in which the differential magnetic field transducer is lodged, characterized in that the current sensor further comprises a magnetic shield (8) having two lateral branches (16a) and a bridging branch, each lateral branch extending from a free end to a connected end, the connected ends joined to opposite ends of the bridging branch (16b), and free ends separated by a gap opposite the bridging branch such that the magnetic shield (8) has a general U-shape, the magnetic shield surrounding, on three sides formed by the lateral branches and the bridging branch, a primary conductor passage (5) through which the primary conductor (1) is installed in use, the differential magnetic field transducer positioned adjacent and in proximity to the bridging branch (16b), between the bridging branch (16b) and primary conductor passage (5), the magnetic field sensing portions (14) being formed on a side of the differential magnetic field transducer facing the primary conductor passage.

2. The current sensor according to the preceding claim further comprising a circuit board (6), the differential magnetic field transducer (4) being mounted on and connected to the circuit board.

3. The current sensor according to the preceding claim wherein the circuit board is positioned adjacent the bridging branch of the magnetic shield (8), either against the bridging branch or separated therefrom by an insulating layer.

4. The current sensor according to any preceding claim wherein the differential magnetic field transducer is in the form of an integrated circuit chip.

5. The current sensor according to any preceding claim wherein a length ( / - / ) of the lateral branches (16a) of the magnetic shield (8) is greater than the sum of the thickness of the differential magnetic field transducer and the circuit board and a thickness (Tp) of the primary conductor configured for mounting in the primary conductor passage (5), such that the free ends (17) of the lateral branches extend beyond the primary conductor (1) which is fully positioned within and overlapped by the lateral branches (16a).

6. The current sensor according to any preceding claim wherein a section of the primary conductor is integrated with and forms part of the current sensor (2).

7. The current sensor according to any preceding claim wherein the magnetic shield has a length (L) in a direction of extension of the primary conductor that is greater than a length of the differential magnetic field transducer (4) so as to completely overlap the differential magnetic field transducer.

8. The current sensor according to any preceding claim in conjunction with the primary conductor, wherein the primary conductor has a measuring section (3) over which the differential magnetic field transducer is mounted that has a reduced cross-section area compared to adjoining sections of the primary conductor.

9. A multi-conductor current sensor system for measuring currents in a plurality of primary conductors, comprising a plurality of current sensors, each for measuring a primary current flowing in a different one of the plurality of primary conductors, wherein at least one of said plurality of current sensors is a magnetically shielded current sensor according to any preceding claim.

10. The multi-conductor current sensor system according to the preceding claim wherein each current sensor comprises a differential magnetic field transducer.

11. The multi-conductor current sensor system according to either of the two directly preceding claims wherein at least one of said plurality of current sensors is without magnetic shield.

12. The multi-conductor current sensor system according to the preceding claim having three primary conductors, a first said current sensor with magnetic shield being coupled to a centre positioned primary conductor of the three primary conductors.

13. The multi-conductor current sensor system according to the preceding claim wherein a second and third said primary conductor are coupled to current sensors without magnetic shield.

14. The multi-conductor current sensor system of any preceding claim 9-13 wherein the current sensors are lodged in a common housing (10) having a plurality of primary conductor passages (5), one for each primary conductor.

15. The multi-conductor current sensor system according to the preceding claim wherein sections of the primary conductors are lodged in the housing.

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