Coreless current probe and a method of measuring direct current

Abstract

A coreless current probe has a U-shaped body with arms, an open end and a cross piece forming a closed end. The opening formed in the U-shaped body has a length not less than the width between the arms. The U-shaped body can engage around a conductor carrying a current to be measured. The U-shaped body has a number of coreless single point magnetic field sensors, usually Hall devices, distributed around the opening. An arrangement with six sensors has two sensors at the ends of the arms of the U-shaped body, two sensors at the closed end, and two sensors mid-way along the arms. Sensing circuitry applies factors to the outputs of the sensors and sums the results to produce a measured current value. The factors are selected so that the summed result is zero in any externally generated homogeneous magnetic field.

1 . A coreless current probe comprising a U-shaped body having arms, an open end, and a cross piece forming a closed end opposite to said open end, said U-shaped body defining an opening having a width between the arms and a length from said closed end to said open end, wherein said length is not less than said width, said width and said length defining a plane of said opening and a central line normal to said plane, whereby a conductor carrying a current to be measured can be engaged by said body of said probe so as to extend through said opening parallel to said central line;
 a plurality of coreless single point magnetic field sensors distributed in said body around said opening including a respective said sensor at an end of each of said arms on each side of said open end of said body and at least one said sensors at said closed end of said body, said sensors each having a respective axis of magnetic field sensitivity and being arranged in said body so that each of said axes is not co-planar with said central line;   and sensing circuitry connected to said sensors which is operative to produce for each of said sensors a respective sensor signal which is a measure of the angle component of magnetic field at the sensor aligned with the respective axis of magnetic field sensitivity of the sensor,   said sensors being arranged such that there are values of c r  for which, in any homogeneous magnetic field,   
       
         
           
             
               
                 
                   
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         where n is the number of said magnetic field sensors,
 h r  is said measure of said magnetic field component for the r th  sensor, 
 
         and c r  is a constant factor for the r th  sensor, 
         said sensing circuitry being further operative to combine said measures h r  to produce a measured current value representing current following in a conductor engaged by said body of said probe. 
       
     
     
         2 . A coreless current probe as claimed in  claim 1 , wherein each of said magnetic field sensors is orientated to have its axis of sensitivity in said plane of said opening. 
     
     
         3 . A coreless current probe as claimed in  claim 1 , wherein said sensing circuitry is adapted to derive as said measured current value the summation 
       
         
           
             
               
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       where the values of c r  are selected to maximise rejection by the probe of external magnetic fields which are not produced by currents in a conductor engaged by the probe. 
     
     
         4 . A coreless current probe as claimed in  claim 3 , wherein the values of c r  are selected such that in any homogeneous magnetic field 
       
         
           
             
               
                 
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         5 . A coreless current probe as claimed in  claim 3 , wherein said length of said opening of said U-shaped body is not less than twice said width of said opening. 
     
     
         6 . A coreless current probe as claimed in  claim 5 , wherein said plurality of said magnetic field sensors comprises at least six said sensors, including
 a) a first pair constituted by said sensors at said ends of said arms of said U-shaped body,   b) a second pair of said sensors at said closed end of said U-shaped body, and   c) a third pair of said sensors located substantially mid-way along said arms;   the sensors of each of said first, second and third pairs of said sensors being disposed spaced uniformly apart symmetrically on opposite sides of a plane of symmetry containing said central line and equally spaced between said arms;   each of the sensors of said first and second pairs of said sensors being orientated in said body to have its axis of sensitivity in said plane of said opening and at a respective acute angle to said plane of symmetry so as to be generally tangential to said opening;   each of the sensors of said third pair of sensors being orientated in said body to have its axis of sensitivity in said plane of said opening and parallel to said plane of symmetry.   
     
     
         7 . A coreless current probe as claimed in  claim 6 , wherein said length of said opening is greater than twice the width, and the spacing between each of said third pair of sensors and a neighbouring sensor of said first or second pair of said sensors is greater than the spacing apart of the sensors of said first pair of said sensors,
 and said sensing circuitry is adapted such that the selected values of c r  for the sensors of said third pair are greater than the selected values of c r  for the sensors of said first and second pairs.   
     
     
         8 . A coreless current probe as claimed in  claim 1 , wherein said plurality of said magnetic field sensors comprises at least four said sensors. 
     
     
         9 . A coreless current probe as claimed in  claim 8 , wherein four said magnetic field sensors are distributed symmetrically in said U-shaped body, about a plane of symmetry which contains said central line and is equally spaced between said arms. 
     
     
         10 . A coreless current probe as claimed in  claim 9 , wherein each of said four sensors is orientated in said body to have its axis of sensitivity in said plane of said opening and at a respective acute angle to said plane of symmetry so as to be generally tangential to said opening. 
     
     
         11 . A coreless current probe as claimed in  claim 10 , wherein said plurality of said magnetic field sensors comprises at least six said sensors, including two additional said sensors located substantially mid-way along said arms and orientated in said body to have their axes of sensitivity in said plane of said opening and parallel to said plane of symmetry. 
     
     
         12 . A coreless current probe as claimed in  claim 11 , wherein said plurality of said magnetic field sensors comprises at least nine said sensors including three further said sensors comprising one said further sensor located substantially mid-way along said cross piece of said body, and one said further sensor located at the end of each of said arms, said three further sensors being orientated to have axes of sensitivity in said plane of said opening and substantially normal to said plane of symmetry. 
     
     
         13 . A coreless current probe as claimed in  claim 12 , wherein said plurality of said magnetic field sensors comprises at least thirteen said sensors including four still further said sensors located on said arms of said body, a respective said still further sensor being located on each side of each of said additional said sensors which are located mid-way along said arms, and said four still further said sensors being orientated to have axes of sensitivity in said plane of said opening. 
     
     
         14 . A coreless current probe as claimed in  claim 1 , further including a housing containing a measured current display connected to said sensing circuitry, a handle fixed to said housing, a tube connecting said U-shaped body to said housing, and connection cables extending through said tube, whereby an operator holding said handle can engage a conductor between the arms of said body of said probe and read a measured current from said display. 
     
     
         15 . A coreless current probe as claimed in  claim 1 , including a housing connected to said U-shaped body, wireless signalling circuitry contained in said housing, said wireless signalling circuitry being connected to said sensing circuitry for wireless signalling said measured current values to a remote location, and a battery compartment for a battery to power said sensing circuitry and said wireless signalling circuitry. 
     
     
         16 . A coreless current probe as claimed in  claim 15 , wherein said probe is formed as a unitary structure incorporating said U-shaped body and said housing with said battery compartment. 
     
     
         17 . A coreless current probe as claimed in  claim 16 , wherein said housing with said battery compartment are integral with said cross-piece of said U-shaped body. 
     
     
         18 . A coreless current probe as claimed in  claim 17 , wherein said arms have parallel internal faces providing a predetermined uniform spacing sized to accommodate a rectangular section bus bar engaged by said probe, and at least one compression tab located on the internal face of at least one of said arms, said tab protruding inwards from said internal face and adapted to be resiliently outwardly compressible on engagement with said bus bar to hold the probe in position on the bus bar. 
     
     
         19 . A coreless current probe as claimed in  claim 1 , wherein said U-shaped body has at least exterior surfaces which are electrically insulating. 
     
     
         20 . A method of measuring direct current flowing in a conductor having minimum and maximum orthogonal cross-sectional dimensions, the method comprising the steps of:
 defining an Ampere's Law integration path around the conductor in an integration plane perpendicular to a central line of the conductor, said path having minimum and maximum orthogonal dimensions which exceed said dimensions of said conductor,   measuring a respective angle component of magnetic field at each of a plurality of locations on said integration path, an adjacent pair of said locations being at one end of said maximum dimension of said path and spaced apart by said minimum dimension of said path, and a third of said locations being at the other end of said maximum dimension of said path, said locations and the orientations of said respective angle components being selected such that there are values of c r  for which, in any homogeneous magnetic field,   
       
         
           
             
               
                 
                   
                     ∑ 
                     
                       r 
                       = 
                       1 
                     
                     n 
                   
                    
                   
                     
                       c 
                       r 
                     
                      
                     
                       h 
                       r 
                     
                   
                 
                 = 
                 0 
               
               , 
             
           
         
         where n is the number of said locations, 
         h r  is the measured value of said magnetic field component at the r th  location, and c r  is a constant factor for the r th  location, 
         and combining said measured values h r  to produce a value of said direct current. 
       
     
     
         21 . A method as claimed in  claim 20 , wherein the respective angle components are orientated in said integration plane. 
     
     
         22 . A method as claimed in  claim 20 , wherein said step of combining performs the summation 
       
         
           
             
               
                 
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                   n 
                 
                  
                 
                   
                     c 
                     r 
                   
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                     h 
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               , 
             
           
         
       
       where the values c r  are selected to maximise rejection in the summation of the influence of external magnetic fields which are not produced by current flowing in the conductor. 
     
     
         23 . A method as claimed in  claim 22 , wherein the values of c r  in said summation are selected such that in any homogenous magnetic field 
       
         
           
             
               
                 
                   ∑ 
                   
                     r 
                     = 
                     1 
                   
                   n 
                 
                  
                 
                   
                     c 
                     r 
                   
                    
                   
                     h 
                     r 
                   
                 
               
               = 
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         24 . A method as claimed in  claim 20 , wherein said maximum orthogonal cross-sectional dimension of said conductor is not less than twice said minimum orthogonal cross-sectional dimension. 
     
     
         25 . A method as claimed in  claim 24 , wherein a respective angle component of magnetic field is measured at at least six locations on said integration path, said locations including
 a) a first pair corresponding to said adjacent pair of said locations at said one end of said maximum dimension of said path,   b) a second pair at said other end of said maximum dimension of said path, and   c) a third pair substantially mid-way along said maximum dimension of said path;   said locations of each of said first, second and third pairs of locations being spaced uniformly apart on opposite sides of a plane of symmetry containing a central line of said conductor and parallel to said maximum orthogonal cross-sectional dimension of said conductor;   the respective angle component of magnetic field being measured at each of the locations of said first and second pairs of locations being in said integration plane and at an acute angle to said plane of symmetry so as to be generally tangential to said conductor;   the respective angle component of magnetic field being measured at each of the locations of said third pair of locations being in said integration plane and parallel to said plane of symmetry.   
     
     
         26 . A method as claimed in  claim 25 , wherein said maximum orthogonal cross-sectional dimension of said conductor is greater than twice said minimum dimension of the conductor, and the spacing between each of said third pair of locations and a neighbouring location along said path of said first or second pairs of locations is greater than the spacing apart of the locations of said first pair of locations,
 and, in said step of combining the selected values of c r  for the locations of said third pair are greater than the selected values of c r  for the locations of said first and second pairs.