Abstract:
A double-clamp earth tester device, having a voltage-inducing transformer (VT) ( 30 ), a current-sensing transformer (CT) ( 32 ), a voltage source adapted to supply a voltage across a primary winding ( 31 ) on the VT core ( 44 ) and a signal processing unit ( 35 ) adapted to measure a resultant signal output from a secondary winding ( 33 ) on the CT core ( 46 ), is susceptible to errors induced by electromagnetic interference between the cores ( 44, 46 ) of the two transformers. Previously, the effects of this interference have been mitigated by surrounding one or both cores ( 44, 46 ) with electromagnetic shielding ( 42 ). This had lead to devices having a bulky head portion and attendant difficulties in accessing sites having restricted access ( 52 ). These problems have been overcome by providing means to modify the signal output form the secondary winding ( 33 ). This may be achieved magnetically by nullifying the parasitic magnetic fields ( 66 ) at source in the core ( 46 ) of the current-sensing transformer, or electrically by the addition of a negative signal to the output signal prior to or during processing in the signal processing unit ( 35 ).

Description:
[0001]    This application claims priority from European Patent Application 09159431.7, filed May 5, 2009, which is incorporated herein by reference. 
       FIELD OF THE INVENTION 
       [0002]    Electrical installations must be grounded or earthed because this is essential for the safety of equipment, users and operating personnel. The earthing is achieved by connecting a suitable conductor, the protective earth, to a soil-buried conductive element such as a rod, bar, pipe or pylon. The resistance of this earthing electrode must be sufficiently low in order to provide requisite electrical protection, i.e. to dissipate an expected fault current thereby to reduce the likelihood of dangerous voltages arising. National and international standards define such required levels of protection. 
         [0003]    The condition of the earthing electrodes and the value of the resistance of earthing electrodes (in short referred to as “earth resistance”) should be inspected periodically, and there are a number of ways in which this can be carried out. 
         [0004]    One technique, of which  FIG. 1   a  is an exemplary configuration, is known as the 3-pole fall-of-potential method. A connection is made to the accessible portion  12 ′ of an earth electrode  12  under test. An auxiliary electrode  22  (the current stake) is driven into soil  16  at a distance B from the electrode under test and is connected to one terminal of an ammeter  26  by a connecting lead  24 . The other terminal of the ammeter  26  is connected to one terminal of a current source  28  and the other terminal of the current source  28  is connected to the electrode under test  12  by a connecting lead  24 ′. A further auxiliary electrode  14  (the potential stake), which is driven into soil  16   a  distance A from the electrode under test  12 , is connected to one terminal of a voltmeter  20  by a connecting lead  18 . The other terminal of the voltmeter  20  is connected to the electrode under test  12  by a connecting lead  18 ′ in order to measure the potential difference between the auxiliary electrode  14  and the electrode under test  12 . 
         [0005]    A measurement of the resistance of the electrode under test is performed as follows: 
         [0006]    The distance of the potential stake  14  to the electrode under test  12  is varied. At each distance, the current source  28  supplies a current I to the current stake  22  which is measured by the ammeter  26 . The current returns through the soil  16  via the electrode under test  12 . The potential difference between the voltage stake  14  and the electrode under test is measured by the voltmeter  20 . The resistance of the electrode under test can be calculated from the ratio of the voltmeter  20  and ammeter  26  readings. A graph is plotted of resistance against distance A of the potential stake  14  and the true resistance of the electrode under test  12  can be determined from the graph by one skilled in the art. 
         [0007]    This technique has several limitations:
       a) If the electrode under test  12  is connected to other earthing electrodes or to equipment through which there is another electrical path to earth then a proportion I′ of the injected current I will not be returned via the current source  28  and the value of resistance for the electrode under test  12  determined by the method will be different from the actual resistance of the electrode under test  12 .   b) If the electrode under test  12  is isolated from the equipment it is protecting so as to obtain a true measurement of its resistance, then that equipment and users of it will suffer from a reduced level of protection against the consequences of electrical faults.       
 
         [0010]    A modification to the technique which counteracts these limitations is shown in  FIG. 1   b  and is known as the 3-pole fall of potential with selective measurement. A current-sensing transformer  10  is clamped around an accessible portion  12 ′ of the electrode under test  12  which measures the proportion I″ of the injected current I which is returned via the electrode  12 . This measured current I″ is used in the calculation of resistance rather than I, i.e. I″=I−I′. 
         [0011]    Such methods can be very time consuming, because they require manual insertion of the auxiliary metal electrodes  14 ;  22  into the soil  16  at least tens of meters away A, B from the electrode under test  12 , with an attendant need for long leads  18 ;  24  to connect those metal electrodes with the electrode under test  12 . Furthermore, such methods can pose a safety risk to the equipment being protected and to the users of that equipment if the electrode under test  12  is to be disconnected at any time and there are substantial leakage currents present. 
         [0012]    In order to overcome some of these problems, a technique was developed which allowed much faster and contactless earth resistance measurements. This well-known method employs a set of two transformers: a voltage inducing transformer and a current sensing transformer. A typical configuration is illustrated in  FIG. 2   a , which is described in greater detail below. Because the transformers are often made as clamps, the technique is often referred to as the ‘double-clamp method’, also known as the ‘stakeless method’. 
         [0013]    In an earthing system with multiple electrodes, as depicted schematically in the circuit diagram of  FIG. 2   b , the parallel resistance of all the electrodes (R 1  . . . Rn) tends to zero. Therefore, the resultant electric circuit simplifies to a loop with equivalent resistance Rx, whose value is very close to the resistance of the electrode under test. This is depicted schematically in the circuit diagram of  FIG. 2   c . As shown in  FIG. 2   a , the loop is energised by the voltage-inducing transformer  30  (VT) and the current in the simplified loop is measured by the current-sensing transformer  32  (CT). Knowing the applied voltage and the current flowing in the equivalent loop it is possible to calculate the resistance Rx of the electrode under test  12  from Ohm&#39;s law. The VT  30  includes a transformer core  44  and the CT  32  includes a transformer core  46 . 
         [0014]    This double-clamp method has a big advantage of performing the measurement without requiring any additional electrodes to be driven into the soil  16  and without disconnecting the rest of the earth system. In fact, the method relies on the rest of the system in order to achieve the simplification of the circuit from that of  FIG. 2   b  to that of  FIG. 2   c  and hence for the resistance calculated on the basis of the simplified loop of  FIG. 2   c  to be valid. 
         [0015]    Initially the two transformers  30 ,  32  used in the above-described double-clamp methods were made as separate clamps  30 ′,  32 ′, connected to a main unit  34  of the measuring device with suitable leads  36 ,  38 , as shown in  FIG. 3 . 
         [0016]    Such an approach was normal because the clamp-on transformers  30 ′,  32 ′ could also be used in other applications, such as the 3-pole fall of potential method discussed above and shown in  FIGS. 1   a  and  1   b . However, this meant a non-optimum approach leading to large size and weight of the whole device. 
         [0017]    Moreover, it has been found that the two clamps  30 ′,  32 ′ cannot be positioned too close to each other because of fundamental measurement inaccuracies induced by electromagnetic interference between the transformers, as set out in greater detail below. 
         [0018]      FIG. 4  schematically illustrates an electromagnetic field  40  that the VT  30  generates around itself. This field  40  induces the required voltage in the loop under test, with the induced current being measured by the CT  32 . However, if the CT  32  is brought into the vicinity of the VT  30 , then the surrounding electromagnetic field  40  interferes with the CT  32 . This is known to adversely affect the measurement accuracy, due to unwanted signals parasitically induced in the CT  32  as a direct result of the electromagnetic field  40  emanating from the VT  30 , in addition to the useful signals induced as a result of the voltage deliberately induced in the loop under test. 
         [0019]    The interfering electromagnetic field  40  generated by the VT  30  and affecting the CT  32  is greatly affected by the presence of a gap  41   a  between the two halves of the transformer core  44  of the VT  30 , and the presence of a gap  41   b  between the two halves of the transformer core  46  of the CT  32 . The amplitude and phase of the field  40  changes with the gap  41   a  in a highly non-linear manner and even a small gap of a few micrometers can adversely affect the measurement to unacceptable level, due to the highly non-linear nature of the interference. 
         [0020]    For stationary devices this is relatively easy to overcome, because the distance between the voltage and current transformers  30 ,  32  can be fixed and they can be designed such that the gaps  41   a ,  41   b  might not be present at all or the gaps will not vary during normal operation. In any case, for fixed devices the level of interference will be constant in amplitude and phase, which is easy to account for during the measurement. For instance, U.S. Pat. No. 6,296,284 B1 describes a use of synchronous rectifier in order to filter out the interference constant in phase. Such a technique could not be used with a portable clamp-on device due to the inherent variability of the gap  41   a  between the two halves of the transformer core  44  of the VT  30 , which translates into high variability of the amplitude and phase of the field  40  and hence the interference, and due to the inherent variability of the gap  41   b  between the two halves of the transformer core  46  of the CT  32 , which translates into high variability of the amplitude and phase of the induced current. 
         [0021]    The clamp-on devices are normally used outdoors in the presence of rain, wind and dust. Such working conditions can cause dirt and other non-magnetic particles to be gathered on the mating surfaces of the clamps causing the gaps  41   a ,  41   b  to vary substantially between measurements. A small variation of the gaps, particularly the gap  41   a , causes large variation of the amplitude and phase of the interference affecting the CT  32 . 
         [0022]    If the clamps  30 ′,  32 ′ of known double-clamp earth tester devices are used as two independent transformers they must be separated by some safe distance, which could be as much as 30 cm in order to obviate such parasitically induced unwanted signals. However, access to the electrode  12  to be tested is often very limited, so the user might be forced to operate the clamps  30 ′,  32 ′ much closer to each other than the optimal, interference-free distance. This inevitably leads to unreliable measurement results. 
         [0023]    A number of double-clamp testers with an integrated head containing both transformers  30 ,  32  are currently on the market. These are provided as hand-held instruments. The integrated nature of these devices was an improvement over prior separate clamp designs as described above with reference to  FIG. 3  in terms of operator ease-of-use and access to the electrode  12  to be tested. 
         [0024]    In order to reduce the electromagnetic interference effects described immediately above and to provide requisite measurement range and accuracy, such hand-held instruments with an integrated head use elaborate electromagnetic shielding  42  of one or both transformer cores  44 ,  46 .  FIG. 5  shows two commonly used arrangements. 
         [0025]    The shielding  42  is made out of ferromagnetic material, almost always with multiple layers. Because of the relatively high interference between the transformers  30 ,  32 , a high quality (therefore expensive) shielding material  42  is required. In some double-clamp devices, up to 6 layers of shielding  42  are used between the two cores  44 ,  46 . This significantly increases the cost of the device and still does not guarantee good accuracy at the high end of the measuring range; the maximum value measurable by such a shielded device is in the range 1250-1550 Ohms. 
         [0026]    However, the relatively high cost of the measuring head is far less important than the bulkiness of the head. Earthing electrodes  12  are normally fixed to the surface of a building  50  or other structure, as illustrated schematically in  FIG. 6 , and there is generally very limited access  52  around the electrode  12  under test. 
         [0027]    For that reason, the size and bulkiness of the head is absolutely critical for the usability of the device. The addition of electromagnetic shielding  42  surrounding at least one of the cores  44 ,  46  (but more often both of them) inevitably increases the overall size of the head considerably. An external casing (not shown) must also protect the whole device. Moreover, the shielding  42  must also be introduced to the inside of the core  44 ,  46  and this decreases the available inner diameter of the clamp  30 ,  32 , thereby severely limiting the size of the conductor or electrode  12  that can be clamped around. 
         [0028]    The integrated head must be designed such as to aid easy clamping around the electrode under test. Therefore, there must not only be the gaps  41   a  and  41   b  extending through the respective cores  44 ,  46 , but also corresponding gaps (not shown) through the shielding  42 . In one type of design the gaps are created between two flat mating surfaces of both head halves similarly to those illustrated in  FIG. 5 . The fact that the gaps must be present also in the shielding  42  severely limits the effectiveness of the magnetic shielding and the cores  44 ;  46  must still be separated by some additional distance even though up to 6 layers of shielding may be used, which leads again to increased bulkiness of the head. It will be appreciated that the respective transformer halves may be linked so that both the CT and the VT are opened and closed simultaneously for clamping around an electrode under test, in which case the gaps  41   a ,  41   b  through the transformer cores  44 ,  46  and through the shielding  42  effectively comprise a single continuous gap. Alternatively, the CT and the VT may be opened and closed independently of one another. 
         [0029]    In another type of design the shield  42  or the cores  44 ;  46  or both the shield and the cores are made with interlocking sections such as to increase the contact area of the mating surfaces and minimise the effective gap between the two halves of the head. This improves the magnetic performance but it has several significant mechanical disadvantages. Dust and dirt collects easily between the interlocking sections, which are more difficult to clean than the flat-face design. More importantly, the gathered dirt can bend the interlocking sections rendering the device completely useless. Finally, as will be apparent to those skilled in the art, the interlocking sections require much wider opening angle of the transformer halves in order to retain the same spacing between the opened halves in order to clamp around an electrode under test  12 . Again, this adversely impacts on the bulkiness of the head and overall accessibility of the device to the electrode under test  12 . 
         [0030]    This bulkiness problem is widely recognised by the users. However, to date the equipment manufacturers have not been able to provide a device that overcomes all these problems. At the moment the working solutions are the expensive and bulky electromagnetic shielding  42  or the interlocking sections discussed above. There are some other concepts such as, for example, a single-core clamp as described in U.S. Pat. No. 7,102,360 B2. Indeed, a single-core head without shielding would allow significant reduction in the head size. However, the associated measurement procedure involves not only extremely complicated mathematics (the description given in U.S. Pat. No. 7,102,360 B2 contains over 40 equations), but also specialised electronic circuitry required for the calibration of the device. Moreover, as is apparent to those skilled in the art, the signal selectivity, sensitivity and accuracy of such a device would be inferior to that of a double-clamp device, even without the magnetic shielding. 
         [0031]    There is therefore a need for a handheld portable device, which can be easily clamped around difficult to access electrodes. Such a device must be capable of repeatable and accurate resistance measurements over a wide operating range and in the presence of the variable intra-core interference arising from the requirement to have de-mountable (hence cut) cores. The construction of the instrument should be economical and compact in order to allow access in confined areas. 
       SUMMARY OF THE INVENTION 
       [0032]    According to one aspect of the invention, there is provided a double-clamp earth tester device comprising:
       a voltage-inducing transformer comprising a split core and a primary winding having one or more turns around said voltage-inducing transformer core;   a current-sensing transformer comprising a split core and a secondary winding having one or more turns around said current-sensing transformer core, wherein the voltage-inducing transformer core and the current-sensing transformer core each include a gap between respective halves and are adapted to be removably clamped around an earth electrode under test;   a voltage source adapted to supply a voltage across the primary winding;   a signal processing unit adapted to measure a resultant signal output from the secondary winding; and   means to modify the signal output from the secondary winding so as to compensate for signals induced by gap-dependent, non-linear electromagnetic interference between the transformers.       
 
         [0038]    The superior usability of the double-clamp method is retained, but without the need for the use of extensive electromagnetic shielding. The compensation technique permits construction of a much more compact head integrating both cores within close proximity to each other and without interlocking sections. The compensation technique can be conveniently applied in a variety of ways, depending on the specific requirements and limitations of the signal processing employed. The technique also substantially improves the accuracy of the resistance measurement over the whole range of the device. 
         [0039]    In one embodiment the means to modify the signal output from the secondary winding comprises magnetic compensation means which is adapted to generate a magnetic field in the current-sensing transformer core that is equal but opposite in magnitude to the portion of a magnetic field induced in the current-sensing transformer core by the gap-dependent, non-linear electromagnetic interference between the transformers. The amplitude and phase of the compensating signal is varied such that the changing interference resulting from varying gap is automatically taken into account. 
         [0040]    This magnetic compensation technique nullifies at source the effects of the parasitic magnetic field that is induced in the current-sensing transformer core by the electromagnetic interference from the voltage-inducing transformer core. In other words, the signal output from the secondary winding is based solely upon the magnetic field induced in the current-sensing transformer core by the current flowing in the electrode under test. 
         [0041]    The magnetic compensation means may comprise an additional winding around the current-sensing transformer core and an associated additional signal source. 
         [0042]    The additional signal source may be part of the signal processing unit. 
         [0043]    The magnetic compensation means may alternatively comprise an auxiliary loop around the voltage-inducing transformer core and the current-sensing transformer core. The electrode under test may comprise part of an earthing system that can be represented by a loop of equivalent resistance and the auxiliary loop may have reversed polarity with respect to the loop of equivalent resistance. The auxiliary loop may contain a configurable impedance Z aux . 
         [0044]    In another embodiment, the means to modify the signal output from the secondary winding comprises electric compensation means which is adapted to generate a compensating signal that is equal but opposite in magnitude to the portion of the signal output from the secondary winding induced by the gap-dependent, non-linear electromagnetic interference between the transformers. The amplitude and phase of the compensating signal is varied such that the changing interference resulting from varying gap is automatically taken into account 
         [0045]    This electric compensation technique functions by nullifying the portion of the output signal that is induced as a result of the parasitic magnetic field that is induced in the current-sensing transformer core by the electromagnetic interference from the voltage-inducing transformer core. In other words, the signal output from the secondary winding is notionally based upon a combination of a magnetic field induced in the current-sensing transformer core by the current flowing in the electrode under test and a parasitic magnetic field that is induced directly by electromagnetic interference from the voltage-inducing transformer core. In effect, the electric compensation means introduces a negative signal that nullifies the portion of the output signal that results from the electromagnetic interference. Thus, the actual output signal that is received at the signal processing unit does not include that portion that results from the electromagnetic interference. 
         [0046]    The electric compensation means may comprise a signal source to supply the compensating signal to the secondary winding. The signal source may comprise an auxiliary channel of the signal processing unit. The signal source may be tapped from the primary winding. This tapping may be achieved by means of a bridge circuit. The electrode under test may comprise part of an earthing system that can be represented by a loop of equivalent resistance and the signal source may comprise a parasitic winding around the voltage-inducing transformer core, which parasitic winding may have reversed polarity with respect to the loop of equivalent resistance. The parasitic winding may contain a configurable impedance Z aux . 
         [0047]    In another embodiment, the electric compensation means comprises an analogue-to-digital converter for digitizing the signal output from the secondary winding, and means to apply a compensation algorithm to the digitized signal. 
         [0048]    An advantage of this approach is that the compensation can be carried out by the implementation of appropriate software, rather than physical components. 
         [0049]    Typically, the voltage source comprises part of the signal processing unit. 
         [0050]    In yet another embodiment, the voltage-inducing transformer core and the current-sensing transformer core are integrated into a head portion of the device. 
         [0051]    The compensation techniques of the invention have particular benefit by enabling the transformer cores to be located closer together than would otherwise be possible, yet retaining or improving accuracy of the device through the variable compensation coupled directly to the variability of the interference caused by the variability of the gap. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    The invention will be described, by way of example, with reference to the accompanying drawings, in which: 
           [0053]      FIG. 1   a  is a schematic illustration of an exemplary known configuration for 3-pole fall of potential earth electrode testing; 
           [0054]      FIG. 1   b  corresponds to  FIG. 1   a , but shows a modified configuration for 3-pole fall of potential with selective measurement earth electrode testing; 
           [0055]      FIG. 2  is representative of the known two-clamp method, with  FIG. 2   a  being a schematic illustration of the physical configuration;  FIG. 2   b  being a schematic diagram of a corresponding resistance network; and  FIG. 2   c  being the equivalent resistance loop; 
           [0056]      FIG. 3  illustrates schematically a typical use of the two-clamp method, using two separate clamps; 
           [0057]      FIG. 4  is a schematic illustration of electromagnetic interference generated by the VT affecting the CT; 
           [0058]      FIG. 5  is a simplified view of part of a double-clamp head, with electromagnetic shielding—the left-hand side showing shielding on just the lower core, the right-hand side showing shielding on both cores; 
           [0059]      FIG. 6  illustrates a typical earth electrode installation, having limited access for testing; 
           [0060]      FIG. 7  is a schematic diagram of a typical double-clamp device; 
           [0061]      FIG. 8  is a schematic diagram of a double-clamp device having magnetic compensation means according to the invention; 
           [0062]      FIG. 9  corresponds to  FIG. 8 , but shows a device having parasitic magnetic compensation; 
           [0063]      FIG. 10  corresponds to  FIG. 8 , but shows a device having output signal compensation; 
           [0064]      FIGS. 11   a  to  11   c  correspond to  FIG. 10 , but show a device having output signal compensation ‘tapped’ from the VT supply; 
           [0065]      FIG. 12  corresponds to  FIG. 10 , but shows a device having output signal compensation by means of a parasitic loop; 
           [0066]      FIG. 13  is a graph illustrating typical results obtained with any of the compensation techniques according to the invention; 
           [0067]      FIG. 14   a  is a schematic diagram of a double-clamp earth tester device having an integrated head portion; and 
           [0068]      FIG. 14   b  is a schematic diagram of a double-clamp earth tester device having an alternative, differently shaped integrated head portion. 
       
    
    
     DETAILED DESCRIPTION 
       [0069]    A typical double-clamp device is shown schematically in  FIG. 7 . The double-clamp device consists typically of a voltage-inducing transformer (VT)  30  with one or more turns of a primary winding  31  and a current-sensing transformer (CT)  32  which has a secondary winding  33  with one or more turns. 
         [0070]    In use, both transformers  30 ,  32  clamp around an electrode under test, which is represented by the loop  60  of equivalent resistance having certain impedance Z loop    62 , as discussed above by reference to  FIG. 2 . The voltage-inducing transformer core  44  and the current-sensing transformer core  46  are typically integrated into a head portion of the device. 
         [0071]    A signal processing unit  35  supplies voltage to the VT  30 . The voltage supplied through the primary winding  31  induces an electromagnetic field in the VT core  44 , which in turn induces a current to flow in the loop  60 . That current flowing in the loop  60  induces a desired magnetic field H loop    64  in the CT core  46 . However, an unwanted magnetic field H noise    66  that is variable in amplitude and phase is also induced directly as a result of the variable electromagnetic interference emanating from the VT  30 , as discussed above by reference to  FIG. 4 . 
         [0072]    The magnetic fields  64 ,  66  induced in the CT core  46  induce a current to flow in the secondary winding  33 . A resultant output signal is received and measured at the signal processing unit  35 . From that output signal, the value of the loop impedance  62  can be calculated. 
         [0073]    The calculation utilises the equation (i): 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     loop 
                   
                   = 
                   
                     
                       C 
                       calibration 
                     
                      
                     
                       
                         V 
                         VT 
                       
                       
                         f 
                          
                         
                           ( 
                           
                             H 
                             CT 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   i 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where:
 
C calibration =calibration constant;
 
V VT =voltage across the primary coil  31  of the VT  30  proportional to the voltage induced in the loop  60 ;
 
ƒ=function relating magnetic field in the core  46  set up by the primary current of the CT  32 , and the output signal of the CT  32 ; and
 
ƒ(H CT )=value of the output signal of the CT  32  related to the magnetic field H CT  caused by the current in the loop  60  to be measured.
 
         [0074]    The value calculated from equation (i) is the modulus of the loop impedance  62 . However, under normal circumstances the resistive component is prevailing. Thus, in practice, the measured value is often synonymous with resistance. However, if the non-resistive component is significant the method can be used also for impedance measurements. 
         [0075]    The magnetic field H CT  comprises a vector combination of the magnetic field H loop    64  generated by the current in the loop  60  as well as the variable magnetic field H noise    66  generated by the electromagnetic interference between the transformers  30 ,  32 . 
         [0000]        {right arrow over (H)}   CT   ={right arrow over (H)}   loop   +{right arrow over (H)}   noise   (ii) 
         [0076]    Therefore, the output signal of the CT  32 , which equates to ƒ(H CT ) is affected by the varying interference. 
         [0077]    It is envisaged that a voltage source separate from the signal processing unit  35  could instead be used to supply the voltage to the VT. However, the supplied voltage should be known in order to calculate the loop impedance  62 . Hence, the signal processing unit  35  must be in communication with such a separate voltage source, if such a separate voltage source were to be used. 
         [0078]    This invention describes in detail methods for convenient removal of the unwanted part of the output signal ƒ(H CT ). This can be achieved either by suppressing the variable magnetic component H noise    66 , thus preventing any influence to the output signal of the CT  32 , or by modifying the output signal of the CT  32  in order to correct for the unwanted signal already induced by the H noise    66 . In the first approach, the compensation is achieved magnetically and thus comprises an indirect means to modify the output signal; in the latter, the compensation is achieved by appropriate analogue or digital signal processing and thus comprises a direct means to modify the output signal. In both approaches the compensation is coupled with the variability of the interference. 
       Magnetic Compensation 
       [0079]    By reference to  FIGS. 8 and 9 , in the magnetic compensation technique, the CT  32  is equipped with an auxiliary winding  70  having one or more turns. The auxiliary winding  70  is supplied with an appropriate signal that induces a current in the auxiliary winding  70  which in turn generates a magnetic field  74  in the CT core  46 . The magnetic field  74  is equal but opposite in magnitude to the interference field  66 . This suppresses the influence of electromagnetic interference to negligible values. The amplitude of the output signal across the secondary winding  33  is therefore free from any unwanted signals caused by the interference, so it can be directly measured by the signal processing unit  35 . By reference to equation (ii), the H noise  component  66  is removed, leaving H CT =H loop ; and since the loop current can be found from H CT  (refer to equation (i)), the measurement of the loop current can be accurate. 
         [0080]    The compensating signal can be generated in a number of suitable ways. This can be achieved actively, as shown in  FIG. 8 , by driving the winding  70  from an additional signal generator, which could be a part of signal processing unit  35  or a separate element  72  and which can generate a compensation signal with variable amplitude and phase to compensate for the instantaneous level of interference from the VT  30  and the effect of the variable gaps  41   a ,  41   b.    
         [0081]    Alternatively, as shown in  FIG. 9 , the signal can be generated parasitically by using a second auxiliary winding  76  with one or more turns wrapped around the VT  30 , thus forming an auxiliary loop  78  comprising the winding  70  around the CT  32  and the auxiliary winding  76 . The auxiliary winding  76  can have a reversed polarity with respect to the loop  60 . The auxiliary loop  78  can contain configurable impedance Z aux    80 , whose role it is to adjust the amplitude and phase of the compensating signal and which can generate a compensation signal with variable amplitude and phase to compensate for the instantaneous level of interference from the VT  30  and the effect of the variable gaps  41   a ,  41   b.    
         [0082]    It is also possible for the auxiliary winding  76  to have reversed polarity with respect to the primary winding  31 . 
       Electric Compensation 
       [0083]    The interfering magnetic field  66  does not have to be suppressed magnetically in order to achieve the compensation. Where the output signal of the CT  32  consists of unwanted as well as useful signal components, electric compensation means can be provided to act on the output signal to nullify the unwanted signal components. The unwanted magnetic field H noise    66  remains present in the core  46 , and only the electric signal induced in the winding  33  is compensated. As shown in  FIG. 10 , a suitably controlled signal source  90  can be inserted in series with the secondary CT winding  33 . The amplitude and phase of the signal source  90  can be then adjusted in order to oppose the unwanted component of the output signal, induced due to the interfering magnetic field  66 . 
         [0084]    As in the magnetic compensation techniques, the compensating signal can be supplied either by an auxiliary channel of the signal processing unit  35  or by a parasitic winding  92  around the VT  30  (as in  FIG. 12 ) or by any other signal generator. The parasitic winding  92  may be of reversed polarity with respect to the loop  60  of equivalent resistance, and/or with respect to the primary winding  31 . 
         [0085]    Another option is for the compensating signal to be tapped from the supply to the primary winding  31 , so as to be driven by the same signals. Optional arrangements for tapping the compensating signal from the primary winding are illustrated in  FIGS. 11   a - 11   c . In essence, these may be special instances of the arrangements of  FIG. 8  or  FIG. 10 , with the signal sources  72  and  90  comprising the signals tapped from the primary winding  31 . 
         [0086]    Similarly to the auxiliary loop  78  of  FIG. 9 , suitable auxiliary impedance  80  can be used for amplitude and phase adjustment in these ‘tapping’ arrangements also. 
         [0087]    Another configuration for tapping the primary winding  31  is a bridge circuit, for example having connections as shown in  FIG. 11   a . The impedance  80  in the configuration of  FIG. 11   a  can be varied in a number of ways (RR, RC, CR, LR circuits, etc).  FIGS. 11   b  and  11   c  show more generic configurations in which the circuit designated “bridge” can be any combination, or a “network”, of various interconnected elements, both passive (RLC) and active. As will be appreciated by the skilled reader, there are numerous combinations of possible connections of tapping, elements, auxiliary coil on CT and the output signal. Other types of “bridge network”, such as an impedance network, differential amplifier, analogue comparator, differential measurement, etc.) are also possible. 
         [0088]    By tapping the signal from the primary winding  31 , an additional signal generator  72 ;  90  is not required. Alternatively, with respect to the configuration of  FIG. 12 , no extra coil  92  is needed. As a result, the size and cost of the device can be reduced even further. 
       Software Compensation 
       [0089]    If the resolution, dynamic range and measurement accuracy of the signal processing unit  35  permit, the signal compensation shown in  FIG. 10  can be achieved entirely by an appropriate digital algorithm (software), which may be implemented as firmware within the apparatus. The output signal across the secondary winding  33  can be digitised by an appropriate analogue-to-digital converter (not shown). Such a signal will then contain two components: the useful signal generated by the magnetic field  64  as well as the variable unwanted signal component generated by the interfering magnetic field  66 . 
         [0090]    The signal processing unit  35  may comprise separate sub-units for the digitising and the processing. For example, the digital compensation may be made before the compensated output signal is passed to a CPU in the signal processing unit  35  for resistance calculation. 
         [0091]    A suitable set of data representing a waveform of required amplitude and phase can be represented in the software. Values of such data can be adjusted so as to represent a signal equal, but with opposing phase, to the unwanted component of the signal induced by the interfering magnetic field  66 . Therefore, the equation employed for calculation of the measured loop impedance  62  will be modified from (i) and will take the following form: 
         [0000]    
       
         
           
             
               
                 
                   
                     Z 
                     loop 
                   
                   = 
                   
                     
                       C 
                       calibration 
                     
                      
                     
                       
                         V 
                         VT 
                       
                       
                         
                           f 
                            
                           
                             ( 
                             
                               H 
                               CT 
                             
                             ) 
                           
                         
                         - 
                         
                           S 
                           compensation 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   iii 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where:
 
S compensation =the data representing the unwanted variable component of the output signal of the CT  30 .
 
         [0092]    The S compensation  can take a form of a set of data (synonymous with a digital waveform containing the variable amplitude and phase information), as well as a single value (synonymous with a digital waveform containing only the amplitude information). The former method allows full compensation to be achieved, whereas the latter technique gives only partial compensation, but is much simpler to implement. In addition, and as described later, this equation has more general applicability in that it in essence describes each of the different compensation techniques, digital or otherwise. 
         [0093]    In order to achieve the in-software compensation the following typical algorithms could be implemented. 
         [0000]    Main algorithm A:
 
Step A1: Introduce the main loop with some impedance.
 
Step A2: Measure VT and CT signals.
 
Step A3: Subtract the digital compensation signal (waveform or value) from the CT signal.
 
Step A4: Use the VT signal and the corrected CT signal to calculate the loop impedance as usual.
 
         [0094]    The main difficulty here is to adjust the amplitude and phase of the digital signal in order to obtain the required level of compensation, and in particular with the presence of a variable gap  41   a ,  41   b . This can be achieved conveniently by implementing the following algorithm and repeating it out for various gap values: 
         [0000]    Compensation calibrating algorithm B
 
Step B1: Introduce main loop to be measured with known impedance.
 
Step B2: Calculate the loop impedance as usual. If the calculated value is sufficiently close to the ideal value (i.e. the known impedance of step B1) go to Step B4.
 
Step B3: Adjust the amplitude and phase of the digital compensation signal and repeat Step B2.
 
Step B4: Store the digital compensation signal for use in algorithm A.
 
         [0095]    The digital compensation signal can be calculated in several ways rather than just with the algorithm B. One of the alternatives could be the following: 
       Compensation Calibration Algorithm C 
       [0096]    Step C1: Do not introduce any loop impedance (synonymous with open loop).
 
Step C2: Calculate the loop impedance as usual. If the calculated value is sufficiently large go to step C4.
 
Step C3: Adjust the amplitude and phase of the digital compensation signal and repeat Step C2.
 
Step C4: Store the digital compensation signal for use in algorithm A.
 
         [0097]    It should be noted that an algorithm similar to the compensation calibration algorithm B can be used also for adjusting the auxiliary loop impedances  80  in the magnetic compensation and in the electric (signal) compensation techniques. This is especially relevant if the adjustment of impedance values of element  80  is carried out by means of digital control by signal processing unit  35 . In such a case the algorithm could take a form: 
       Compensation Calibrating Algorithm D 
       [0098]    Step D1: Introduce main loop to be measured with known impedance.
 
Step D2: Calculate the loop impedance  62  as usual. If the calculated value is sufficiently close to the ideal value (i.e. the known impedance of step D1) go to Step D4.
 
Step D3: Adjust the amplitude and phase of the digital compensation signal and repeat Step D2.
 
Step D4: Use the value of auxiliary loop impedance  80  for use with the main algorithm A.
 
         [0099]    As described in detail above, there are a number of embodiments and associated methods capable of compensating for interference. However, all these methods stem from the same concept—equation (iii) describes the effect of each and every embodiment described above. 
         [0100]    There are a number of advantages associated with the application of these compensation techniques. In particular, by virtue of the fact that the compensation that is obtainable allows the shielding  42  used in current devices to be eliminated, a much simplified head design is allowed, which leads to reduced manufacturing costs. Moreover, this invention allows a substantial reduction in the size of the head, as well as increased accuracy of the device, especially in the presence of a variable gap  41   a ,  41   b.    
         [0101]    Any of the variations of the invention described above are capable of achieving the results shown in FIG.  13 —provided that sufficient control over stability and resolution of the appropriate component values is obtained. 
         [0102]    It will be appreciated that the compensation methods can be used completely without shielding. However, they can also be applied to a design with limited electromagnetic shielding in order to improve the accuracy and immunity to interference still further whilst retaining the benefits of a less bulky head. 
         [0103]    The cores  44 ,  46  described above are illustrated as being toroidal. It will be appreciated, however, that the shape of the cores is not significant and that many alternative shapes could be used.  FIG. 14   a  is illustrative of a head portion of a double-clamp device having toroidal cores  44 ,  46 .  FIG. 14   b  is illustrative of an alternative double-clamp device, having differently-shaped cores. 
         [0104]    Whereas the split cores have been described as comprising two halves, it will be appreciated that the gaps  41   a ,  41   b  may not necessarily be at a geometric mid-point of the respective cores  44 ,  46 , and one ‘half’ of the split core may be larger than the other ‘half’. 
         [0105]    Whereas the auxiliary loops or windings  70 ,  76 ,  78  have been described as single-turn windings throughout, it will be appreciated that these may alternatively comprise multi-turn windings.