GB2259783A - Four-terminal current measuring devices - Google Patents

Abstract

A four-terminal shunt (200) for measuring currents comprises two large ends (202, 204) and a reduced cross-section bridging section (206) having connection points (212, 214) to which two voltage-sensing conductors (216) are connected. The ends are apertured (208, 210) to receive bolts or rivets for attachment to current-carrying conductors (not illustrated). Each connection point (212, 214) in the bridging section is formed by an annular indentation, and a solder connection is made thereto. Sensed voltages (216) are applied to a voltage-frequency converting arrangement (86, Figure 3 not shown). <IMAGE>

Description

Title: Improvements in ad relating to current measuring devices Field of invention This invention concerns current measuring devices such as may be incorporated into power measuring meters as are employed to meter the power consumed from a centrally generated electricity supply by individual consumers.

Background to the invention In order to measure current it is necessary to observe and measure some effect which is directly related to the passage of electric current through a circuit element.

Thus historically the increase in length of a wire brought about by heating, itself caused by the passage of current through the wire core has been used. Likewise the deflection of a magnetic element by a magnetic field produced by the passage of an electric current has been used.

With the tendency towards solid state voltage and current metering, attention has been focussed on an alternative technique in which the current is caused to pass through a so-called shunt resistor (usually of very low ohmic value) so that a voltage (albeit small in value) is developed across the shunt, the value of which is proportional to the current passing therethrough.

It is one object of the present invention to provide an improved shunt for use in a current measuring apparatus.

It is another object of the invention to provide an improved current measuring meter employing such a shunt.

It is another object of the invention to provide an improved power measuring meter employing such a shunt.

Summary of the invention According to one aspect of the present invention a shunt for a current measuring device comprises an elongate element of electrically conductive material having enlarged cross-section ends and a reduced cross-section bridging section therebetween, the two ends being adapted to be connected in a current carrying circuit and the bridging section having two connection points therein spaced apart in the direction the current will tend to flow in the bridging section, to which two points, two conductors can be connected, for deriving an electrical voltage the value of which is proportional to the magnitude of the current flowing through the said bridging section.

According to a preferred feature of the invention the thickness of the shunt element is similar throughout but the width of each of the two ends is considerably greater than the width of the bridging section, so as to produce the required difference in cross-section size between the said ends and the said bridging section.

Typically the two ends are apertured to allow bolts or rivets to extend therethrough to permit attachment of the shunt to conductive connectors.

The shunt may be formed from a copper based alloy such as Manganin and be of sufficient thickness and cross-section as not only to be self supporting but also serve to serve as a stand-off support for one of the conductive connectors.

According to a preferred feature of the invention the two conductors are connected to the two connection points by soldering.

According to another feature of the invention, each connection point is formed by annularly indenting the surface of the material forming the bridge, so as to define an island in the surface of the bridging section and the solder connection is made to the top the said island.

preferably the indentation is such as to produce an upstanding generally conically shaped island, and the apex of the cone defines the solder point.

The indenting of the material is selected so that the two islands are symmetrical relative to the bridging section.

Each island is preferably located halfway across the width of the bridging section.

Where the ends are apertured, the distance between the nearest point on the perimeter of each aperture and its adjoining island is preferably at least equal to one half the width of the bridging section.

To further improve the electrical characteristics of the shunt each island may be located at a position in the bridging section which is located along its length from one end towards the middle thereof, by a distance equal to at least one half the width of the bridging section.

The diameter of each annular indentation is preferably many times smaller than the diameter of the adjoining aperture in the end of the shunt.

Electrical conductors connected to the two islands may be formed as a twisted pair.

The invention also lies in a meter adapted to measure current comprising voltage measuring means for measuring the voltage between the two islands of a shunt as aforesaid.

The invention also lies in a meter for measuring electrical power in which two signals one corresponding to a supply voltage and the other to the current flowing therefrom to a load, are generated, and a product thereof formed to provide a measure of the power being supplied to the load, wherein a voltage proportional to the current is generated using a shunt as aforesaid and conductive connectors are provided for connecting the shunt in series with a conductor carrying the current to the said load.

The invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a simplified block circuit diagram of a meter which can embody the invention, Figure 2 is a block circuit diagram of the elements which make up the processor employed in Figure 1, Figure 3 comprises block circuit diagram of the voltage and current to frequency converters of Figure 1, Figure 4 illustrates a simplified form of meter based on Figure 1 in which no opto-isolation of the processor is required from the voltage to frequency and current to frequency circuits, Figure 5 illustrates how two or more meters can be connected to a so-called substandard meter for simultaneous calibration, Figure 6 illustrates a card controlled meter embodying the invention in its protective housing, Figure 7 is a perspective view of the interior of the meter with the front housing cover and card reader removed, Figure 8 is a perspective view of the card reader from one side, Figure 9 is a similar view of the card reader from the other side, Figure 10 is a perspective view of the rear of the meter housing with the pcb and card reader removed, to reveal the contactor and current shunt, Figure 11 is a perspective view of a shunt constructed in accordance with the present invention, Figure 12 is a plan view of one face of the shunt to an enlarged scale, Figure 13 is a cross-section to an enlarged scale through the shunt, Figures 14a and 14b show how the shunt can be further decoupled from stray electric and magnetic fields by crossing over resistors located in the lines between the shunt and a signal amplifying device, and Figure 15 shows how the alternative tracks on a pcb between the resistors and the twisted pair can be crossed over.

In order to better understand how a shunt may be incorporated into a meter such as a power measuring meter, reference will first be made to Figures 1 to 10 which show a solid state power meter.

Referring to Figure 1, the instantaneous potential of the mains supply is measured between the live line 3 and the neutral line 11 of a domestic supply. The voltage is converted to a pulse train by the voltage to frequency converter 5 which has a free running frequency of approximately 2,000 Hz which is modulated up or down in frequency depending on the polarity and amplitude of the potential excursion. Typically the base frequency is modulated by the supply voltage waveform signal to produce a frequency modulated signal with a deviation of plus or minus 200 Hz. The deviation is proportional to the instantaneous voltage of the waveform.The pulse train produced by the frequency to voltage converter 5 is coupled to input 1 of a processor 8 via an opto-isolator The instantaneous current flowing is measured by measuring the potential generated across a shunt resistor 2 connected in series with a live wire 3 of the supply to the consumer. This voltage is converted to a pulse train by a voltage to frequency converter 4 which also has a free running oscillator operating at a frequency of approximately 2,000 Hz. This base frequency is modulated by the supply current waveform signal to produce a frequency modulated signal with a deviation of plus or minus 200 Hz full scale. The deviation is proportional to the instantaneous current flowing to the consumer and the pulse train is coupled via an opto-isolator 6 to a second input of the processor 8.

The processor 8 provides signals for driving a display 106 (typically liquid crystal display) to indicate under normal operation, the accumulated numerical value of units of power measured by the meter, (as will be described in more detail with reference to Figure 2).

Likewise contactor 108 may also be controlled by an output signal from the processor.

The contactor is preferably a pulse operated device held into its last switched state by a spring or permanent magnet or both.

The contactor is only required if ON/OFF control of the supply to the consumer/load is required as in the case of a coin or a card controlled meter or a meter which is to be remotely controlled by the supply authority as by power line modulation.

there provided, a power line signalling receiver or transmitter/receiver 112 provides control signals for the processor or receives data from the processor for transmission to the supply authority - as for example to indicate a fault condition in the meter or relay to the authority the accumulated value of measured power.

The card reader 114 likewise is controllable by signals from the processor and in turn produces electrical signals serving as signal inputs to the processor. Thus the processor may produce a signal to enable the card reader, cancel data on a card after it has been read, may receive data read from an inserted prepayment card for validating the card and can indicate to the processor the number of units to be permitted before requiring a further card to be inserted.

DC power for the voltage/current to frequency converter circuits 4 and 5 and the driving elements of the optoisolators is derived from a halfwave rectifying circuit made up of diode 116 and reservoir/smoothing capacitor 118. Typically the DC voltage requirement is of the order of a few volts for example 5-15 volts, and since the ac supply voltage will normally be 240 volts DSS or the like, a tapping 120 on the primary winding 122 of a transformer 124 can provide the lower ac voltage needed as input to the rectifying citcuit 116/118.

where isolation of the processor is required (as is essential if user accessible ports are provided on the meter), the dc power for the processor (and display, card reader, powerline communication receiver/transmitter, contactor etc when fitted) is derived from a second rectifying circuit supplied from the secondary winding 126 of the transformer 124 and comprising diode 128 and smoothing/reservoir capacitor 130.

The processor may be an integrated circuit capable of performing all the functions in Figure 2, or an array of devices which collectively can perform the said functions.

In order to obtain the values of voltage and current, the processor has to measure the time between pulses. In the example shown this is achieved by the same method for both current and voltage, and will be described with reference to Figure 2.

For simplicity the processor will be assumed to be a single integrated circuit ie a customised microprocessor chip.

Also for simplicity the controlling (central processing) element of the customised device of Figure 2 is not shown, nor are the signal paths between it and the individual processing elements shown in Figure 2.

A 16 bit counter 40 is continuously clocked by a crystal controlled oscillator 12 running at 5MHz.

The output of the counter is connected in parallel to the inputs of two 16 bit latches 14 and 15. Latch 14 relates to the voltage and latch 15 to current.

The clock inputs of the respective latches are fed with the pulse trains of lines 9 and 10 respectively.

On the positive edge of a pulse on line 10, the value of the counter 40 is stored in the latch 14. The edge also provides an interrupt via the interrupt request register 13. The interrupt causes the value stored in the latch 14 to be read. To obtain a number indicative of the period between pulses1 the value of the previous count is subtracted from the newly counted value in a subtractor 20. To produce a value proportional to frequency, the period is inverted by inverter stage 21 to produce the arithmetical reciprocal.

Simultaneously with the above counting and latching sequence, another counter 18 and latch 19 average the incoming voltage related frequency over a 40 second period determined by a timer 17. This average voltage related frequency is then subtracted by 22 from the instantaneous voltage related frequency to give numerical output value at 38 proportional to the instantaneous voltage. This feature allows the the voltage "base" frequency to be continuously autozeroed to cater for long term drift.

A second channel accepts the signal representative of current along line 9 and produces a value 39 proportional to the instantaneous current in exactly the same way as the signal 38 is produced corresponding to voltage.

The current value channel has a similar arrangement to the voltage channel for producing an average value of the current related frequency, but typically this is only performed once during calibration, during manufacture.

The items concerned are labelled 34, 35 and 36. The average or base current related frequency value is subtracted from the values of instantaneous current frequency which arise during use, giving a frequency value proportional to the instantaneous current, for supply as signal 39.

The two signals 38 and 39 are the inputs to a four quadrant multiplier 26. As these two signals are asynchronous, the multiplication is caused to occur at regularly occuring intervals of time displaced by, for example, 500 microseconds. A suitable timing or interrupt circuit 27 produces the necessary control signals for the multiplier. The latter uses the input signal present at 38 and 39 at each instant in time and each result is then passed to an accumulator 28 which keeps a total of the power consumed. The running total in the accumulator is compared in the comparator 29 with a number, from register 30 which is equivalent to a 1/1000th of a kilowatt hour.

When this value is reached or exceeded a current pulse is generated causing the kilowatt hour register 33 to be incremented by one and if desired a front panel light emitting diode (LED) 37 can be triggered to flash. The value in 30 is also subtracted from the register (accumulator) 28 in response to the generation of the count pulse.

It is to be noted that if the value in the register 28 is greater than the value from 30 the excess will remain in the register 28 and will count towards the new accumulating value in the register. This significantly improves the accurancy of the measuring technique since in this way no part of any power signal computed by multiplier 26 will be lost and over a long period of time the shortfall in the kilowatt hour register 33 would be quite considerable if the overflow amounts left in the accumulator register 28 were (in fact) to be disregarded.

The number used to indicate a 1/1000th of a kilowatt hour is at least initially adjustable to enable calibration of the meter against a standard. This provides a method for calibration of the meter during manufacture and if required following any subsequent refurbishment.

The processor 8 may include memory means (not shown) in which one or more programmes or instructions can be stored for recall in response to appropriate interrupts and/or input signals, to cause the processor to perform the functions described in relation to Figure 2.

Figure 3 illustrates a preferred circuit for providing two frequency modulated signals corresponding to the instantaneous values of supply voltage and load current of Figure 1.

Certain of the components and connections are common to elements in Figure 1 and to this end the same reference numerals have been employed.

The shunt should develop the smallest possible potential difference vI. To this end a differential amplifier 86 is employed to generate a larger signal VI for supply to pin 5 of a type 555 timer 88.

Mean frequency controlling circuit elements 90 and 92 provide potentials for pins 2, 6 and 7 and a charging/discharging capacitor 94 is connected between pin 2 (which is also connected to pin 6) and the live line.

Pin 3 provides the FI output signal which may be supplied directly to the processor 8 (as shown in Figure 4) or via an opto-isolator 6 as shown in Figure 1.

A second 555 timer 96 forms the basis of the supply voltage to frequency converter 5. A small fraction of the supply voltage (between the LIVE line 3 and the NEUTRAL line 11) is produced by a potential divider made up of resistors 84 and 98. The desired small fraction appears across the resistor 98. This potential difference appears between pins 1 and 3 of 96. As before the mean frequency of operation of 96 is controlled by RC elements 100, 102 and 104 and typically the resistors 100 or 102 or both are made adjustable so that the frequency of 5 can be made the same as that of 4. (Alternatively or additionally the resistor 90 or 92 or both may be made adjustable).

As before the IF signal is derived from pin 3 of the 555 device and is supplied either directly (as shown in Figure 4) or via an opto-isolator 7 as shown in Figure 1, to the second input of the processor 8.

Figure 4 merely shows diagrammatically how in a simple meter (in which there is no user accessible port such as coin freed mechanism or card reader) but merely a display with or without a contactor for local or remote ON/OFF control of the supply, the need for opto-isolators is removed. To this end the processor 8 is at LIVE rail polarity as is consequently the display 106 and actuator coil of the contactor 108 if provided. there the latter is provided the LOAD terminal is connected to the terminal A of Figure 4 via the contacts 110. The latter is to advantage a spring and/or permanent magnet assisted contactor requiring positive and negative pulses only for operation to open and close the contacts 110.

where the contactor is not required, the load is connected direct to terminal A.

Calibration is usually performed by comparing the power measured by a meter under test with the power measured by a "standard meter" set to measure the same voltage and current parameters over the same period of time. Although a so-called standard quality meter is ideally used, in practice meters which are not quite up to standards quality may be used as the reference and such meters are commonly referred to as sub-standard meters.

Such a meter is that produced by Landis and Gyr under the code TVE 102/1. These meters deliver an electrical pulse for each 1/500,000th of a kilowatt hour measured by the meter. Each such pulse is called a unit power pulse.

As described with reference to Figure 2 the numerical value with which the accumulated value being registered at 28 is to be compared (to determine when a 1/1000th of a kilowatt hour has been registered by the meter) can be adjusted for calibration purposes. This numerical value is held in the register 30.

Since the pulses to be accumulated by the meter under test should correspond to 1/1000th kilowatt hour, an interface 74 is provided which includes divider devices (not shown), typically CMOS type CD 4510B connected to provide a 500:1 ratio, so that one value is delivered by the interface for every 500 pulses received from the "standard" meter 50.

The permanent value for register 30 is arrived at by feeding via optical port 32 pulses from the interface 74 to a counter 41. The value in counter 41 is initially set to zero by a reset pulse on line 44. This reset pulse may for example be the first to arrive of a sequence of pulses from the sub-standard meter or a specially generated reset pulse. The incrementing value in register accumulator 28 is also reset to zero by the same reset pulse on line 44.

If (as is arranged) both meters are set to measure the same voltage and current, pulses arriving from the substandard meter via interface 74 and port 32 increment the counter 41, and in a similar manner the numerical value in the register accumulator 28 is incremented by the action of the power measuring circuits of the meter under test as described with reference to Figure 2.

Counter 41 is set to generate an output pulse when N pulses have been received from the interface 74 and this trigger is supplied to the divider 42 to divide the numerical value which has been accumulated in the accumulator register 28 by the value N to produce a numerical value for latching into the register 30.

The value of Nmay be 256 to simplify the division step and ensure a relatively long period in which the substandard meter output is compared with the meter or meters under test. To this end the arrival of the 257th pulse can be used to serve as the trigger to generate the divider instruction pulse along line 45.

It is to be understood however that the numerical value N is quite arhitrary and any value can be chosen which is convenient and sufficiently large enough to ensure that enough unit power pulses have been received to ensure an accurate value after division by N for insertion in the register 30.

After the comparator register 30 is latched it is preferably WRITE-inhibited in any known manner to prevent unauthorised recalibration of the meter.

As shown in Figure 5 a sub-standard meter 50 may be connected between the L and N terminals of a supply 52 and is received current from the terminals 68, 70 of the secondary of a current transformer 54.

One terminal 70 of the current transformer secondary is connected to the L terminal of the sub-standard meter 50 and in order to ensure that the same current passes through the current measuring circuits of all the meters, the LOAD terminal 56 of the sub-standard meter is connected to the LIVE terminal 60 of the first meter under test 58 and the LOAD terminal 62 of that meter is connected to the LIVE terminal 64 of the next meter under test 66 and so on, until the last meter in the chain where the LOAD terminal is connected to the terminal 68 of the load.

In Figure 5 only two meters are shown under test and it is therefore the LOAD terminal 72 of the second meter 66 which is connected to the load terminal 68.

In order to convey unit power pulses from the sub-standard meter 50 to a number of meters under test 58 and 66 etc, the interface unit 74 pulses serve to drive a chain of LEDs 76, 78 etc and cause the latter to flash in synchronism therewith. By positioning each, LED 76, 78 etc opposite the opto communication ports 80, 82 respectively of the meters under test, so the pulses derived from the unit power pulses from the sub-standard meter 50 can be used to calibrate all of the meters in the chain.

The assembled meter of Figures 1 and 2 is shown in Figure 6 within a two part housing comprising a base unit 132 and front cover 134. The latter is adapted to be panel or wall mounted and the front cover includes a panel containing a viewing window 136 through which a display 106 can be seen. The slot of a card reader 106 is shown at 138 and finger operable control buttons 140 and 142 allow the meter to be programmed after appropriate instructions have been entered via an opto communications port 144 containing an LED 146 and light sensitive transistor 148.

The LED 37 which flashes when unit power pulses are generated is also visible through the window 150.

Removing the front cover and card reader allows the inside of the meter to be seen as shown in Figure 7. Here the display 106 is shown mounted on a small pcb 156 carrying also the receiver and transmitter units 146, 148 of the opto communications port 144, switches 152 and 154 operable by the press pads 140 and 142 in Figure 6, and the LED 37 (of Figure 2). The small pcb 156 is mounted by standoffs 158 and 160 from the main pcb 162 on which is mounted a central processor chip 164 and related power supplies and buffer circuit elements, the opto isolators, the 555 timer devices 88, 96, the differential amplifier 86 and related decoupling and signal coupling paths and devices. A slot 166 is provided into which the inboard end of a card reader can be fitted and located.

Cable connections are provided at 168, 170, 172 ad 174 for connecting the Live, Neutral In, Neutral Out and Load (ie for example the live busbar of a domestic supply). Figures 8 and 9 show the card reader as comprising a shallow box-like member 176 defining a slot 138 at one end and having on one face a DC motor 178 adapted to draw in and eject cards via a claw drive 180 and on the other face an erasing device comprising a pivotted arm 182 bearing a permanent magnet 184 which during reverse movement of the card (not shown) is moved under the action of the motor drive into contact with the magnetic stripe on the card to erase magnetic data stored thereon.

Reading and writing control circuits for receiving signals from and supplying signals to a read/write head (not shown) also mounted on one face of the box 176, are also carried by the latter.

Beneath the main pcb 162 as shown in Figure 10, is located a contactor 186 which for convenience supports the shunt 2 (of Figure 1) itself connected between one terminal 167 of the contactor and one of the cable connectors 168 etc of Figure 7.

Figure 11 shows a preferred shunt constructed in accordance with the present invention comprising a generally rectilinear element 200 of a copper based alloy manganin having two larger cross-section ends 202, 204 joined by a smaller cross-section bridge 206. Each end is apertured at 208, 210 respectively to allow the ends to be formed as by rivetting or bolting to electrical connectors such as are shown in Figure 10 at 167, 168. Thus in that meter, the shunt not only connects one of the contactor connections to the connector 168 but also provides mechanical suport for either the connector block 168 (when bolted thereto) or the contactor (when bolted to it), depending on whether the block 168 or the contactor 186 is rigidly held in place in the casing.

Between the apertures 208, 210 are two circular indentations 212, 214 which serve as solder connections for a twisted pair of conductors 216.

The positions of the solder connections relative to the bridge 206, ends 202, 204 and apertures 208, 210 is best seen in Figure 12. Here it will be seen tht the distance is equal to the 1/2M (where M = the width of the bridge 206), and the solder connections 212, 214 are located halfway across the width of the bridge 206.

Figure 13 shows in cross-section the conical centres 216, 218 left by the annular indentations 220, 222, and which serve as solder connections for the conductors 212, 214.

It has been found that the electrical connection of the cables 212, 214 to the shunt becomes much less critical by forming the solder points as conical upstands 216, 218 in accordance with the invention, defined by annular indentations in the shunt surface.

As shown in Figure 14(a) and Figure 14(b) the shunt 200 may be further decoupled from stray fields by connecting the two points 214, 216 to two input terminals 224, 226 on a pcb 228 by means of a twisted pair 230.

Further decoupling is achieved as shown in Figure 14(b) by mounting line resistors 232, 234 feeding input terminals 236, 238 of signal amplifying device 240, so that the one cross the other as shown.

Alternatively as shown in Figure 15 tracks 242, 244 on the pcb 228 may be crossed by employing a double sided board or any other convenient way of arranging one track to cross another. The resistors 232, 234 may then be mounted conventionally as shown in Figure 15 or cross mounted as in Figure 17(b).

Claims (18)

1. A shunt for a current measuring device comprising an elongate element of electrically conductive material having enlarged cross-section ends and a reduced crosssection bridging section therebetween, the two ends being adapted to be connected in a current carrying circuit and the bridging section having two connection points therein spaced apart in the direction the current will tend to flow in the bridging section, to which two points, two conductors can be connected, for deriving an electrical voltage the value of which is proportional to the magnitude of the current flowing through the said bridging section.

2. Shunt as claimed in claim 1, wherein the thickness thereof is similar throughout but the width of each of the two ends is greater than the width of the bridging section, so as to produce the required difference in cross-section size between the said ends and the said bridging section.

3. Shunt as claimed in claim 1 or claim 2, wherein the two ends are apertured to allow bolts or rivets to extend therethrough to permit attachment of the shunt to conductive connectors.

4. Shunt as claimed in any of claims 1 to 3, when formed from a copper based alloy such as Manganin.

5. Shunt as claimed in any of claims 1 to 4, which is of sufficient thickness and cross-section as not only to be self-supporting but also serve to serve as a stand-off support at least for one of the conductive connectors.

6. Shunt as claimed in any of claims 1 to 5, wherein two conductors are connected to the two connection points by soldering.

7. Shunt as clained in any of claims 1 to 6, wherein each connection point is formed by an annular indentation in the surface of the material forming the bridging section so as to define an island in the surface of the bridging section and the solder connection is made to the top of the said island.

8. Shunt as claimed in claim 7, wherein each indentation is such as to produce an upstanding generally conically shaped island, and the apex of the cone defines the solder point.

9. Shunt as claimed in claim 7 or 8, wherein the indenting of the material is selected so that the two islands are symmetrical relative to the bridging section.

10. Shunt as claimed in any of claims 7 to 9, in which each island is located halfway across the width of the bridging section.

11. Shunt as claimed in any of claims 7 to 10, wherein the ends are apertured and the distance between the nearest point on the perimeter of each aperture and its adjoining island is at least equal to one half the width of the bridging section.

12. Shunt as claimed in any of claims 7 to 10, wherein each island is located at a position in the bridging section which is located along its length from one end towards the middle thereof, by a distance equal to at least one half the width of the bridging section.

13. Shunt as claimed in any of claims 7 to 12, wherein the diameter of each annular indentation is smaller than the diameter of the adjoining aperture in the end of the shunt.

14. Shunt as claimed in any of claims 1 to 13, when connected to a circuit component by electrical conductors connected to the two islands, wherein the two conductors are formed as a twisted pair.

15. A meter adapted to masure current comprising voltage measuring means for measuring the voltage between the two islands of a shunt as claimed in any of the preceeding claims.

16. A meter for measuring electrical power in which two signals one corresponding to a supply voltage and the other to the current flowing therefrom to a load, are generated and a product thereof formed to provide a measure of the power being supplied to the load, wherein a voltage proportional to the current is generated using a shunt as claimed in any of claims 1 to 14 and conductive connectors are provided for connecting the shunt in series with a conductor carrying the current to the said load.

17. A shunt constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.

18. A power measuring meter when fitted with a shunt as claimed in claim 1 constructed and arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.