Conventional kWh meters typically use relatively costly current transformers for measuring mains current. Current shunts are also used, but due to the need for a high current rating, low resistance and thermal stability, the one or more conductors making up such current shunts are generally large and unwieldy, and cannot be fitted into a conventional DIN or similar housings. Accuracy requirements also result in such shunts having to be precisely engineered and calibrated, adding to their cost.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a current shunt comprising a plurality of elongate conductors arranged in a bundle, first and second terminal assemblies provided at opposite ends of the bundle of conductors, and first and second corresponding shunt wires extending from each terminal assembly, with the bundle of elongate conductors having a resistance which provides a measurable voltage or current output proportional to mains current and which remains substantially constant over an operating current range.

In a preferred form of the invention, the bundle of elongate conductors have a non-uniform resistance, the shunt including at least one external compensating resistance which is used to compensate the non-uniform resistance to arrive at a predetermined precise resistance to provide a measurable voltage or current input which is uniformly proportional to mains current.

Preferably, the compensating resistance includes at least one compensating fixed resistor in series with the shunt wires.

Conveniently, each of the terminal assemblies includes a clamping collar for holding the bundle of conductors together, and a terminal extending from each clamping collar.

Typically, the terminal includes a clamping leg which extends into the clamping collar and clamps the bundle of conductors in position within the clamping collar via a clamping screw.

Advantageously, the conductors are formed from a nickel copper alloy containing from 54% to 58% copper and from 40% to 44% nickel, and more preferably are formed from 56% copper and 42% nickel respectively.

Typically, the bundle of conductors comprises four rigid round cylindrical conductors bundled together in a square two-on-two configuration.

Normally, the operating current range is from 0 to 80 Amps, and the non- uniform resistance is preferably from 350 to 450 micro-ohms, and more preferably from 240 to 320 micro-ohms.

Preferably, the bundle of conductors has a positive temperature co-efficient of less than 10~70hms/°C, more preferably the bundle of conductors has a positive temperature co-efficient of less then 5 x 10-gohms/°C, and even more preferably less than 3 x 10~8ohms/°C.

Typically, the current shunt is sized to fit into a distribution board-mountable housing, which is typically a DIN housing.

Advantageously, the ends of the bundle of conductors and the clamping collars are dipped in solder so as to provide a heat-conducting infill.

Z-terminals may extend from the clamping collars, the Z-terminals constituting heat sinks.

The invention extends to a single- or multi-phase electricity meter comprising a current shunt of the type described above, electronic metering circuitry fed by the shunt wires, preferably a display, and a distribution board-mountable housing for housing the shunt, the circuitry and the display.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a pictorial view of a current shunt of the invention.

Figure 2 shows a cross-section on the line 2 - 2 of Figure 1 ; Figure 3 shows a partly cross-sectional side view of a DIN housing incorporating the current shunt of Figure 1 as part of a kWh meter ; and Figure 4 shows a schematic circuit diagram of the current shunt circuit of the kWh meter of Figure 3.

DESCRIPTION OF EMBODIMENTS Referring first to Figure 1, a current shunt 10 of the invention comprises four round cylindrical conductors 12. 1, 12. 2, 12. 3 and 12. 4 each having a diameter o- 3.2m1n and a length of 45mm. The conductors are formed from a copper/nickel alloy manufactured under the Avance0 trademark. The copper/nickel alloy contains approximately 56% electrolytic copper and 42% pure nickel. The alloy has a relatively high specific resistance of 0.05889Q/m for a 3. 2mm diameter conductor and a negligible positive temperature co- efficient of resistance.

Terminal assemblies 14 and 16 are fitted to opposite ends of the wire conductors 12. 1 to 12. 4. Each of these terminal assemblies include square or rectangular clamping collars 18 defining a central square or rectangular apertures 20 through which opposite ends of the wire conductors 12. 1 to 12. 4 pass. The flat ends of Z-terminals 22 and 24 are also fitted through the upper portions of the apertures 20, and clamping screws 26 pass through complementally threaded apertures 28 formed in each of the collars 18. Insulated shunt wires 30 and 32 have bared ends which also extend into the apertures 20 just beneath the flat ends 22. 1 and 24. 1 of the Z-terminals 22 and 24. The clamping screws 26 are tightened up against the flat ends 22. 1 and 24. 1 of the Z-terminals which serve as clamping plates so as to clamp the four conductors 12. 1 to 12. 4 firmly within the collars 18 in a square configuration.

In order to avoid excessive heat generation in the terminal assemblies 14 and 16 at the tangential contact points between the conductors, the terminal assemblies 14 and 16 are each dipped into a silver solder bath, thereby ensuring that the conductors 12. 1 to 12. 4 and the shunt wires 30 and 32 are completely surrounded by solder 34.

Referring now to Figure 3, the current shunt 10 is shown fitted within a shell half 36 forming part of a DIN housing having an integral DIN connection bracket 38. The noise-reducing twisted pair 33 formed by the shunt wires 30 and 32 extends onto pcb 40 which carries electronic circuitry 41 and a mechanical display 42 for a kWh meter. The DIN housing 36 is made from phenolicformaldehyde which is durable and has a high dielectric strength. The twisted pair 33 leads to a dedicated SAMES 9406 metering IC 43 which processes the current and voltage readings and provides a pulsed output to the mechanical display 42. The metering IC 43 has current sensing input pins 1 and 2 which are each designed to sense a 16 micro-amp electrical current at a maximum rated load current of 80 amps.

During manufacture, the resistance of the current shunt 10 is measured.

Typically, due to variations in manufacturing tolerances, the resistance of the current shunt is non-uniform and may vary from 240 micro-ohms to 320 micro- ohms. In the case of a 240 micro-ohm resistance being measured, the resistances of current sensing resistors R1 and R2 are calculated as follows : <BR> <BR> <BR> <BR> <BR> <BR> imaxxRsh <BR> <BR> R1 = R2 = 2 x 16 µA 80 x 240 µ# 32 µA = 600# In the case of a shunt resistance of 320 micro-ohms, and applying the above calculation, 800 ohm resistors would be required for resistors R1 and R2.

In addition to the current sensing resistors Rl and R2, voltage sensing resistors R3. R4 and R5 are used to sense incoming voltage via voltage input lines 44. A voltage sensing pin 19 of the metering IC 43 is designed to receive a 14 micro- amp asymmetrical current.

Assuming that: VD}V(R3/R4) = 14V then R5= 14V/14HA= 1MQ R3 = R4 (Vrated-14V)/14 For a rated voltage of 230VAC: R3 = 15.43 x R4 <BR> <BR> <BR> = 510 k# and<BR> <BR> <BR> <BR> <BR> R4 = 33k# The voltage supply lines 44 feed a power supply IC 44. 1 having an output DC voltage Vss of-2.5V. The output voltage is fed via a resistor R6 and a potentiometer VR1 to reference voltage pin 3 of the metering IC 43.

Typical reference voltage and current values are 1. 2V and 50uA respectively.

A typical reference resistance is thus : R,. er = 1.2V/5011A =24kQ For calibration purposes, Rref = 24kQ 10% = 21. 7kQto26. 4kQ.

As a result, R6 = 22kQ and the potentiometer VR1 = 5kQ.

The potentiometer VR1 is thus used for calibration purposes to adjust the overall gain of the metering IC 43. Coarse tuning of the overall resistance to achieve the correct current input to the IC takes place via the resistors Rl and R2 rather than via the shunt itself. Finer tuning and calibration is then achieved indirectly by adjusting the overall gain of the IC using the potentiometer VR1.

The metering IC is also provided with a voltage input 44, and kWh output 46 which leads to amplification circuitry 48 for enabling the mechanical display 42 to be driven, together with an LED display 50. A divider circuit 52 is also fed from the amplification circuit 48, and produces a pulsed power signal output 54 for remote monitoring purposes. It will be appreciated that the metering IC can be replaced by any other current or power metering circuit which requires an accurate input voltage signal proportional to current. It will also be appreciated that the metering circuit need not necessarily include a display, but may merely include a pulsed output 54 which can be transmitted to a remote metering station.

The Z-terminals 22 and 24, as well as connectors 56 fitted to the terminals provide a heat sink for heat generated across the shunt conductors 12. 1 to 12. 4.

The silver solder infill 34 assists in conducting heat to the heat sinks constituted by the Z-terminals 22 and 24 and connectors 56. Each of the connectors 56 comprise a box connector or clamping collar 58 fitted with a clamping screw 60, the end of which bears against the upper surfaces of the Z-terminals 22 and 24. The bared ends of mains wires 62 are clamped between the undersurfaces of the Z-terminals 22 and 24 and the bases of the clamping collars or box connectors 58, with the clamping screws 60 serving to clamp the bared ends of the mains wires 62 in position. The clamping collars 58 in conjunction with the Z-terminals 24 form a heat sink for directing heat away from the conductors 12. 1 to 12. 4.

The nickel/copper alloy conductors 12. 1 to 12. 4 are sized to ensure that the full rated current of 80 Amps does not generate excessive heat. According to IEC 1036, the maximum permissible nominal power loss is 2. 5W at 20A. In the present invention, the nominal power loss is 1. 25W at 20A, which falls well within the specification. This is achieved using an overall resistance of 400 micro-ohms, which will provide a measurable voltage input for the SAMES IC 43 for a current range of 0 to 80A. The variation in resistance over a temperature rise of 30°C is 0. 2%, which represents a negligible positive temperature co-efficient of 2. 667 x 10-gohms/°C. The overall resultant accuracy of the meter is 1%.

A significant advantage of the current shunt is that, because of the characteristics of the alloy of which it is made, the shunt can be made compact enough to fit within the conventional DIN housing without sacrificing accuracy or stability over a relatively wide temperature range. A further advantage is that the resistor network constituted by resistors R1 to R5 and VR1 can be used to compensate for inherent variations in the shunt resistance 10 and the metering IC 43, thereby avoiding costly and precise manufacturing procedures associated with the mass production of current shunts having uniform resistance, and precision IC's.

As a result of the bundled structure of the four conductors, the surface area-to- volume ration is almost twice the surface area to volume ratio of a single cylindrical conductor having the same cross sectional area/volume. As a result, the bundled conductors have a greater heat sink capacity, thereby reducing the temperature range under which the current shunt operates.