Abstract:
A residual current detection device comprising a plurality of resistive shunts for connection in respective ones of a plurality of lines though which current can flow to and from a load, and detector means sensitive to the voltage developed across each of the shunts to detect any imbalance between the currents flowing though the shunts.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a residual current detection device for use in a circuit breaker. 
     DESCRIPTION OF THE PRIOR ART 
     Conventionally, residual current is detected utilizing a current transformer having primary windings through which, in the case of a single phase device, load current flows in opposite directions so that if the return current is different from the outwardly flowing current because of current leakage an output current signal is induced in a secondary winding of the transformer. In the case of a multi-phase device, primary windings of the transformer are connected in all of the phase lines and the neutral line. In normal situations, when there is no current leakage, the net current induced in the secondary winding is zero and therefore no output is detected. 
     Sophisticated materials have been developed for the core of the current transformer, which enable considerable accuracy to be obtained when the currents flowing in the primary windings are substantially sinusoidal. However, switch mode power supplies are often used for computers and other equipment and there is an increasing tendency for such equipment to cause dc offsets in the currents. Such developments have made detectors utilizing current transformers less reliable and prone to false tripping or failure to detect a dc current leakage. 
     This is a particular problem in the case of directly actuated electro-mechanical devices, where the current transformer secondary winding actually drives an actuator. The situation is not much improved, when including an electronic detection and amplification means connected to the secondary winding, as there are still problems with high frequency transients and de offsets. A very small dc current level can cause the core to saturate thereby seriously impairing the ability of the detector to detect current leakage. 
     OBJECTIVES AND SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a residual current detection device in which the above mentioned problems are substantially overcome in a simple and efficacious manner. 
     In accordance with the invention there is provided a residual current detection device comprising a plurality of resistive shunts for connection in respective ones of a plurality of lines though which current can flow to and from a load, and detector means sensitive to the voltage developed across each of the shunts to detect any imbalance between the currents flowing though the shunts. 
     Preferably, the detector means comprises an analog to digital converter for each shunt and a processor for receiving the digital signals from the converters and determining whether a current imbalance exists. 
     Each shunt preferably takes the form of a composite strip having conductive portions at its ends and a resistive portion interconnecting the conductive portions. Such composite strips can be mass produced inexpensively to very high tolerances which makes them extremely suitable for this purpose. 
     The analog to digital converter for each shunt may include a delta-sigma modulator, which generates a high frequency single digital data stream which is converted by decimation filtering to a multibit digital data stream at a lower frequency. 
     The analog to digital converter for each shunt is preferably connected to the processor through an isolation barrier so that the converter can float at the voltage level of the shunt which it serves. The decimation filtering may be effected entirely in the converter, entirely in the processor or split between the converter and the processor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a diagrammatic perspective view of an example of the invention as applied to a single phase device, 
     FIG. 2 is a block diagram of an another example of the invention as applied to a three phase device, 
     FIG. 3 is a perspective view showing one of the current sensing devices, 
     FIG. 4 is a sectional view of the current sensing device of FIG. 3, 
     FIG. 5 is an elevation of the device of FIG. 3, 
     FIG. 6 is a block diagram of a simple form of the electronic circuit of a single current sensor device, 
     FIG. 7 is a block diagram of an alternative form of the electronic circuit, and 
     FIG. 8 is a block diagram of yet another form of the electronic circuit. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the device shown in FIG. 1, a substrate  10  supports two composite conductor strips  11 ,  12 . Each of these includes end portions  13  of copper and an intermediate portion  14  of a resistive material such as manganin. The strips are formed by slicing up a sandwich formed by electron beam welding the copper portions to opposite sides of the manganin portion. The shunts formed by the resistive portions manufactured by this method can have a nominal resistance of 0.2 mΩ to a tolerance of less than 5%. If the two shunts  14  used on one device are pressed from adjacent portions of the sandwich stock, they are matched to within 2%. Calibration of the shunts built into a unit at two different temperatures can virtually eliminate shunt errors. 
     In the example shown in FIG. 1, there is a separate signal pre-processing ASIC  15  mounted on each of the shunts  14  and connected to the copper end portions  13  of the associated conductor strips. The two ASICs  15  are connected to via an isolation transformer array  16  to a main processor  17 . The ASICs  15  operate to convert the two voltages across the shunts into a digital signal stream which is communicated to the processor  17  via the isolation transformer array. The main processor is programmed to provide a drive signal to a trip actuator  18 . 
     The actual preferred structural configuration of the sensors is shown in FIGS. 3 to  5 . These show leads  40  connecting two analog input terminals of the ASIC to the two copper end portions. Other leads connect other terminals of the ASIC  15  to a lead frame by means of which all other external connections are made. FIG. 5 shows in dotted lines a block  42  of encapsulation material and FIG. 4 shows an electrically insulative adhesive layer  41  by means of which the ASIC is attached to the intermediate portion  14  of the composite strip  14 ,  15 . 
     FIG. 2 shows in rather more electrical detail a three phase device. In this case there are four shunts  14 , one in each phase line and a fourth in the neutral line. The ASICs  15  of FIG. 1 are shown as four separate blocks  20 ,  21 ,  22 , and  23 , and there is a power supply unit  24  which draws power from the phase lines on the mains side of the shunts  14  and provides controlled voltages to the processor  17 . Power is supplied to the four blocks  20  to  23  via isolation barriers  25  which make up the array  16 . Each block of the ASIC includes an analog to digital converter in the form of a delta-sigma modulator which provides a high frequency one bit digital data stream. A multiplexer may be included in each converter so that the converter can provide to the processor, through the respective isolation barrier, signals representing both current in the associated shunt and the voltage at one end of it. The processor uses these signals to monitor the current in each shunt and to operate the actuator  18  if an imbalance occurs. 
     It will be noted that the voltage sensing connections to the ASICs are made via resistor chains connected between each phase line and the neutral. Each such resistor chain comprises an outer pair of precision resistors of relatively low ohmic value and an intermediate resistor of relatively high ohmic value. These resistor chains allow the RCD to be provided with an independent reference. If the neutral ADC is taken as the selected system reference, then the operating software of the main processor can use the multiple signals derived from the several resistor chains to calibrate each phase against the neutral reference. 
     The CPU is programmed to carry out the necessary calculations to determine the existence of an imbalance and can determine the true RMS value of the residual current, which conventional devices fail to do correctly particularly in the case of non-sinusoidal current waveforms. The CPU may be programmed to enable it to determine from the data it receives whether a particular event is, in fact, an unacceptable leakage more reliably than conventional devices. For example, the CPU can take into account the historic performance of the unit when setting the leakage current threshold and may ignore events which have a recognisable “signature”.In this way improved tolerance to nuisance tripping can be obtained. 
     Decimation filtering of the high frequency one bit data stream is required to reduce each data stream to a multi-bit digital signal at a predetermined sample frequency. By way of example, each current signal maybe a 23-bit signal at a sample rate of 64 times the mains frequency, but lower resolution at lower sample rates can be employed when non-linear, rather than linear conversion is acceptable. The decimation filtering is typically a function of the processor, filtering of the four data streams being executed simultaneously so that sample values are derived for all four shunts simultaneously. A circuit employing such an arrangement is shown in FIG.  6 . 
     In an alternative embodiment as shown in FIG. 7, one or more stages of the decimation filtration may be executed by hardware included within the ASIC. Multi-bit digital words are transmitted serially across the isolation barriers instead of a one-bit signal stream. The filtration stages may be split between the ASIC and the processor. 
     Where current and voltage are both to be monitored as in the system shown in FIG.  2 , the circuit  15  may be as shown in FIG. 8 with separate modulations and filtering components for the two signal streams and a common serial interface. Alternatively separate serial interfaces may be employed. 
     The arrangements described enable very accurate detection of current imbalance to be effected even in the presence of switching transients and DC offsets. The problems which arise from potential saturation of the current transformer core are avoided completely. 
     Since the CPU receives actual line current and voltage data from each of the blocks  20  to  23 , it can be programmed to perform other calculations, such as current limit and power consumption. Thus an RCD device constructed as described above can also provide the functions of a conventional circuit breaker and/or those of a power consumption meter without any additional sensing or analog-to-digital components being required.