GB2183348A - Detecting tampering with meters - Google Patents

Abstract

To detect tampering, an electricity meter (1) comprises a device (11,12) providing a first output representative of the current through the live conductors (6,8) and a device (26,28) providing a second output representative of the current through the neutral conductors (7,9). A third output is generated (29,30) representative of the difference between the currents in the live and neutral conductors. The second and third outputs are summed (34) to provide a fourth output, and the first and fourth outputs are compared (38). A tamper detection output is generated (39) when the difference between the first and fourth outputs exceeds a predetermined threshold. The third output, alone, may be used to supply an earth leakage detector circuit. <IMAGE>

Description

SPECIFICATION Electricity meter The present invention relates to a device for detecting tampering with an electricity meter.

Electricity meters are in nearly all cases ofthe induction type and millions of such meters have been installed in domestic and industrial premises to record the consumption of electrical energy. Such meters are constructed so that a torque is exerted on an aluminium disc, the torque being proportional to the product of the voltage supplied and the current supplied,that product being equaltoth a power consumed. The disc is driven at a speed which is proportional tothetorque and the numberofdisc rotations is integrated.

The known meters are not always very accurate due to, for example, de-magnetisation of a brake magnet which is provided forfine speed adjustment and friction ofthe bearings supporting rotating parts. In addition the meters are relatively bulky and heavy. These problerns are however relatively minor when compared with the problems of deliberate tampering intended to reduce the indication of power consumed and therefore enable the person responsibleforthetampering to steal effectively energy.It is difficult to quantify the losses resulting from tampering with electrical meters but they are substantial The problem is that the meters are accessible to the potential thief continuously whereas the utility responsible for the meters has access only on those infrequentoccasionswhen the meter is read.Ametercan only be read withthe cooperation ofthe person occupying the premises with which the meter is associated except in those limited circumstances where the meters are mounted so esto be accessible from outside the premises. Proposals have been madeto enable meters to be read from a remote location but utilities have resisted such proposals to date because it has been difficultto accomplish and additionally fear of tampering has also caused concern.

There are various known methods fortampering with an electricity meter so that it shows a reduced power consumption. These methods will not be described herein for security reasons but will be known to selected persons with knowledge ofthe electricity supply industry. It is an object ofthe present invention to detect such tampering.

According to the present invention there is provided a device for detecting tampering with an electricity meter measuring electrical energy delivered to a load from an electrical powersupply via live and neutral conductors, comprising a first current measuring device providing a first output representative of the currentflowing thorough the live conductor terminals of the meter a second current measuring device providing a second output representative of the current flowing through the neutral conductorterminals of the meter, means providing a third output representative ofthe magnitude ofthe difference between the current flowing through the live conductorterminais and the currentflowing through the neutral conductor terminals meansforsumming the second and third outputs to provide a fourth output, means for comparing the first and fourth outputs, and means for providing atamperdetection output when the difference between the first and fourth outputs exceeds a predetermined threshold.

Thefirst output may be usedto drivethe meterso that the first current measuring device is used both to provide a measure of power supplied and to provide an output required to detect tampering.

Preferably, means are provided to add the absolute value ofthefourth output to thefirst output with the resultant signal being used to drive the meter so that tampering with the meter does not reduce the measurement of electricity consumption.

The tamper detection output may be arranged to energise an alarm in theform of a loud siren orto simply activate an alarm indicator providing an alarm output which will only be apparent to a utility employee for example reading the electricity meter.

Detection oftampering may also be signalled to a remote location.

In circumstances where no tampering has occurred there may be some differences between the currents in the live and neutral conductors as a result of earth leakage currents. It is knownforexampleto monitor such earth leakage currents and to disconnect the electrical supply to a load when the detected leakage current exceeds a predetermined threshold. An earth leakage current results in the first and second outputs ofthe device according to the present invention differing in magnitude. These differences are however compensated for by the third output which is representative of the earth leakage current.The third output also could be used howevertotrip an earth leakage circuit breaker if so desired Each of the current measuring devices preferably comprises a Hall effect device although alternative devices such as simple windings supported on toroidal cores may also be used.

Embodiments ofthe present invention will now be described by way of example with reference to the accompanying drawings, in which Figure lisa simplified circuit diagram of current monitoring components of a known electrical energy meter incorporating a Hall Effect device; Figure 2 is a block diagram of circuitry which in association with the arrangement of Figure 1 is used to derive a measure of energy consumed; Figure 3 illustrates current measuring components of a first embodiment of the present invention; Figure 4 illustrates circuitry associated with the current measuring devices of Figure3; Figure 5illustrates a modified form of the embodimentshown in Figure 3; and Figure 6illustrates circuitry associated with the embodiment of Figure 5.

Referring to Figures 1 and 2, the illustrated device is a known Hall effectwattmeter. The meter comprises a casing 1 and terminals 2,3, 4and 5to which terminals incoming live conductor6, incoming neutral conductor 7, outgoing live conductor 8 and outgoing neutral conductor 9 are connected. The conductors 6 and 7 are connected to the power supply and the conductors 8 and 9 are connected to the consumer's load.

A conductor 10 extends between live terminals 2 and 5 and threads a toroidal core 11 which defines a gap in which a Hall effect probe 12 is located. The conductor 10 is a short length of thick copper conductor having avert low resistance value. An auxiliarytransformerhasaprimarywinding 13 connected between the incoming live and neutral conductors 6,7 and a secondary 14supplying terminals 15. An adjustable resistor 16 is connected in series with the secondary winding 14 so asto enable adjustment ofthe output appearing across terminals 15.

Referring in particularto Figure 2, the Hall probe 12 is represented by a square and the magnetic field induced acrosstheHall probe is represented bythe arrow B. The terminals 15 are connected to respective opposite sides of the Hall probe 12 and an output is generated across the other two sides ofthe Hall probe and appears at terminals 17. The signal across terminals 15 is a control current (generally of the order of a few milliamps) which can be adjusted for calibration purposes using the variable resistor 16, One ofthe signals making upthe control current is a DC quantity, other signals being alternating quantities related to offset and double frequency components consequent on the multiplying processes in the Hall elements.

The output of the Hall probe is applied via an differential amplifier 18to a low passfilter l9which removes all alternating quantities above 5Hz. The resultant DC signal appearing at the output ofthe lower passfilter 19 is applied to a voltageto frequencyconverterwhich produces a series of squared pulses the frequency of which is proportional to the DC magnitude derived from the Hall effect probe. In a conventional mannerthese pulses are counted, divided and memorised in a calculating circuit21 and the resultant measure of energy consumption is displayed on a light emitting display 22.

Referring now to Figures 3 and 4, equivalent components to those described above with reference to Figures 1 and 2 carry the same reference numerals.Thesecondaryl4issplitintothree sections to provide outputs across terminals 23,24 and 25. A second ferromagnetic core 26 is provided which is threaded by a conductor 27 linking the neutral terminals 3 and 4. The toroidal core 26 defines a gap in which a Hall effect device 28 is located. Athird ferromagnetic core 29 is provided which is threaded by both the live conductor 10 and the neutral conductor 27. The core 29 defines a gap in which a Hall effect device 30 is located.

Referring in particularto Figure 4 a summing circuit 31 is connected between the low pass filter 19 and the voltage to frequency converter 20. The output of the Hall effect device 28 is applied by a differential amplifier 32 and low pass filter 33 to a summing circuit 34. The output of the Hall effect device 30 is applied to a differential amplifier 35, a low pass filter 36 and an absolute value circuit 37 to another input of the summing circuit 34. The output ofthe summing circuit 34 is applied to a subtractor 38 the other input of which is received from the low passfilter 19. The subtractor38 supplies a comparator 39 the output of which controls an alarm buzzer 40. The output ofthe subtractor 38 is also applied to the summer 31 via absolute value circuit 41.The outputs of the absolute value circuits are as their name implies always positive regardless ofthe sign ofthe signal applied to them.

When currentthrough the conductors 10 and 27 is the same the outputs ofthe low pass filters 19 and 33 are also the same, the output ofthefilter36 iszero, and accordinglytheoutputofthesubtractor38is zero. The output ofthe subtractor accordingly has no effect on the operation ofthesummer31 and therefore the circuit behaves exactly as in the case of the simple meter circuit according to Figure 1 and 2.

If howeverthe currentthrough the conductor 10 is lessthanthatthroughtheconductor27theoutputs offilters 19 and 33 and the output offilter 36 is not zero. Differences between the currents in conductors 10 and 27 may be the resultoftampering which must be detected or alternatively is the result of earth leakage currents which must be distinguished from the effects oftampering.

The provision of the Hall effect device 30 which is sensitive to earth leakage current ensures that the effects of earth leakage are distinguished from the effects oftampering. To avoid disclosing known tampering methods which are effective against some prior art meters the response ofthe described circuitry to such methods is not described in detail herein. The operation of the circuitry will be apparent to persons familiar with electronic components of the type described.

With regard to Figures 5 and 6, the illustrated embodiment is very similarto that of Figures3 and 4 except forthefact that the toroid 29 and Hall effect probe 30 are replaced by a closed toroid 42 and a simple secondary winding 43. As shown in Figure 6 the output of the winding 42 is applied to the differential amplifier 44 which supplies a rectifier45.

The output of the rectifier is applied via an absolute value circuit46to the summer34. The output ofthe summer 34 is supplied to the subtractor 38 in exactly the sameway as inthecase ofthe circuit of Figure 4.

It would be possible to replace each of the Hall effect probes of the devices described above by an arrangement equivalentto the core 42 and winding 43 of Figure 5.

The output ofthe Hall effect device 30 of Figure 3 will not normally be exactly equal in magnitudeto the difference between the outputs ofthe devices 12 and 28. It is not fundamental to the present invention forthis difference to exist however and in a simpler arrangement in accordance with the present invention the Hall effect device 30 can be dispensed with and the earth leakage currentdetermined from the outputs ofthe devices 12 and 28, thereby removing the need forthe core 29 and its associated components.

Claims (8)

1. A device for detecting tampering with an electricity meter measuring electrical energy delivered to a load from an electrical power supply via live and neutral conductors, comprising a first current measuring device providing a first output representative of the current flowing through the live conductorterminals ofthe meter, a second current measuring device providing a second output representative of the current flowing through the neutral conductor terminals ofthe meter, means providing a third output representative of the magnitude of the difference between the current flowing through the live conductorterminals and the currentflowing through the neutral conductor terminals, means for summing the second and third outputs to provide a fourth output, means for comparing the first and fourth outputs, and means for providing a tamper detection output when the difference between the first and fourth outputs exceeds a predetermined threshold.

2. A device according to claim 1, wherein the meter is connected to the first output so that thefirst current measuring device is used both to provide a measure of power supplied and to provide an output ofthecomparing means.

3. Adevice according to claim 1 or2, comprising means for adding the absolute value of the fourth outputto the first output and supplying the resultant signal to the motorsothattampering with the meter does not reduce the measurement of electricity consumption.

4. A device according to any preceding claim, comprising means responsive to the tamper detection output to energise an alarm indicator,

5. A device according to claim 4, comprising meansforsignalling energisation ofthealarm indicatorto a remote location.

6. A device according to any preceding claim, comprising an earth leakage circuit breakerwhich is arranged to be trapped by the third output.

7. A device according to any preceding claim, wherein each ofthecurrentmeasuring devices comprises a Hall effect device.

8. Adevicefor detecting tampering with an electricity meter substantially as hereinbefore described with reference to Figures 3 and 4 and Figures 5 and 6 ofthe accompanying drawings.