Electricity meters are typically employed for, among other things, measuring electricity consumption for the purpose of billing customers for energy usage. Tampering with a meter to hamper its ability to accurately measure consumption results in energy theft. Accordingly, it is necessary to employ methods of detecting and/or inhibiting tampering. Tampering can take many forms. One method of tampering consists of tampering with the measurement ability of the current transformer (CT). CTs are used in many meters to measure current flow to, and hence energy usage by, a customer load.
FIG. 1 shows a schematic diagram relevant portion of a prior art electricity meter 10 connected to the electrical service of a load 20, wherein the CT 12 of the meter 10 that has been tampered with using a resistor 36. In general, the load 20 is connected to the power lines 22, 24 via respective service lines 26, 28. The service lines 26, 28 collectively carry the current provided to the load 20. To measure energy consumption, the meter 10 measures voltage and current signals on the service lines 26, 28. The meter 10 measures the current consumption using the CT 12, which detects the current flowing through the service lines 26, 28 on its secondary winding 30. The secondary winding 30 carries a measurement current signal that is proportional to the current signal on the service lines 26, 28. Ideally, the burden resistor 32 receives the entire measurement current signal from the secondary winding 30. The processing circuit 34 receives the voltage drop over the burden resistor 32, which ideally represents the current signal in the secondary winding 30 and hence the current in the service lines 26, 28.
In the tampered meter 10 of FIG. 1, however, the burden resistor 32 does not receive all of the measurement current from the secondary winding 30 of the CT 12. The meter 10 has been subject to tampering by placement of a resistor 36 across the secondary winding 30. The resistor 36 consumes some of the current from the secondary winding 30, thereby reducing the amount of current dropped over the burden resistor 32. As a consequence, the processing circuit 34 receives a signal that has a smaller magnitude than the measurement current flowing through the secondary winding 30.
The present method used by the utility to detect this type of tampering is comparing month-to-month consumption to look for anomalous behavior or trends. This method can be subjective and inaccurate, however, and often results in misdiagnosis of tampering. Another method is to perform a visual check when a meter is inspected at the installation site. However, visual inspection will not always result in detection of a resistor such as the resistor 36 of FIG. 1.
Accordingly, there is a need for a method and/or apparatus for detecting tampering with a current measurement in an electricity meter.