One common non-intrusive way to steal electricity is to apply a strong permanent magnet or an AC magnet near the electricity meter, thus tampering with the meter. A permanent magnet or an AC magnetic field can affect meter components like current transformer current sensors, shunt current sensors (shunts are only affected by AC magnets), or any power-supply transformer. As a result of the weaknesses of these components to magnetic tampering, utility customers can be undercharged for the energy consumption, thereby allowing consumers to essentially steal electricity.

Due to the susceptibility of meters to magnetic tampering, magnetic sensors are often used in electricity meters to detect external magnetic fields to take appropriate action, such as disconnecting services to the meter or applying a penalty fee for magnetic tampering. In this design, magnetic tamper detection is done with the TMAG5273 linear 3D Hall-effect sensor, which has the following advantages compared to other magnetic sensing devices and designs:

• Ease of assembly: Hall-effect sensors in general are not as fragile as reed switches, the latter of which can break during assembly.
• Only one surface mount IC is needed: Sensing in three directions with the TMAG5273 requires only one surface mount IC for 3D linear Hall-effect sensors instead of the three ICs in the case of 1D Hall-effect sensors. 3D linear Hall-effect sensors therefore enable a more compact printed circuit board (PCB) layout. In addition, having a surface mount-only implementation can reduce PCB manufacturing costs compared to a 1D Hall-effect sensor implementation that can require through-hole sensors for detecting some of the directions.
• Flexibility for defining magnetic tampering threshold: Since 3D linear Hall-effect sensors provide information about the actual sensed magnetic flux density value, it is possible to select the magnetic tampering threshold of each axis to anything within the magnetic sensing range of the 3D linear Hall-effect sensor. This enables configuring how to define what is tampering, which can vary between designs since the magnetic flux density sensed depends on the distance from the magnet to the sensor as well as the characteristics of the external magnets to be detected. This type of flexibility is not possible for Hall-effect switches with fixed magnetic operating point (B_OP) thresholds. Finding the appropriate tamper threshold definition can be done by using a magnetic calculation tool to determine what is the resulting magnetic flux density observed for the different magnet-to-sensor distances and magnet types that must be detected. The magnetic threshold can be then set to something lower than the magnetic flux density detected by the sensor when exposed to the desired tamper conditions. Typically, setting the threshold to be small enough to detect tamper magnets but also large enough so that the system does not see any false positives from any nearby equipment that causes a magnetic field that does not affect the functionality of the meter is desired. The magnet-to-sensor distance depends on where the sensor is placed on the PCB as well as the dimensions of the e-meter case. For small-sized systems, the magnetic sensor can be placed near the center of the board for symmetrical sensing coverage across the meter case, or the sensor can be placed near any components that are affected by magnetic tampering. For large-sized systems like certain polyphase meters, sometimes it is not possible for one magnetic sensor to sense tampering across the entire meter surface, so multiple 3D Hall-effect sensors can be used and placed spread out with respect to each other on the PCB to cover a large sensing area. The TMAG5273 has four sets of device orderables that are factory programmed with different I2C addresses, which enable multiple devices to share the same I2C bus.
• Ability to change between multiple device power modes: The TMAG5273 supports switching between multiple power modes, depending on if it is desired to reduce system current consumption. The TMAG5273 has an active mode for taking measurements, a sleep mode for minimizing current consumption, and a duty-cycle mode that automatically switches between active and sleep modes. The following list describes the typical use-cases of the different power modes for electricity meters:
  • Active mode is used for taking measurements and requires the most power out of the different power modes. An example scenario where active mode is typically used is when the Mains are available and the meter is running off the AC/DC power supply. When running off the AC/DC power supply, the relatively high active mode current consumption (2.3mA) of the TMAG5273 is negligible.
  • In duty cycle mode, the device takes measurements and then automatically goes to sleep for a user-specified amount of time. Duty-cycle mode is good for minimizing current consumption while still detecting magnetic tampering, such as when low-speed magnetic tamper detection is necessary when running off a backup battery. To reduce average current consumption in duty cycle mode, select a long sleep time. When selecting the sleep time, set the sleep time to be less than the desired response time for magnetic measurements. As an example, to sense magnetic tampering every 2ms using wakeup and sleep mode, set the sleep time to 1ms instead of 1 second.
  • In sleep mode, the device does not take any magnetic measurements. An alternative to wakeup and sleep mode is to have the MCU manually set the sensor to sleep mode and then manually set the sensor to wake up after the desired sleep time has passed. This requires more overhead from the MCU; however, this option can reduce the system current consumption if the MCU is going to have the wakeup and sleep mode that allows the MCU to reconfigure the TMAG5273 during each wakeup and sleep mode cycle. For systems that do not require detecting magnetic tampering when running off a backup battery, the TMA5273 can just be put in sleep mode to reduce system current consumption when running off a battery and then put back in active mode when the system is able to run off the AC/DC power supply again.
• GPIO pin interrupts when magnetic tampering detected (depends on device): The TMAG5273 has the ability to set an interrupt pin when the sensed magnetic flux density of any axis goes beyond a user-defined magnetic switching threshold. To detect tampering, the user can set the magnetic switching point for interrupts to the desired magnetic tampering threshold. Since the interrupt pin of the Hall-effect sensor can wake up the microcontroller when the MCU is in low-power mode, and since the microcontroller does not have to read the Hall-effect sensor to determine magnetic tampering, the MCU can go to low-power mode when running off a backup power supply until woken up by the interrupt pin of the Hall-effect sensor. Used simultaneously, the general-purpose input/output (GPIO) pin interrupt feature and duty-cycle power mode can reduce system current consumption and extend the lifetime of the backup power supply. Once the GPIO pin of the Hall-effect sensor wakes up the microcontroller, the MCU can then retrieve the value of the sensed magnetic field reading that caused the interrupt and then enable wakeup and sleep mode with GPIO interrupts again.
• Detection of AC magnets: AC magnets do not only affect current transformers. AC magnets can also affect shunt and Rogowski coil current sensors. To detect AC magnets, a linear 3D Hall-effect sensor can also be used. Figure 2-3 shows that detecting AC magnets requires a fast-enough effective sampling period and a small enough sleep time to properly capture enough samples along a cycle of the AC magnet waveform. The effective sampling period corresponds to the time needed to get one set of samples, which is dependent on the internal sampling rate of the device. Since linear Hall-effect sensors provide information on the actual sensed magnetic flux densities, the sensors are better able to detect AC magnets than a low-sample rate Hall switch.