Meter anti-tampering: Stopping those pesky meter tamperers

With electricity providers striving to reduce energy loss, energy theft prevention is a major concern.  Specifically, utility companies seek to prevent energy meter tampering that may lead to obtaining electricity that is not properly billed by the meter to the customer. To prevent energy theft due to the unauthorized modification of energy meters, many markets require that meters include anti-tamper mechanisms to not only detect different meter tampering methods but also to prevent or mitigate their effects.

There are multiple techniques for tamper detection and prevention.  First, for tamper prevention the first line of defense should always be the meter case. The meter case should be sealed so that easy access to the meter is not allowed and there are no spaces that may allow entering objects to bypass the sensed current. As an additional precaution, a button can be added within the case so that a trigger is generated whenever a case is opened.  For some processors such as the ultra-low power MSP430F67791A microcontroller (MCU), there are specific capture pins on the chip that can be used to log the time of the trigger even when the main power to the DVCC is not available.  An example of this functionality can be found in the TIDM-AUX-MODULE design.

To deal with meter tamperer specifically bypassing current on the live channels, one anti-tamper technique is to measure the current on the neutral channel. For three-phase meters the neutral current should ideally be zero if loads are balanced for all phases. If there is a large current on the neutral channel this can indicate tampering. For single-phase meters, under ideal conditions, the live current should be equal to the neutral current. If there is a significant mismatch between the measured live and neutral currents, this also may indicate that a tamper event has occurred. In the drawing below, the method of measuring both neutral and live currents is shown.

Figure 1: Single phase meter diagram

In the drawing, the red arrow connected to the first terminal on the left corresponds to Live_In, the black arrow connected to the second terminal on the left corresponds to Neutral_In, the red arrow connected to the third terminal on the left corresponds to Live_Out, the black arrow connected to the fourth terminal corresponds to Neutral_Out, and the set of green arrows represent someone trying to bypass current at the terminals pointed to by the green arrows. In this configuration, the energy consumption is calculated using the voltage between Live_In and Neutral _In and the current sensed across a shunt connected between Live_In and Live_Out.  A similar calculation is also done using the same voltage between Live_In and Netural_In as well as the current produced at the secondary of a current transformer that has its primary connected between Neutral_In and Neutral_Out. By measuring both line and neutral currents, this can help detect someone trying to tamper a meter by bypassing current. Additionally, by calculating average active power as a signed number, tampering with a meter to count in reverse can be detected by checking if the signed average active power is negative (assuming net metering is not supported).

There are multiple options to combat magnetic tampering. First, any items that are vulnerable to magnetic tampering should be orientated such that it is difficult to place a magnet near these susceptible items. Susceptible components should also be magnetically screened to help lessen the effects of this type of tampering. If it is difficult or not cost-effective to provide magnetic screening, another option is to use a hall-effect sensor such as the DRV5033 to detect the magnetic field and take special action, as is done in the "Magnetic Tamper Detection Using Low-power Hall Effect Sensors TIDA-00839 reference design."

Because transformer-based power supplies are susceptible to magnetic tampering, RC power supplies such as the PMP6960 design are often used instead of transformer-based power supplies. Alternatively, transformer-based power supplies can use more magnetically immune air-core transformers.

Due to the effects of magnetic tampering on current transformers (CTs), shunts resistors may be used as current sensors instead of current transformers. For single-phase meters that measure both neutral and live current for anti-tamper purposes, many times one current channel would be measured with a CT and the other channel would be measured with a shunt to help deal with magnetic tampering of the CT. For poly-phase meters, shunts can also be used as current sensors, as is done in the "Multi-phase Energy Measurement with Isolated Shunt Sensors TIDA-00601 reference design." In order to allow shunts to be used in poly-phase systems, an isolated sigma delta modulator such as the AMC1304M05 is used to compensate for shunts not having inherent isolation.

Lastly, another anti-tamper technique consideration is to make a meter robust against electrostatic discharge (ESD) and electromagnetic immunity (EMI) attacks. There are many design considerations needed to harden a meter against these attacks. The three-phase metrology TIDM-3PHMTR-TAMP-ESD, TIDM-3PH-ENERGY5-ESD, and single-phase metrology TIDM-1PHMTR-ESD designs provide more details about these design considerations.

With some of the common anti-tamper methods discussed, you now have knowledge on how to defend against common meter tampering techniques.  In order to make your job easier, here are some designs and devices from TI that simplify the tamper detection and prevention process:

  • TI Design reference designs:
    • Magnetic Tamper detection using low-power hall effect sensors reference design (TIDA-00839)
    • Multi-phase energy measurement with isolated shunt sensors reference design (TIDA-00601)
    • Three-phase metrology with enhanced ESD protection and tamper detection reference design (TIDM-3PHMTR-TAMP-ESD)
    • Class 0.5 three-phase energy measurement system with enhanced ESD protection reference design (TIDM-3PH-ENERGY5-ESD)
    • One-phase metrology with enhanced ESD protection reference design (TIDM-1PHMTR-ESD)
    • Battery management and auxiliary power supply options for e-meters (TIDM-AUX-MODULE)
    • Relevant TI products:
      • DRV5033 2.5 to 38 V digital omnipolar-switch hall effect sensor
      • Reinforced isolated modulator with LDO regulator, ±50mV Input, and CMOS interface AMC1304M05
      • Ultra-low power MSP430F67791A mixed signal microcontroller
  • Visit TI's Power Quality Analyzers solution page and a Power Quality Meter system diagram that uses TI MCUs, low noise amplifiers, precision ADCs, and low-power industrial interfaces to measure and communicate power quality data to a host system.