Electricity meter sticks on circuit breakers

Berkeley Lab researchers have developed a peel-and-stick electricity meter which allows individual circuits in the home and industry to be accurately monitored.

It is an alternative to current transformers and live voltage connections that can be installed without mains voltage working expertise as the sensors are simply stuck to the front of each circuit breaker in a consumer unit. The only critical steps are aligning the sensor before sticking, and identifying the breaker type to the system so that calibration parameters can be loaded.

Each sensor is 19 x 12mm including a capacitive voltage pick-up and a Hall-effect current probe. They are daisy-chained with ribbon cable which supplies power (5V 16mA) and takes serial data back to a local wireless node – based on a Raspberry Pi in the experiment. This links with a remote laptop running system monitoring algorithms, written in Python.

Most circuit breakers have similar internal structures, which makes the technique broadly applicable. There are two types in the US, according to the Lab: bimetallic thermal and solenoid magnetic. The thermal type was chosen for testing because its lower magnetic field (10-20x) made it the toughest challenge for current sensing.

Inside, the voltage probe is the bottom layer of the internal PCB and the Hall sensor is an Allegro A1301. Both signals are conditioned by op-amp-based filters and fed into the 10bit ADC of a MSP430G2131 microcontroller for serialisation and transmission along the ribbon cable.

1,920sample/s are sent – a multiple of the 60Hz line frequency to simplify subsequent fast Fourier transforms, and fast enough for mains harmonics to be captured.

Time averaging is included to increase the signal-to-noise ratio of the Hall effect probe, and the laptop runs fairly complex power estimation algorithms that include software phase locked loops to track input waveforms. Calibration is required once per breaker type.

There is considerable cross-talk from adjacent circuit breakers, and an adaptive algorithm reduces errors from this to 5% after 15min of operation without any need for calibration via known loads.

Voltage measurements are within 1% of conventional contact probing, power below 10W can be resolved, and complex loads like triac dimmers can be handled.

More information on the algorithms is available in ‘COTS-Based Stick-On Electricity Meters for Building Submetering‘, published in the IEEE Sensors Journal.

steve bush

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