The EN62056 standard describes local data exchange and describes how to use COSEM over a local port (optical or current Single Phase Power Meter Basic Requirements). There are several features required for today’s PM design. All requirements must conform to regulation EN50470-3, including:.
a) Input voltage: PM should work with U.S. and European main voltage of 230 (120)V, 50 (60)Hz.
b) Overall power consumption: This must not exceed 2W. This limits maximal resistance of the current sensing element and consumption of the electronic circuit. The power consumption of a PM (electronic circuit) itself is less than 100mW, and the majority of the power is consumed on shunt.
Shunt resistance is then R = P/I 2 = aprox.312 micro-ohms. A standard 300 microOhm shunt resistor is used in the design.
Therefore, PM consumption is restricted to 10VA as well, which restricts the amount of current drawn from the capacitive power supply. Using a capacitive divider power supply gives approximately 8mA power budget, if a 470nF capacitor is used.
c) Accuracy: PM accuracy is set to 5(80)A at one percent (B class of EN50470-3).
For today’s single phase PM, it is sufficient to meet 5(80)A specification, meaning the dynamic range of the measured current is 1:320.
There are some current thresholds, such as Istart, Imin, Itr that border on the accuracy needed by EN 50470. Maximal current Imax measured with full accuracy (one percent) is set to 80A. PM must withstand short over current Imax * 30 = 2400A.Reference current Iref is set to 5A. Iref is referenced to Imin, Itr and Istart by EN50470-3.
Imin = Iref / 20 = 250mA. Imin is minimal current where the PM must measure with determined accuracy (+-1.5% for B class). Out of those values, the dynamic range of the current measurement is Imax / Imin = .25 / 80 = 1:320.
Istart = Iref / 250 = 25mA. The PM must start to register energy when the current is bigger than Istart – accuracy is not measured.
Irt =Iref/10 = 500mA (Itr is threshold for full accuracy – one percent).
d) Standard metering functionality: Voltage RMS, Current RMS, Active Power, Active Energy, Real Time Clock
e) Optoisolated and optical (LED) energy pulse output must be present to enable power meter calibration and accuracy verification.
f) LCD interface and button need to show all measured values. The button should switch among displayed values.
g) Tamper switch to detect lid removal: mains connectors should be placed under the lid. The PM monitors any manipulations with the mains connection, even if there is no voltage on the mains.
h) Infrared interface and RS232 opto-isolated serial interface are used for metering data reading.
Blocking out the design
As mentioned above, the design in Figure 1 below shows static single phase PM with direct measurement - shunt. The measurement segment is made up of the several blocks.
Figure 1. A reference design based on the 8-bit Freescale MC9S08LH64 MCU, suitable for metering applications as a single phase power meter.