The American National Standard that sets the physical aspects and acceptable performance criteria for 0.1, 0.2, and 0.5 accuracy class electricity meters meeting Blondel’s Theorem, ANSI C12.20-2015 – Electricity Meters – 0.1, 0.2, and 0.5 Accuracy Classes, has been revised.
Blondel’s Theorem, which derives its name from its discoverer, Andre E. Blondel, actually traces its origins back to 1893, when the engineer and physicist established the basic rules for metering alternating current circuits. Simply put, Blondel’s Theorem states that one less stator than the total number of wires in the circuit is required to correctly meter the energy flowing in the circuit. Under this rule, a two-wire circuit requires a single stator meter, a three-wire circuit requires a two-stator meter, and so forth.
The 0.1, 0.2, and 0.5 accuracy class electricity meters established within ANSI C12.20-2015 are accurate to within +/-0.1%, +/-0.2%, and +/-0.5% of true value at a full load, respectively. Beyond the designations of these three meter types, the standard covers voltage and frequency ratings, test current values, service connection arrangements, pertinent dimensions, form and display designations, environmental tests, and acceptable performance of the meters and associated equipment.
It should be noted that Blondel’s Theorem is not strictly adhered to in all metering practices. For reference, non-Blondel metering installations are listed in Table 2A of ANSI C12.20-2015, and these explicitly are not covered by the standard.
In fact, the clarification that non-Blondel applications are not covered by the document is one of the significant changes made to the new revision. Including this alteration, the updates made to the standard were made to keep it modern with an industry that is facing dramatic changes driven by advancements in technology and regulatory matters. Additional major changes to ANSI C12.20-2015 include testing under harmonic conditions, the addition of a 0.1% accuracy class, and the addition of specifications for the optical test output port.
ANSI C12.20-2015 – Electricity Meters – 0.1, 0.2, and 0.5 Accuracy Classes is available on the ANSI Webstore.