***Update: This standard has been revised by ANSI C12.1-2022.
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 was ANSI C12.20-2022 – Electricity Meters – 0.1, 0.2, and 0.5 Accuracy Classes.
What Is the Current Edition of ANSI C12.20?
While ANSI C12.20 was previously its own standard, its content has been revised by and merged into ANSI C12.1-2022, so there is now a singular document that covers the entire code for electricity metering. Blondel and non-Blondel meters are both covered by ANSI C12.1-2022: Electric Meters – Code For Electricity Metering, and ANSI C12.20 is now a historical standard. You can learn more here: ANSI C12.1-2022: Code for Electricity Metering
Anyone interested in the past edition of ANSI C12.20 and the new ANSI C12.1 can get them together as the ANSI C12.1 / ANSI C12.20 – Electric Meters Set.
What Is Blondel’s Theorem?
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.
Which Electric Meters Are Covered by ANSI C12.21?
The 0.1, 0.2, and 0.5 accuracy class electricity meters established within ANSI C12.21-2022 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 C12.21-2022, and these explicitly are not covered by the standard.
Changes to ANSI C12.20-2015
In fact, the clarification that non-Blondel applications are not covered by the document is one of the significant changes made to the 2015 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 C12.21-2022 – Electricity Meters – Code For Electricity Metering (Incorporates ANSI C12.20-2015) is available on the ANSI Webstore.