Blondel’s Theorem
Metering theory for alternating current circuits was in its infancy in the early
1890’s. In 1893 an engineer by the name of Andre E. Blondel set forth the basic
rules for metering all alternating current circuits.
Simply stated:
Count the number of wires in any circuit (including the neutral). One less stator than the total number of wires in the circuit is required to correctly meter the energy flowing in the circuit.
For example:
A 2-wire circuit requires a single stator meter.
A 3-wire circuit requires a two-stator meter.
A 4-wire circuit requires a three-stator meter.
In practice, Blondel’s Theorem is not strictly adhered to in all metering applications. Meter manufacturers have found ways to design special meters that allow adequate accuracy without the required number of stators. One such meter is the common (form 2S) house meter. It is a single stator meter that is specifically designed to meter a 3-wire circuit. In addition, some metering circuits may be connected in special configurations, which may also provide adequate levels of accuracy without the required number of stators.
A quite appropriate question to ask is, "Was Blondel wrong"? The answer is, "No"! Any time a metering circuit does not strictly adhere to Blondel’s Theorem, it is subject to inaccuracies when the voltages present in the circuits are not balanced. Once again, to emphasize the previous sentence, when cheating on Blondel’s Theorem, it is the balance of the voltages that are important, not the balance of currents flowing in the circuits! Most voltages in modern alternating current circuits are adequately balanced to permit these deviations from the rules laid forth by Blondel.