Polyphase AC meters. Practical implementation
Alternating current meters - Electromechanical meters

According to Blondel's theorem, two single-phase meters are needed for a three-phase, three-wire circuit, and three single-phase meters for a three-phase, four-wire circuit. But for reasons of space, these single-phase meters are usually grouped together in a single housing. Electromechanical three-phase meters have a single motor shaft and one or more discs on which two or three motor elements act.

For reasons of economy, however, it is possible in some cases to deviate from the strict application of Blondel's principle, but the result is a reduction in measurement accuracy. An example of such a deviation is given by a single-phase meter for a three-wire circuit connected as shown in figure