The network can be thought as compo

The network can be thought as composed of n – 1 single-phase independent systems, with the common return on any one of the wires (e.g., the sth wire).
Then, the absorbed power can be measured as the sum of the readings of n – 1 wattmeters, each one inserted with the current circuit on a different wire and the voltmeter circuit between such a wire and the sth one (Figure 3.15):