Classical Nonquantum AC Power Measurements With Uncertainties Approaching 1 ${\rm \mu}\hbox{W/VA}$

A recent upgrade of the primary standard for electric alternating power at the Physikalisch-Technische Bundesanstalt improved the measurements, indicating that standard uncertainties of the order of 1 ${\rm \mu}\hbox{W/VA}$ are attainable. Investigations done to ensure such low uncertainty figures are presented, and the evaluation of measurement uncertainties is discussed in detail.