Testing Methods are set by national and international standards. The relevant standards subdivide the instrument transformers into classes, for which the amplitude and phase errors are fixed.
The amplitude errors are defined by the relative deviation of the measured amplitudes of the current- or voltage vectors from the nominal values. The phase error is the phase angle of the output minus the phase angle of the input, for a sinusoidal input and output at a fixed frequency. As well known, phase angles will contribute to the measured values of power and thus also the energy. To verify both types of errors, reference standards are necessary, the measuring uncertainty of which is much smaller than those of the devices to be tested