The moving-iron panel meters are mainly used for measuring AC currents and voltages in the frequency range of 15 … 100 Hz. They indicate the rms value of the wave, even with high harmonics, with a minor ininfluence on the accuracy. Our instruments are normally calibrated for sinusoidal AC.  For current above 100A or high voltages, the instruments must be connected through measuring transformers.
Movin iron instruments can be connected in any order without observation of polarity (ki) on the current transformer.