Three-phase voltmeters are used for measuring the voltages between phases, or phase-phase and phase-neutral in a line. They incorporate a switch in order to select the wires between which the measurement is desired. The EQ..n SWT3 also has a position of disconnection in the switch (OFF). Their frequency range is 25 to 100 Hz. They indicate the rms value of the voltage, even with high harmonics, with a minor influence on the accuracy.