Frequencymeter : Frequencymeter is the device, which measures the frequency. Frequencymeter shows the number of the cycles in 1 second and its unit is cycle/second or Hertz. Frequencymeters are connected parallel to circuit which frequency will be measured like voltmeters. They are produced for being connected between phases and notr-phases. Measurement Device Classes : Percentage expression of the failure ratio of the measurement devices at the maximum voltage. 0,1- 0,2 Class: devices, which will be used at producing of measurement devices. 0,5-1 Classes: devices, which are generally, used movable. 1,5-2,5 Classes: cable type devices, which are used at industrial measurements.