Select a voltage transformer

Defining the transformer ratio Up/Us

  • Preferably select as primary voltage the nominal service voltage of the network
  • Select as secondary voltage the nomial input voltage of the equipment to be fed

Defining the accuracy class against the application

Standard reference = Class 0,1 or 0,2
Tariff metering = Class 0,5 or 0,2
Divisional industrial metering = Class 1
Measure display = Class 1 or 3
Protection = Class 3P or 6P

Defining the voltage factor

  • The voltage factor is determined by the maximum operating voltage, which depends on the neutral system and the voltage transformer primary winding earthing conditions.
  • When the voltage transformer is connected between two phases, the voltage factor is 1.2Un continuous.
  • When the voltage transformer is connected between phase and earth, the voltage factor depends on the neutral system :
    – 1.9 Un for 30 s, in a non-effectively earthed neutral system with automatic earth-fault tripping.
    – 1.9 Un for 8 h, in an isolated neutral system, or in a resonant earthed system, without earth-fault tripping.