Defining the transformer ratio Up/Us
• Preferably select as primary voltage the nominal service voltage of the network.
• Select as secondary voltage the nominal 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.