In electricity , the rated voltage of an electrical device is the voltage that it must not exceed in normal operation.The nominal voltage is expressed in volts by the ratio of two values Uo / U; Uo is the effective value of the voltage between an external conductor. U is the effective value between the two external conductors of a multi-conductor cable.
The main purpose of a nominal voltage is to ensure that the user of the wiring must not encounter danger for the user or damage the device.In the standard set by the voltage, the chances of overheating and electric fires decrease considerably.The nominal voltage also serves to classify the various electrical systems that can be considered very low voltage.
How to calculate the nominal voltage
The nominal voltage is calculated differently for single-phase and three-phase systems.On the contrary in three-phase systems, common in industrial plants, they have 2 nominal voltages which must both be specified. The ” line voltage ” is calculated between 2 phases, while the ” phase voltage ” is calculated between phase and neutral.
The choice of nominal voltages
The nominal voltages are divided into four bands, each characterized by a technical aspect that prevails over all the others. Within each band, for technical and economic reasons, there was convergence on some values
- In electrical uses the main aspect is the safety of people. Depending on the power and the environment of use, the values are 12 V, 24 V, 48 V, 110 V, 230 V, 400 V.
- In electrical machines for power plants or large industries the dominant element is related to problems of insulation in quarry and reliability. The adoptable voltages are 3, 6, 10 15 and 20 kV
- In the large transmission lines the limits are of technological-environmental type and the adopted values are 220 kV (240), 380 kV (400) kV. For very long distances 750 kV, 1000 kV and 1500 kV
- For power distribution lines, voltage levels arise from technical-economic optimizations linked to the powers involved. Three bands are distinguished:
- sub transmission: 60 kV, 130 kV
- medium voltage distribution: 10 kV, 15 kV, 20 kV, 33 kV
- low voltage distribution: 230/400 V.
Wiring that is used in the electrical system of a home for the elderly must meet a specific voltage rating.