According to international rules, there are only two voltage levels:
Low voltage: up to and including 1 kV AC (or 1,500 V DC)
High voltage: above 1 kV AC (or 1,500 V DC)
Most electrical appliances used in household, commercial and industrial applications work with low voltage. High voltage is used not only to transmit electrical energy over very large distances, but also for regional distribution to the load centers via fine branches.
However, because different high voltage levels are used for transmission and regional distribution, and because the tasks and requirements of the switchgear and substations are also very different, the term “medium voltage” has come to be used for the voltages required for regional power distribution that are part of the high voltage range from 1 kV AC up to and including 52 kV AC.
Most operating voltages in medium voltage systems are in the 3 kV AC to 40.5 kV AC range.
Operating voltages between 3 kV and 15 kV are frequently found in industrial supply systems.
In power supply and distribution systems, medium voltage equipment is available in following forms:
Power stations, for generators and station supply systems.
Transformer substations of the primary distribution level (public supply systems or systems of large industrial companies), in which power supplied from the high voltage system is transformed to medium voltage
Secondary distribution level
Local supply, transformer or customer transfer substations for large consumers (secondary distribution level), in which the power is transformed from medium to low voltage and distributed to the consumer.
Reference: Siemens Energy Sector – Power Engineering Guide Edition 7.0