3.2.1 Introduction
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 (fig. 3.2-1). Most operating voltages in medium-voltage systems are in the 3 kV AC to 40.5 kV AC range.
The electrical transmission and distribution systems not only connect power plants and electricity consumers, but also, with their “meshed systems,” form a supraregional backbone with reserves for reliable supply and for the compensation of load differences. High operating voltages (and therefore low cur