Electric power distribution systems are designed to serve their customers with reliable and high-quality power. The most common distribution system consists of simple radial circuits (feeders) that can be overhead, underground, or a combination. From the distribution substation, feeders carry the power to the end customers, forming the medium-voltage or primary network, operated at a medium voltage level, typically 5–35 kV. Feeders range in length from a few kilometers to several tens of kilometers. As they must supply all customers in the designated distribution area, they often curve and branch along the assigned corridors.[1][2] A substation typically supplies 3–30 feeders.[citation needed]Distribution transformers or secondary transformers, placed along feeders, convert the voltage from the medium to a low voltage level, suitable for direct consumption by end customers (mains voltage).[3] Typically, a rural primary feeder supplies up to 50 distribution transformers, spread over a wide region,[4] but the figure significantly varies depending on configuration. They are sited on pole tops, cellars or designated small plots.[2] From these transformers, low-voltage or secondary network branches off to the customer connections at customer premises, equipped with electricity meters.[3]