An electric grid is made up of a complex network of power plants, transformers, transmission and distribution lines, sensing systems, protective equipment, and control devices. The transmission lines are the links between high-voltage substations. The distribution lines are the links between high-and medium-voltage substations. They are also the links between medium-and low-voltage circuits. At power plants, the transformers are used to step up the voltage of the transmission lines to high levels (220-1200 kV) to reduce the current passing through the transmission lines, thus reducing the cross section of the transmission conductors and consequently reducing the size of the towers and increasing their spans. Near load centers, the voltage of the transmission lines is stepped down to medium levels (15-25 kV) for the distribution of power within city limits without the need for large towers. In residential areas, the voltage is further stepped down to a phase voltage of 100-240 V for household use worldwide.