the national power grid (e.g. , ), they convert it to 55-0-55 kV (or 66-0-66 kV) AC at . The 0 V point is connected to earth through an inductance so that each conductor of the single phase AC power line has a voltage of (or ) with respect to earth potential. This is similar to split-phase electric power systems and results in a balanced line transmission. The inductance through which the earthing is done is designed to limit earth currents in cases of faults on the line. At the transformer substations, the voltage is transformed from (or ) AC to AC and the energy is fed into the overhead line. Asynchronous converters The frequency of depends on the necessity to avoid synchronism in parts of the rotary machine, which consists principally of a three phase asynchronous motor and a single phase synchronous generator. Since synchronism sets in at a frequency of (according to the technical details) in the single phase system, the frequency of the centralized system was set to . Power plants providing , , are either dedicated to generating this specific single phase AC or have special generators for the purpose, such as the Neckarwestheim nuclear power plant or the Walchensee hydroelectric power station. Synchronous converters The power for the decentralized system is taken directly from the national power grid and directly transformed and converted into , by synchronous-synchronous-converters or static converters. Both systems need additional transformers. The converters consist of a three-phase synchronous motor and a single-phase synchronous generator. The decentralized system in the north-east of Germany was established by the Deutsche Reichsbahn in the 1980s, because there was no centralized system available in these areas. Facilities for 15 kV AC railway electrification in Germany, Austria and Switzerland Germany, Austria and Switzerland operate the largest interconnected 15 kV AC system with central generation, and central and local converter plants. Norway In Norway all electric railways use 15 kV 16 Hz AC (except the Thamshavnbanen museum railway which uses 6.6 kV 25 Hz AC). The Oslo T-bane and tramways use 750 V DC power. Sweden In Sweden most electric railways use 15 kV 16 Hz AC. Exceptions include: Saltsjöbanan and Roslagsbanan (1.5 kV DC), the Stockholm Metro (650 V and 750 V DC) and tramways (750 V DC). The Oresund Bridge linking Sweden and Denmark is electrified at 25 kV, Danish standard; the split is located on