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Inside the locomotive, a transformer steps the voltage down for use by the traction motors and auxiliary loads. Separate low-voltage transformer windings supply lighting and the motors driving auxiliary machinery. The low-frequency AC system may be powered by separate generation and distribution network or a network of converter substations, adding the expense, also low-frequency transformers, used both at the substations and on the rolling stock, are particularly bulky and heavy. However, the higher voltages used in many AC electrification systems reduce transmission losses over longer distances, allowing for fewer substations or more powerful locomotives to be used. Electric locomotives may easily be constructed with greater power output than most diesel locomotives. For passenger operation it is possible to provide enough power with diesel engines (see e.g. 'ICE TD') but, at higher speeds, this proves costly and impractical. Power-only rails can be mounted on strongly insulating ceramic chairs to minimise current leak, but this is not possible for running rails, which have to be seated on stronger metal chairs to carry the weight of trains.

Modern electronics makes this possible with relative ease and cross voltage travel is now possible without changing locomotives. These drives can run equally well on DC or AC of any frequency, and many modern electric locomotives are designed to handle different supply voltages and frequencies to simplify cross-border operation. These systems could either use standard network frequency and three power cables, or reduced frequency, which allowed for return-phase line to be third rail, rather than an additional overhead wire. These motors may either be DC motors which directly use the DC or they may be three-phase AC motors which require further conversion of the DC to variable frequency three-phase AC (using power electronics). Also, the energy used to blow air to cool transformers, power electronics (including rectifiers), and other conversion hardware must be accounted for. Energy efficiency and infrastructure costs determine which of these is used on a network, although this is often fixed due to pre-existing electrification systems. In the UK, the London, Brighton and South Coast Railway pioneered overhead electrification of its suburban lines in London, London Bridge to Victoria being opened to traffic on 1 December 1909. Victoria to Crystal Palace via Balham and West Norwood opened in May 1911. Peckham Rye to West Norwood opened in June 1912. Further extensions were not made owing to the First World War.

As alternating current is used with high voltages, this method of electrification is only used on overhead lines, never on third rails for safety reasons. Insulation: This part keeps the conductors separated thereby preventing short-circuiting or an unwanted current flow path. However, elastomeric rubber pads placed between the rails and chairs can now solve part of the problem by insulating the running rails from the current return should there be a leakage through the running rails. While part of the SkyTrain network, the Canada Line does not use this system and instead uses more traditional motors attached to the wheels and third-rail electrification. The Expo and Millennium Line of the Vancouver SkyTrain use side-contact fourth-rail systems for their 650 V DC supply. On "French system" HSLs, the overhead line and a "sleeper" feeder line each carry 25 kV in relation to the rails, but in opposite phase so they are at 50 kV from each other; autotransformers equalize the tension at regular intervals. Power conversion for a DC system takes place mainly in a railway substation where large, heavy, and more efficient hardware can be used as compared to an AC system where conversion takes place aboard the locomotive where space is limited and losses are significantly higher.

Electricity for electric rail systems can also come from renewable energy, nuclear power, or other low-carbon sources, which do not emit pollution or emissions. Electricity powers all kinds of things in your home, from everyday appliances like refrigerators and washing machines to your computer, smartphone, and other devices. Like SER, The cable is mostly used as a panel feeder in multi-family residential buildings and branch circuits. NEMA L14 are three-pole and ground connectors rated for 125/250 V. Intended for three-pole, four-wire hot-hot-neutral-ground circuits with a nominal supply voltages of 240 or 208 V hot-to-hot and 120 V hot-to-neutral. Different size wires may be used in the same raceway so long as they are all insulated for the maximum voltage of any of these circuits. The neutral wire is identified by gray or white insulated wire, perhaps using stripes or markings. Although the supply has an artificially created earth point, this connection is derived by using resistors which ensures that stray earth currents are kept to manageable levels.

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