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Inside the locomotive, a transformer steps the voltage down for use by the traction motors and auxiliary loads. The majority of modern electrification systems take AC energy from a power grid that is delivered to a locomotive, and within the locomotive, transformed and rectified to a lower DC voltage in preparation for use by traction motors. 2⁄3 Hz to 16.7 Hz which is no longer exactly one-third of the grid frequency. 2⁄3 Hz (the 50 Hz mains frequency divided by three) single-phase AC. The standard-frequency AC system may introduce imbalance to the supply grid, requiring careful planning and design (as at each substation power is drawn from two out of three phases). Various railway electrification systems in the late nineteenth and twentieth centuries utilised three-phase, what is electric cable rather than single-phase electric power delivery due to ease of design of both power supply and locomotives. Two lines opened in 1925 under the Southern Railway serving Coulsdon North and Sutton railway station. Powerline workers work on electrically energized (live) and de-energized (dead) power lines. The NEC is not any jurisdiction's electrical code per se; rather, it is an influential work of standards that local legislators (e.g., city council members, state legislators, etc. as appropriate) tend to use as a guide when enacting local electrical codes.

Soon after the shelling of Fort Sumter, the South cut telegraph lines running into D.C., which put the city in a state of panic because they feared an immediate Southern invasion. A few lines of the Paris Métro in France operate on a four-rail power system. United States to have either a leakage current detector interrupter (LCDI) or a ground-fault circuit interrupter (GFCI) protective device built into its power cord. The transmission system usually does not have a large buffering capability to match loads with generation. Network effects are a large factor with electrification. Large fossil fuel power stations operate at high efficiency, and can be used for district heating or to produce district cooling, leading to a higher total efficiency. The DC system, apart from being limited as to the maximum power that can be transmitted, also can be responsible for electrochemical corrosion due to stray DC currents. 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.

In the US, the New York, New Haven, and Hartford Railroad, the Pennsylvania Railroad and the Philadelphia and Reading Railway adopted 11 kV 25 Hz single-phase AC. The higher power of electric locomotives and an electrification can also be a cheaper alternative to a new and less steep railway if train weights are to be increased on a system. 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. 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. Also, the energy used to blow air to cool transformers, power electronics (including rectifiers), and other conversion hardware must be accounted for. This is especially useful in mountainous areas where heavily loaded trains must descend long grades. The rating must be marked on the plug, and in the case of non-rewirable plugs the marking must be the value of the fuse fitted by the plug manufacturer in accordance with table 2 of the standard.

Designs exist for 15-ampere (L11-15), 20-ampere (L11-20), and 30-ampere (L11-30) devices, and L11-20 and L11-30 devices were commercially available from at least one manufacturer (Bryant Electric). All NEMA 6 devices are three-wire grounding devices (hot-hot-ground) used for 208 and 240 V circuits and rated for 250 V maximum, with the 6-15, 6-20 and 6-30 being grounding versions of the 2-15, 2-20 and 2-30, respectively. These are deprecated due to the lack of grounding but L10-20 and L10-30 devices are specified by NEMA and are commercially available. NEMA 6 devices, while specified as 250 V, may be used for either 208 or 240 V circuits, generally depending on whether the building has a three-phase or split-phase power supply, respectively. However, electric rolling stock may run cooling blowers when stopped or coasting, thus consuming energy. The electric train can save energy (as compared to diesel) by regenerative braking and by not needing to consume energy by idling as diesel locomotives do when stopped or coasting. Regenerative braking returns power to the electrification system so that it may be used elsewhere, by other trains on the same system or returned to the general power grid.