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Major engineering and economic challenges face any solar power satellite project. The adaptor was screwed into the lightbulb socket, leaving a flat face with two holes to conveniently attach the plug. U.S. patent 1,180,648 fig. 4 and 5, showing a concept for polarized receptacle and plug. 1,180,648. The flat blade plug which in subsequent years evolved to become NEMA 1-15 was born. This number is followed by the letter 'R' to indicate a receptacle or 'P' to indicate a plug. Overall, China takes first place, with around 100,000 km (62,000 mi) of electrified railway, followed by India with over 60,000 km (37,000 mi) of electrified railway, and continuing with Russia, what is electric cable with over 54,000 km (34,000 mi) of electrified railway. This was despite almost nine in ten respondents admitting they're aware that the domestic extension leads should not be used outside - and over half saying they use them even when it's raining. The European Union contains the longest amount of electrified railways (in length), with over 114,000 km (71,000 mi) of electrified railway, however only making up around 55% of the total railway length. P. M. Kalla-Bishop, Future Railways and Guided Transport, IPC Transport Press Ltd. Princeton: Princeton University Press.

Twist-locking types are used for heavy industrial and commercial equipment, where increased protection against accidental disconnection is required. Most are equipped with a retaining screw or clip to prevent accidental disconnection. These cables are stranded to lower skin effects. Telephone and other communications cables are supported in the same manner. High-voltage direct current (HVDC) systems require relatively costly conversion equipment that may be economically justified for particular projects such as submarine cables and longer distance high capacity point-to-point transmission. Birds may perch on parts with different charges, and animals may also touch the electrification system. Future DC railway electrification system Go for 9kV (PDF). In most of the world's railway networks, the height clearance of overhead electrical lines is not sufficient for a double-stack container car or other unusually tall loads. In hot and cold climates home air conditioning and heating loads affect the overall load. These special horns have a small air pressure tube attached which, if the pressure is lost, will cause the pan to lower automatically and so reduce the possible wire damage. Small specialized padlocks are available to fit these holes, allowing "lockout" of hazardous equipment, by physically preventing insertion of locked plugs into a power receptacle.

Not so for electric trains, which can never run on non-electrified lines, and which even on electrified lines can run only on the single, or the few, electrical system(s) for which they are equipped. The 20th century's rapid industrialization made electrical transmission lines and grids critical infrastructure. Transmission lines can be used to carry data: this is called power-line carrier, or power-line communication (PLC). High-voltage fuses of the expulsion type have fiber or glass-reinforced plastic tubes and an open end, and can have the fuse element replaced. Medium-voltage fuses may have liquid-filled envelopes to assist in the extinguishing of the arc. However, due to ventilation issues, diesel trains may have to be banned from certain tunnels and underground train stations mitigating the advantage of diesel trains somewhat. Since at least 1949, numerous receptacle devices have also been invented to use these holes to hold the prongs inside the receptacle slots, using a corresponding latch or locking mechanism.

The metal conductive blades are often informally called "prongs" (as in "3-prong plug"). This design was improved once again in 1915, introducing a polarized plug under the U.S. Later in 1904, he changed the design to flat blades (a design later incorporated in the NEMA 2 series), filled under the U.S. In 1910, Hubbel worked on improving his popular flat blade design, filled U.S. NEMA non-locking connectors all use blades of various flat and folded shapes (except for the round pins used on grounding connectors). Since NEMA was a forum between manufacturers and not an governmental body, any decision had to be thoroughly discussed and agreed upon by its members, which substantially slowed the standardization process. In 1919, Hubbell unsuccessfully tried to prevent other manufacturers from making receptacles and plugs to the dimensions used by Hubbell. NEMA connectors are power plugs and sockets used for AC mains electricity in North America and other countries that use the standards set by the US National Electrical Manufacturers Association.