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To allow maintenance to sections of the overhead line without having to turn off the entire system, the overhead line system is broken into electrically separated portions known as sections. Sections often correspond with tension lengths as described above. The transition from section to section is known as a section break and is set up so that the locomotiveA locomotive is a vehicle that provides the motive power for a railway train. Traditionally, the locomotive or locomotives are positioned at the front of a train, pulling passenger carriages and/or freight vehicles. This requires the locomotive to be move's pantograph is in continual contact with the wire.
For bow-collectors and pantographs, this is done by having two contact wires run next to each other over a length about four wire supports: a new one dropping down and the old one rising up until the pantograph smoothly transfers from one to the next. The two wires never touch (although the pantograph is briefly in contact with both wires). In normal service the two sections are electrically connected, but this can be broken for servicing.
On overhead wires designed for trolleypoles this is done by having a neutral section between the wire, but this requries an insulator. The driver of the tramcar turn off the power when the trolleypole passes through to prevent arching from burning-out the insulator.
Sometimes on a larger electrified railway, tramway, or trollebusway, it is necessary to power different areas of track from different power grids, the synchronisation of the phases of which cannot be guaranteed. (Indeed, sometimes the sections are even powered with different voltages or frequencies!) There may be mechanisms for having the grids synchronised on a normal basis, but events may cause desynchronisation. This is no problem for DC systems, but for AC systems it would obviously be quite undesirable to connect two unsynchronised grids together, even momentarily. A normal section break is insufficient to guard against this since the pantograph briefly connects both sections.
Instead, a phase break is used. This consists of two section breaks back-to-back so that there is a short section of overhead line that belongs to neither grid. If the two grids are synchronised, this stretch of line is energised (by either supply) and trains run over it normally. If the two supplies are not synchronised, the short isolating section is disconnected from the supplies, leaving it electrically dead, ensuring that the two grids cannot be connected to each other.
The sudden loss of power over the phase break would jar the train if the locomotive was at full throttle, so special signals are set up to warn the crew. Normal instructions are to put the controller (throttle) into neutral and coast through an isolated phase break section.
On the Pennsylvania RailroadThe Pennsylvania Railroad ( AAR reporting mark: PRR was an American railroad existing 1846 1968, after which it merged into the Penn Central. Commonly referred to as the Pennsy the company was headquartered in Philadelphia, Pennsylvania. The company's sym, phase breaks were indicated to train crews by a metal sign hung in the overhead with the letters PB on it, created by holes drilled in the metal. When the phase break is "dead", a signal consisting of eight lit lights in a circular pattern indicates this to the crew.
The use of overhead cables conducting electricity, invented by Granville T. Woods in 1888, amongst several other improvements by Woods, led to the development of electrified railways, the first of which was operated at Coney Island from 1892.
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