Editing Overhead line (section)

=== Neutral section (phase break) ===
[[File:Neutral Gap Sign.svg|thumb|Neutral Section Indication Board used on railways in the [[UK]]. Six of these would be required at [[#Crossings|crossings]]]]
[[File:ED9M-pantograph passing neutral section of 25 kV catenary.webm|thumb|A [[Pantograph (transport)|pantograph]] of EMU passes neutral section of [[25 kV AC railway electrification|25 kV 50 Hz AC overhead line]] without lowering but with switching off a circuit breaker]]

Sometimes on a larger electrified railway, tramway or trolleybus system, it is necessary to power different areas of track from different power grids, without guaranteeing synchronisation of the phases. Long lines may be connected to the country's national grid at various points and different phases. (Sometimes the sections are powered with different voltages or frequencies.) The grids may be synchronised on a normal basis, but events may interrupt synchronisation. This is not a problem for [[Direct current|DC]] systems. [[Alternating current|AC]] systems have a particular safety implication in that the railway electrification system would act as a "Backdoor" connection between different parts, resulting in, amongst other things, a section of the grid de-energised for maintenance being re-energised from the railway substation creating danger.

For these reasons, Neutral sections are placed in the electrification between the sections fed from different points in a national grid, or different phases, or grids that are not synchronized. It is highly undesirable to connect unsynchronized grids. A simple section break is insufficient to guard against this as the pantograph briefly connects both sections.<ref>{{Cite journal |last1=Morais |first1=Vítor A. |last2=Martins |first2=António P. |date=2022-03-01 |title=Traction power substation balance and losses estimation in AC railways using a power transfer device through Monte Carlo analysis |journal=Railway Engineering Science |language=en |volume=30 |issue=1 |pages=71–95 |doi=10.1007/s40534-021-00261-y |s2cid=256402259 |issn=2662-4753|doi-access=free |bibcode=2022RailE..30...71M }}</ref>

In countries such as France, South Africa, Australia and the United Kingdom, a pair of permanent magnets beside the rails at either side of the neutral section operate a bogie-mounted [[transducer]] on the train which causes a large electrical circuit-breaker to open and close when the locomotive or the pantograph vehicle of a multiple unit passes over them.<ref>{{cite web|url=http://www.vortok.com/signalling/train-control-magnets/automatic-power-control-apc-magnet|title=Vortok Automatic Power Control Magnet|access-date=25 July 2018|archive-date=25 July 2018|archive-url=https://web.archive.org/web/20180725183756/http://www.vortok.com/signalling/train-control-magnets/automatic-power-control-apc-magnet|url-status=dead}}</ref> In the United Kingdom equipment similar to [[Automatic Warning System]] (AWS) is used, but with pairs of magnets placed {{em|outside}} the running rails (as opposed to the AWS magnets placed midway between the rails). Lineside signs on the approach to the neutral section warn the driver to shut off traction power and coast through the dead section.

A neutral section or phase break consists of two insulated breaks back-to-back with a short section of line that belongs to neither grid. Some systems increase the level of safety by the midpoint of the neutral section being earthed. The presence of the earthed section in the middle is to ensure that should the transducer controlled apparatus fail, and the driver also fail to shut off power, the energy in the arc struck by the pantograph as it passes to the neutral section is conducted to earth, operating substation circuit breakers, rather than the arc either bridging the insulators into a section made dead for maintenance, a section fed from a different phase, or setting up a Backdoor connection between different parts of the country's national grid.

[[File:Neutral zone of overhead catenary.jpg|thumb|25&nbsp;kV AC neutral zone in Romania]]

On the [[Pennsylvania Railroad]], phase breaks were indicated by a position light signal face with all eight radial positions with lenses and no center light. When the phase break was active (the catenary sections out of phase), all lights were lit. The position light signal aspect was originally devised by the Pennsylvania Railroad and was continued by [[Amtrak]] and adopted by [[Metro-North Railroad|Metro North]]. Metal signs were hung from the catenary supports with the letters "PB" created by a pattern of drilled holes.