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A railway electrification system supplies electrical energy to railway locomotives and multiple units so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world. Railway electrification has many advantages but requires significant capital expenditure for installation.
ill_p3 Electric locomotives under the wires in Sweden
ill_p4 Overhead wire and catenary in Bridgeport, Connecticut, USA
Characteristics of electric traction
The main advantage of electric traction is a higher power-to-weight ratio than forms of traction such as diesel or steam that generate power on board. Electricity enables faster acceleration and higher tractive effort on steep gradients. On locomotives equipped with regenerative brakes, descending gradients require very little use of air brakes as the locomotive's traction motors become generators sending current back into the supply system and/or on-board resistors, which convert the excess energy to heat.
Other advantages include the lack of exhaust fumes at point of use, less noise and lower maintenance requirements of the traction units. Given sufficient traffic density, electric trains produce fewer carbon emissions than diesel trains, especially in countries where electricity comes primarily from non-fossil sources.
A fully electrified railway has no need to switch between methods of traction thereby making operations more efficient. Two countries that approach this ideal are Switzerland and Hong Kong, but both use more than one system, so unless multi-system locomotives or other rolling stock is used, a switch of traction method may still be required.
The main disadvantages are the capital cost of the electrification equipment, most significantly for long distance lines which do not generate heavy traffic. Suburban railways with closely-spaced stations and high traffic density are the most likely to be electrified and main lines carrying heavy and frequent traffic are also electrified in many countries. Also, if the overhead wiring breaks down in some way, all trains can be brought to a standstill.
Electrification systems in Europe:
1) High speed lines in France, Spain, Italy, United Kingdom, the Netherlands, Belgium and Turkey operate under 25 kV.
Electrification systems are classified by three main parameters:
(3) Contact System:
Six of the most commonly used voltages have been selected for European and international standardization. These are independent of the contact system used, so that, for example, 750V DC may be used with either third rail or overhead lines (the latter normally by trams).
There are many other voltage systems used for railway electrification systems around the world, and the list of current systems for electric rail traction covers both standard voltage and non-standard voltage systems.
The permissible range of voltages allowed for the standardized voltages is as stated in standards BS EN 50163 and IEC 60850. These take into account the number of trains drawing current and their distance from the substation.
[ Electrification system:
750 V DC
1,500 V DC
1 3 kV DC
2 15 kV AC, 16.7 Hz 25 kV AC, 50 Hz ]
[ Lowest non-permanent voltage:
17.5 kV ]
[ Lowest permanent voltage:
19 kV ]
[ Nominal voltage:
25 kV ]
[ Highest permanent voltage:
27.5 kV ]
[ Highest non-permanent voltage
29 kV ]
Early electric systems used low-voltage DC. Electric motors were fed directly from the traction supply and were controlled using a combination of resistors and relays that connected the motors in parallel or series.
The most common DC voltages are 600 V and 750 V for trams and metros and 1,500 V, 650/750 V third rail for the former Southern Region of the UK and 3 kV overhead. The lower voltages are often used with third or fourth rail systems, whereas voltages above 1 kV are normally limited to overhead wiring for safety reasons. Suburban trains (S Bahn) lines in Hamburg, Germany, operate using a third rail with 1,200 V, the French SNCF Culoz-Modane line in the Alps used 1,500 V and a third rail until 1976, when a catenary was installed and the third rail was removed. In the UK, south of London, 750 V third rail is used while, for inner London, 650 V is used to allow inter-running with London Underground which uses a 650 V fourth rail system but with the 4th (center) rail connected to the running rails in inter-running areas.
During the mid-20th century, rotary converters or mercury arc rectifiers were used to convert utility (mains) AC power to the required DC voltage at feeder stations. Today, this is usually done by semiconductor rectifiers after stepping down the voltage from the utility supply.
The DC system is quite simple but it requires thick cables and short distances between feeder stations because of the high currents required. There are also significant resistive losses. In the United Kingdom, the maximum current that can be drawn by a train is 6,800 A at 750 V. The feeder stations require constant monitoring and, on many systems, only one train or locomotive is allowed per section. The distance between two feeder stations at 750 V on third-rail systems is about 2.5 km (1.6 mi). The distance between two feeder stations at 3 kV is about 25 km (16 mi). If auxiliary machinery, such as fans and compressors, is powered by motors fed directly from the traction supply, they may be larger because of the extra insulation required for the relatively high operating voltage. Alternatively, they can be powered from a motor generator set, which offers an alternative way of powering incandescent lights which otherwise would have to be connected as series strings (bulbs designed to operate at traction voltages being particularly inefficient). Now solid-state converters (SIVs) and fluorescent lights can be used.
ill_p8 The Tyne and Wear Metro is the only UK system that uses 1.5kVDC.
ill_p9 Nottingham Express Transit in United Kingdom uses a 750 V DC overhead, in common with most modern tram systems.
1,500 V DC is used in the Netherlands, Japan, Hong Kong (parts), Ireland, Australia (parts), India (around the Mumbai area alone, to be converted to 25 kV AC like the rest of the country), France, New Zealand (Wellington) and the United States (Chicago area on the Metra Electric district and the South Shore Line interurban line). In Slovakia, there are two narrow-gauge lines in the High Tatras (one a cog railway). In Portugal, it is used in the Cascais Line and, in Denmark, on the suburban S-train system.
In the United Kingdom, 1,500 VDC was used in 1954 for the Woodhead trans-Pennine route (now closed); the system used regenerative braking, allowing for transfer of energy between climbing and descending trains on the steep approaches to the tunnel. The system was also used for suburban electrification in East London and Manchester, now converted to 25 kV AC. 3 kV DC is used in Belgium, Italy, Spain, Poland, the northern Czech Republic, Slovakia, Slovenia, western Croatia, South Africa and former Soviet Union countries (also using 25 kV 50 Hz AC). It was also formerly used by the Milwaukee Road's extensive electrification across the Continental Divide and by the Delaware, Lackawanna & Western Railroad (now New Jersey Transit, converted to 25 kV AC) in the United States. 600 VDC is used by Milan's network of tramways and trolleybuses.
ill_p10 A bottom-contact third rail on the Amsterdam Metro, the Netherlands
Most electrification systems use overhead wires, but third rail is an option up to about 1,200 V. While use of a third rail does not require the use of DC, in practice, all third-rail systems use DC because it can carry 41% more power than an AC system operating at the same peak voltage. Third rail is more compact than overhead wires and can be used in smaller-diameter tunnels, an important factor for subway systems.
Third rail systems can be designed to use top contact, side contact or bottom contact. Top contact is less safe, as the live rail is exposed to people treading on the rail unless an insulating hood is provided. Side- and bottom-contact third rail can easily have safety shields incorporated, carried by the rail itself. Uncovered top-contact third rails are vulnerable to disruption caused by ice, snow and fallen leaves.
DC systems (especially third rail systems) are limited to relatively low voltages and this can limit the size and speed of trains and cannot use low-level platform and also limit the amount of air-conditioning that the trains can provide. This may be a factor favoring overhead wires and high voltage AC, even for urban usage. In practice, the top speed of trains on third-rail systems is limited to 100 mph (160 km/h) because, above that speed, reliable contact between the shoe and the rail cannot be maintained.
Some road operating trams (streetcars) used conduit third-rail current collection. The third rail was below street level. The tram picked up the current through a plough ( U.S. "plow") accessed through a narrow slot in the road. In the United States, much (though not all) of the former streetcar system in Washington, D.C. (discontinued in 1962) was operated in this manner to avoid the unsightly wires and poles associated with electric traction. The same was true with Manhattan's former streetcar system. The evidence of this mode of running can still be seen on the track down the slope on the northern access to the abandoned Kingsway Tramway Subway (in central London, United Kingdom), where the slot between the running rails is clearly visible, and on P and Q Streets west of Wisconsin Avenue in the Georgetown neighborhood of Washington DC, where the abandoned tracks have not been paved over. The slot can easily be confused with the similar looking slot for cable trams/cars (indeed, in some cases, the conduit slot was originally a cable slot). The disadvantage of conduit collection included much higher initial installation costs, higher maintenance costs, and problems with leaves and snow getting in the slot. For this reason, in Washington, D.C. cars on some lines converted to overhead cable on leaving the city center, a worker in a "plow pit" disconnecting the plow while another raised the trolley pole (hitherto hooked down to the roof) to the now-present overhead wire. In New York City for the same reasons of cost and operating efficiency outside of Manhattan overhead wire was used. Finally, a new approach to avoiding overhead wires is that taken by the "second generation" tram/streetcar system in Bordeaux, France (entry into service of the first line in December 2003; original system discontinued in 1958)with its APS (alimentation par sol -- ground current feed). This involves a third rail which is not in a slot but runs flush with the surface like the tops of the running rails. The circuit is divided into segments with each segment energized in turn by sensors from the car as it passes over it, the remainder of the third rail remaining "dead". Since each energized segment is completely covered by the lengthy articulated cars, and goes dead before being "uncovered" by the passage of the vehicle, there is no danger to pedestrians. At least initially there were teething troubles in terms of maintaining current feed, however, and the fact that the system is used exclusively in the historic center, with the cars on leaving this zone converting to conventional overhead pickup, underlines how, esthetics aside, for streetcars/trams it is hard to beat the overhead wire system in terms of overall efficiency.
ill_p12 Arcs like this are normal and occur when the collection shoes of a train drawing power reach the end of a section of power rail.
ill_p13 With top-contact third (and fourth) rail a heavy shoe suspended from a wooden beam attached to the bogies collects power by sliding over the top surface of the conductor rail.
The London Underground in England is one of the few networks that uses a four-rail system. The additional rail carries the electrical return that, on third rail and overhead networks, is provided by the running rails. On the London Underground, a top-contact third rail is beside the track, energized at +420 V DC and a top-contact fourth rail is located centrally between the running rails at -210 V DC, which combine to provide a traction voltage of 630 V DC. The same system was used for Milan's earliest underground line, Milan Metro's line 1, whose more recent lines use an overhead catenary.
This scheme was introduced because of the problems of return currents, intended to be carried by the earthed (grounded) running rails, flowing through the iron tunnel linings instead. This can cause electrolytic damage and even arcing if the tunnel segments are not electrically bonded together. The problem was exacerbated because the return current also had a tendency to flow through nearby iron pipes forming the water and gas mains.
Some of these, particularly Victorian mains that predated London's underground railways, were never constructed to carry currents and had no adequate electrical bonding between pipe segments. The four-rail system solves the problem. 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.
ill_p1 4 London Underground track at Ealing Common on the District Line, showing the third and fourth rails beside and between the running rails
London's sub-surface underground railways also operate on the four-rail scheme since, in a number of areas (for example the Piccadilly Line and Metropolitan Line services to Uxbridge), sub-surface and deep-level stock run on the same tracks.
On lines shared with National Rail third-rail stock, the center 'negative' rail is connected to the return running rail, allowing both types of train to operate.
A system proposed (but not used) by the South Eastern and Chatham Railway around 1920 was 1,500 V DC four-rail. Technical details are scarce but it is likely that it would have been a mid-earth system with one conductor rail at +750 volts and the other at -750 volts. This would have facilitated conversion to 750 V DC three-rail at a later date.
A few lines of the Paris Métro in France also operate on a four-rail power scheme but for a very different reason. It is not strictly a four-rail scheme as they run on natural rubber tyres running on a pair of narrow roadways made of steel and, in some places, concrete.
Since the tyres do not conduct the return current, two conductor rails are provided outside of the running 'roadways', so at least electrically it fits as a four-rail scheme. The trains are designed to operate from either polarity of supply, because some lines use reversing loops at one end, causing the train to be reversed during every complete journey (intended to save having to "change ends" by having the operator walk to the other end of the train to make the former last car the lead car in the new direction).
These are overhead electrification systems. Alternating current can be transformed to lower voltages inside the locomotive. This allows much higher voltages and therefore smaller currents along the line, which means smaller energy losses along long railways.
Low-frequency alternating current
ill_ 15 15 kV 16.7 Hz AC traction current used in Switzerland
ill_ 16 The world's first AC locomotive in Valtellina (1898-1902). Power supply: 3-phase 15 Hz AC, 3000 V, (AC motor 70 km/h). It was designed by Kálmán Kandó in Ganz Company, Hungary.
Common DC commutating electric motors can also be fed with AC (universal motor), because reversing the current in both stator and rotor does not change the direction of torque. However, the inductance of the windings made early designs of large motors impractical at standard AC distribution frequencies. In addition, AC induces eddy currents, particularly in non-laminated field pole pieces, that cause overheating and loss of efficiency. In the previous century, five European countries, including Germany, Austria, Switzerland, Norway and Sweden, standardized on 15 kV 12 2/3 Hz (one-third of the normal mains frequency) single-phase AC in an attempt to alleviate such problems.
On 16 October 1995, Germany, Austria and Switzerland changed the designation from 12 2/3 Hz to a nominal frequency of 16.7 Hz (though the actual frequency has not changed, its designation has). In the United States (with its 60 Hz distribution system), 25 Hz (an older, now-obsolete standard mains frequency) is used at 11 kV between Washington, D.C. and New York City and between Harrisburg, Pennsylvania and Philadelphia. A 12,500 V 25 Hz section between New York City and New Haven, Connecticut was converted to 60 Hz in the last third of the 20th century.
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. Two lines opened in 1925 under the Southern Railway serving Coulsdon North and Sutton railway station. The lines were electrified at 6.7 kV 25 Hz. It was announced in 1926 that all lines were to be converted to DC third rail and the last overhead electric service ran in September 1929.
In such a system, the traction motors can be fed through a transformer with multiple taps.
Changing the taps allows the motor voltage to be changed without requiring power wasting resistors. Auxiliary machinery is driven by small commutating motors powered from a separate low-voltage winding of the main transformer.
The use of low frequency requires that electricity be converted from utility power by motor-generators or static inverters at the feeding substations, or generated at altogether separate traction power stations.
Since 1979, the three-phase induction motor has become almost universally used. It is fed by a static four-quadrant converter which supplies a constant voltage to a pulse-width modulator inverter that supplies the three-phase variable frequency to the motors.
Polyphase alternating current systems
p17 3-phase pantograph on a Corcovado Rack Railway train in Brazil
p18 Train using a multiphase electrification system on the Petit train de la Rhune, France
The majority of the Italian State railway system three-phase system was 3,300 V at 15- 16.7 Hz. With such a low frequency, the locomotives did not need gearing. It is also possible to use the polyphase system regeneratively, as on the Italian State railway's mountain lines, where a loaded train descending could supply much of the power for a train ascending. Experimental polyphase installations in Italy in the 1930s used higher voltage (10 kV) at industrial frequencies (45 or 50 Hz). In the United States, the Great Northern Railway's (Cascade Tunnel) first electrified line (1909-1927) was at 6,600 V, 25 Hz.
The main complexity with three-phase systems is the need for three conductors (including the rails), hence two overhead conductors. Early locomotives on the Italian State Railways used a wide bow collector which covered both wires but later locomotives used two pantographs side-by-side. In the United States, a pair of trolley poles were used.
They worked well with a maximum speed limit of 15 mph. The dual conductor pantograph system is used on four mountain railways that continue to use three phase power (Corcovado Rack Railway in Rio de Janeiro, Brazil, Jungfraubahn and Gornergratbahn in Switzerland and the Petit train de la Rhune in France).
Standard frequency alternating current
Only in the 1950s after development in France (20 kV then 25 kV) and former Soviet Union countries (25 kV) did the standard-frequency single-phase alternating current system become widespread, despite the simplification of a distribution system which could use the existing power supply network.
The first attempts to use standard-frequency single-phase AC were made in Hungary since 1923, by the Hungarian Kálmán Kandó on the line between Budapest-Nyugati and Alag, using 16 kV at 50 Hz. The locomotives carried a four-pole rotating phase converter feeding a single traction motor of the polyphase induction type at 600 to 1,100 V. The number of poles on the 2,500 hp motor could be changed using slip rings to run at one of four synchronous speeds. The tests were a success so, from 1932 until 1960s, trains on the Budapest-Hegyeshalom line (towards Vienna) regularly used the same system. A few decades after the second world war, the 16 kV was changed to the Russian and later French 25 kV system.
Today, some locomotives in this system use a transformer and rectifier to provide low voltage pulsating direct current to motors. Speed is controlled by switching winding taps on the transformer. More sophisticated locomotives use thyristor or IGBT circuitry to generate chopped or even variable-frequency alternating current (AC) that is then supplied to the AC induction traction motors.
This system is quite economical but it has its drawbacks: the phases of the external power system are loaded unequally and there is significant electromagnetic interference generated as well as significant acoustic noise.
A list of the countries using the 25 kV AC 50 Hz single-phase system can be found in the list of current systems for electric rail traction.
p20 Close-up view of catenary on Northeast Corridor, USA
The United States commonly uses 12.5 and 25 kV at 25 Hz or 60 Hz. 25 kV, 60 Hz AC is the preferred system for new high-speed and long-distance railways, even if the railway uses a different system for existing trains.
To prevent the risk of out-of-phase supplies mixing, sections of line fed from different feeder stations must be kept strictly isolated. This is achieved by Neutral Sections (also known as Phase Breaks), usually provided at feeder stations and midway between them although, typically, only half are in use at any time, the others being provided to allow a feeder station to be shut down and power provided from adjacent feeder stations. Neutral Sections usually consist of an earthed section of wire which is separated from the live wires on either side by insulating material, typically ceramic beads, designed so that the pantograph will smoothly run from one section to the other. The earthed section prevents an arc being drawn from one live section to the other, as the voltage difference may be higher than the normal system voltage if the live sections are on different phases and the protective circuit breakers may not be able to safely interrupt the considerable current that would flow. To prevent the risk of an arc being drawn across from one section of wire to earth, when passing through the neutral section, the train must be coasting and the circuit breakers must be open. In many cases, this is done manually by the driver. To help them, a warning board is provided just before both the neutral section and an advanced warning some distance before. A further board is then provided after the neutral section to tell the driver to re-close the circuit breaker, although the driver must not do this until the rear pantograph has passed this board. In the UK, a system known as Automatic Power Control (APC) automatically opens and closes the circuit breaker, this being achieved by using sets of permanent magnets alongside the track communicating with a detector on the train. The only action needed by the driver is to shut off power and coast and therefore warning boards are still provided at and on the approach to neutral sections.
On French high-speed rail lines, the UK High Speed 1 Channel Tunnel rail link and in the Channel Tunnel itself, neutral sections are negotiated automatically.
p21 Info-graphic map of Railway electrification in Europe by country
In 2006, 240,000 km (25% by length) of the world rail network was electrified and 50% of all rail transport was carried by electric traction.
Advantages + disadvantages
+ lower running cost of locomotives and multiple units
+ lower maintenance cost of locomotives and multiple units
+ higher power-to-weight ratio, resulting in:
++ fewer locomotives ++ faster acceleration
++ higher practical limit of power
++ higher limit of speed
+ less noise pollution (quieter operation)
+ reduced power loss at higher altitudes
+ lack of dependence on crude oil as fuel
+ less environmental pollution, even if electricity is produced by fossil fuels
+ upgrading brings significant cost, ++ especially where tunnels and bridges and other obstructions have to be altered for clearance ++ alterations or upgrades will be needed on the railway signaling to take advantage of the new traffic characteristics
p22 Large cargo may require special cars
p23 Most overhead electrifications do not allow sufficient clearance for a double-stack car.
+ Maintenance costs of the lines may be increased, but many systems claim lower costs due to reduced wear-and-tear from lighter rolling stock. There are additional maintenance costs associated with the electrical equipment but, if there is sufficient traffic, reduced track and engine maintenance costs can exceed the costs of this maintenance.
+ Network effects are a large factor with electrification. When converting lines to electric, the connections with other lines must be considered. Some electrifications have eventually been removed because of the through traffic to non-electrified lines. If through traffic is to have any benefit, time consuming engine switches must occur to make such connections or expensive dual mode engines must be used. This is mostly an issue for long distance trips, but many lines come to be dominated by through traffic from long-haul freight trains (usually running coal, ore, or containers to or from ports). In theory, these trains could enjoy dramatic savings through electrification, but it can be too costly to extend electrification to isolated areas, and unless an entire network is electrified, companies often find that they need to continue use of diesel trains even if sections are electrified. The increasing demand for container traffic which is more efficient when utilizing the double-stack car also has network effect issues with existing electrifications due to insufficient clearance of overhead electrical lines for these trains, but electrification can be built or modified to have sufficient clearance, at additional cost.
Additionally, there are issues of connections between different electrical services, particularly connecting intercity lines with sections electrified for commuter traffic, but also between commuter lines built to different standards. This can cause electrification of certain connections to be very expensive simply because of the implications on the sections it is connecting. Many lines have come to be overlaid with multiple electrification standards for different trains to avoid having to replace the existing rolling stock on those lines. Obviously, this requires that the economics of a particular connection must be more compelling and this has prevented complete electrification of many lines. In a few cases, there are diesel trains running along completely electrified routes and this can be due to incompatibility of electrification standards along the route.
Summary of advantages and disadvantages:
+ Lines with low frequency of traffic may not be feasible for electrification (especially using regenerative braking), because lower running cost of trains may be overcome by the higher costs of maintenance. Therefore most long-distance lines in North America and many developing countries are not electrified due to relatively low frequency of trains.
+ 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 but, at higher speeds, this proves costly and impractical. Therefore, almost all high speed trains are electric.
+ The high power of electric locomotives gives them the ability to pull freight at higher speed over gradients; in mixed traffic conditions this increases capacity when the time between trains can be decreased. The higher power of electric locomotives and an electrification can also be a cheaper alternative to a new and less steep railway if trains weights are to be increased on a system.
There is a significant amount of published material that concludes that electric trains are more energy efficient than diesel-powered trains and, with suitable energy production, can have a smaller carbon dioxide footprint. Some of the reasons include:
+ electric trains are generally lighter than self powered versions (e.g. diesel traction); o they do not have to carry the weight of prime movers, transmission and fuel.
++ this is partially offset, however, by the weight of electrical control equipment, and in the case with high-voltage AC by the weight of traction transformers, which may be particularly heavy with low frequency AC (e.g. 16.7 Hz.).
+ the electricity may be generated from various energy sources which are more efficient than a diesel engine, as well as lessening reliance on petroleum products and reducing carbon dioxide emissions, including:
++ nuclear power, o renewable resources (e.g. hydroelectricity, wind generation, etc.), o large fossil fuel using power stations with greater efficiency (although they may still have a relatively large carbon footprint).
+ under certain conditions, some suitably equipped electric trains can use regenerative braking to return power to the electrification system so that it may be used elsewhere; o by other vehicles within the network section;
+ often implemented in tram networks, where there is a high density of vehicles in each fairly short powered section,
+ on high voltage mainlines where there may be several trains within each long section,
+ on mountainous lines where trains may be scheduled such that one is ascending whilst another descends; o in some form of energy storage, such as flywheel energy storage so that it may be used later (e.g. to accelerate a train from a station at which it has recently stopped)
++ some systems, such as most 25 kV AC systems in the UK, are able to return excess energy to the public network.
According to widely accepted global energy reserve statistics, the reserves of liquid fuel are much less than gas and coal (at 42, 167 and 416 years respectively). Most countries with large rail networks do not have significant oil reserves and those that did, like the United States and Britain, have exhausted much of their reserves and have suffered declining oil output for decades. Therefore, there is also a strong economic incentive to substitute oil for other fuels. Rail electrification is often considered an important route towards consumption pattern reform.
The external cost of railway is lower than other modes of transport but the electrification brings it down further if it is sustainable.
Also, the lower cost of energy from well to wheel and the ability to reduce pollution and greenhouse gas in the atmosphere according to the Kyoto Protocol is an advantage.
Research and development
Another result of electrification is the effect on locomotive and wagon productivity and it is going to be more effective by more railway research in this field. The trend of technology in railway electrification is very important to adopt the efforts for better results, for example the trend from GTO (Gate turn-off thyristor) to IGBT (Insulated-gate bipolar transistor) for more powerful locomotives with higher reliability is one of the elements of Technology roadmap (TRM) and the loop to have a mature system as in Maturity road mapping with the Technology transfer provision.
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