acdchistoryvoltage
As the title states, in the rail industry, why are the majority of railway's third line or overhead line in DC volts and not AC volts? My initial guess would be that it is in AC since it is easier to distribute AC over long distances. I do know there are a few overhead lines that are 25KV AC, but the majority are 600-750 V DC.
EDIT: I found this article which explains the difference a little bit, but it still doesn't explain why the majority are DC.
There are several reasons. One: power loss in a wire is I^2 * R. Therefore it is better to transmit power at very high voltage and low current. AC is much more easily boosted to high voltage (no electronics are needed). To boost industrial loads using silicon electronics is not practical.
Another is ease of switching under load. If you turn off a load connected to DC, the arcing at the switch due to wire inductance and load inductance becomes problematic. This forces DC switches to be more robust.
The 60 Hz noise created by transformers is much less than the switching noise that would be created by all the electronics required to buck and boost DC and then convert it to AC at point of load as you propose.
Many appliances in the USA and elsewhere have two-prong plugs because they are "double insulated." The third prong is for ground fault protection except where outlets have been designed with protective shutters on the current-carrying slots that are opened by the ground prong. Double insulation provides very effective ground fault protection that is less expensive for most products.
The fact that the US voltage for domestic wall-plug use is 120 volts offers a small amount of additional safety vs. 220 volts, but that is generally not taken into consideration in US safety standards.
Double insulated or class 2 electrical appliances are products that have been designed in a way so as not to require a safety connection to electrical earth (These products must NOT have a safety connection to Earth).
These products are required to prevent any failure from resulting in dangerous voltage levels becoming exposed causing a shock etc. This must be done without the aid of an earthed metal casing. Ways of achieving this include double layers of insulating material or reinforced insulation protecting any live parts of the fitting.
There are also strict requirements relating to the maximum insulation resistance and leakage to any functional earth or signal connections of such appliances. Products of this type are required to be labelled "Class 2," "Class II,", "double insulated" or bear the double insulation (square in a square) symbol.
The safety of electrical product designs are certified by an independent testing agency such as Underwriters Laboratories (UL) or the Intertek ETL laboratory (originally Edison Testing Laboratory). In order to apply the UL or ETL label, the manufacturer must have product design evaluated and samples tested in the independent laboratory or have their own testing and review results evaluated by the independent laboratory. In either case, they must maintain a contract with the independent testing laboratory that includes periodic, un-announced inspection of their manufacturing facility and its quality assurance procedures.
See the product marking illustration below.
Best Answer
Railway infrastructure is expensive. It is relatively rare to create totally new tracks and when you do, they most often conform to the engineering norms of the existing tracks (gauge etc) to allow for flexibility in rolling stock usage etc.
Therefore decisions about electrification were made in the 19th century (e.g. 1890 in London). At that time, speed control of large motors was probably easier for DC than for AC where the speed is linked to AC frequency.
Also at that time DC distribution had advantages over AC.
Subsequent technological revolutions are generally hampered by the need to maximise return on very long term investment in large-scale infrastructure.
An interesting case is London's Thameslink which has trains that operate on overhead 25KV AC for the northern part of the journey and on third-rail 750V DC tracks for the portion of the journey south of Farringdon station. The costs of introducing incompatible infrastructure can be considerable.