Technical Details
These locomotives were equipped to operate from the 650 or 750 V DC third-rail or an on-board diesel engine to allow it to operate over non-electrified routes.
Electrical power was gathered in the normal manner of collector shoes which ran along the top of the third rail. There are two shoes mounted on either side of each bogie which could be retracted when not in use to avoid damage as they were prone to drop out of gauge slightly when not under pressure.
A major design flaw with the class was that the traction current circuits of each bogie were not isolated. The 3rd rail supply is generated close to the rail line and there are many areas where it changes from one substation to another. Although great effort is made to ensure the voltages are the same between sections, at the extreme ends of supply there can be very large differences due mainly to voltage drops caused by other electric trains. There are examples of fires in class 73 where the locomotive has acted as an electrical bridge between two different substations causing the traction circuit inside the locomotive to take the brunt of the huge surges in supply as trains move in and out of the sections behind and in front. This problem was particularly noticeable at junctions where conductor rails cannot be separated by a good distance and many of the class were damaged by fire caused in this manner over the years. Class 73 was well known for incredible displays of arcing at the collector shoes for this exact reason.
Read more about this topic: British Rail Class 73
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