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Sub stations are required to supply electricity locally to sections of overhead wire from the main supply network.

The electrical power for a tramway is usually taken from the national grid at too high a voltage to be safely used in the streets and is Alternating Current (A.C.). This does not suit most tramcars which operate on Direct Current (D.C.).

A.C. has several advantages for distribution.(Click here for an explanation) It is only converted to D.C. near the trackside where realtively small enclosures called sub-stations are provided to house the equipment.

The Board of Trade regulations used to insist on the overhead electrical system being divided so that half-mile sections could be individually isolated. The utility of this approach is still understood and followed.
This means that a single sub-station could feed a length of overhead wire half a mile in each direction without any additional feeder cables. This would necessitate 25 sub-stations for the proposed Bath system, an uneconomically large number.

By using feeder cables, each sub-station could feed more than two sections of overhead and the number of sub-stations reduced. This has the following advantages:


The disadvantages of too few sub-stations would be:

Sub-stations do not have to be mounted immediately adjacent to the tracks, but the cost of extra cabling should be carefully weighed up against any advantages offered by using a more remote site.

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