Three Wire System.
A system of distribution of electric current for multiple arc or
constant potential service. It is the invention of Thomas A. Edison.
It includes three main wires which start from the central station or
generating plant, and ramify with corresponding reduction in size,
everywhere through the district or building to be lighted. As ordinarily
carried out when dynamos are used, the dynamos are arranged in groups of
two. One lateral lead starts from the negative binding post of one
dynamo. The positive terminal of this dynamo connects to the negative of
the other. Between the two dynamos the central or neutral lead is
connected. The other lateral lead starts from the positive binding post
of the second dynamo.
The lamps or other appliances are calculated for the potential
difference of a single dynamo. They are arranged between the neutral
wire and the laterals, giving as even a disposition as possible to the
two laterals.
Fig. 336. DIAGRAM OF THREE WIRE SYSTEM SHOWING NEUTRAL WIRE.
If evenly arranged and all burning or using current, no current goes
through the neutral wire. If all the lamps situated on one lateral are
on open circuit all the current goes through the neutral wire. In other
cases the neutral wire receives the excess of current only.
The advantages of the system are that it uses smaller wire than the two
wire system for lamps of the same voltage. If lamps of double the
voltage were used the two wire system would be most economical.