40. Car Wiring:

Must be always run out of reach of the passengers, and must be insulated with a waterproof insulation.

41. Lighting and Power from Railway Wires:

Must not be permitted under any pretence, in the same circuit with trolley-wires with a ground return, nor shall the same dynamo be used for both purposes, except in street-railway cars, electric-car houses, and their power-stations.

42. Car-houses:

Must have special cut-outs located at a proper distance outside, so that all circuits within any car-house can be cut out at one point.

43. Ground Return Wires:

When ground return is used it must be so arranged that no difference of potential will exist greater than 5 volts to 50 feet, or 50 volts to the mile between any two points in the earth or pipes therein.

"Feeders" are the wires that lead from the station to points where current is taken off by the different branches. If all the current required to operate a large street-railway had to be conducted from the station through the trolley-wires, either these wires would have to be of tremendous size, or else there would be great loss in forcing a large current

through a small wire. If the wires were small they would greatly overheat, and the pressure would be different at different parts of the system. After a large current had been forced through the trolleywire, there would not be so much energy left to operate the cars. Where much current is needed it is consequently the practice to use the trolleywire only as a sort of local conductor to carry current from the ends of the feeders to the cars. 75.)

(Fig.

FEEDERS

TROLLEY WIRE

FIG. 75.- Feeders.

Street railways that use a trolley running along a wire overhead, almost invariably make the rails and the earth serve as return conductors to the station. Figure 76 shows the connections, and also indicates a stationary motor being operated from the trolley line. Quite aside from any danger, it is unsatisfactory to take current for power and light from a street-railway trolley-wire, because the electrical pressure varies when the current taken by the cars varies, and because when for any reason the circuit-breaker at the station cuts off the current from the line,

the service is interrupted. It is more dangerous to operate power or lights from a trolley line than from a regular power or light circuit, for the following reasons:

One side of the circuit is always connected with the earth. With other circuits every precaution is taken to keep both wires thoroughly insulated from the earth, but with the trolley circuit one side is grounded intentionally, permanently, and as well as possible. Where both wires of a circuit are insulated from the ground, to have trouble with earth connections it is necessary to have two connections with the earth, one on each wire at the same time. With a trolley circuit, a bad leak or short circuit is caused by one connection between the one wire and the earth

at any time. With With any circuit the electrical pressure is constantly striving to force current through the insulation there are always minute leaks. : When the pressure is doubled these minute leaks double and are much more likely to become rapidly worse. With a circuit having both wires insulated, only half the total pressure is acting to break down the insulation on each wire; but where one side of the circuit is grounded, the total pressure is acting to break down the insulation on the single wire. With a grounded circuit there is likely to be more trouble from lightning because there is a connection to the earth through motors and lamps. The discharge in

passing through these is apt to cause damage. It is difficult satisfactorily to operate motors from the trolley-wire when a proper fuse is used, because the fluctuations in the pressure cause the motor often to take rushes of current that will melt the fuse unless it be too large for good protection.

The foregoing are the principal reasons why light and power from trolley lines are so strongly objected to by the underwriters. Incandescent lamps operated from railway wires would be prohibited by Rule 14 (c) as well as by Rule 41, because with a 500 volt circuit it is necessary to connect five lamps in series and there is thus a multiple-series system.

Of course it does not make an insulated metallic circuit if two wires are run from the motor to the power-house, because the only change this makes is the connection of one wire to the earth at the dynamo instead of at the motor.

Figure 76 also illustrates the way in which the current in a trolley circuit returns to the generator. A part returns through the rails, and a part flows into the earth, and returns through buried pipes, moist earth, or any other conducting substance. This return current divides according to the conductivity of the different paths. (Page 38.) Since there is resistance it takes a part of the pressure of the dynamo to force this current from the car back to the dynamo;

thus there will be a difference of pressure between any two points in the return circuit, say between A and B. If a gas-pipe should run near the point A and a water-pipe should run near the point B, and if these two pipes should touch each other somewhere, there might be dangerous arcing due to the current finding its way back through the pipes. This is one of the reasons for Rule 43. The other and greater reason

FIG. 76.-Illustrating Street-railway Circuit and Paths taken by Current.

is, that if there are considerable differences of potential there is destructive action called "electrolysis" wherever the current leaves the pipe for the earth, as at C, Figure 76. The iron of the pipe is carried away into the earth toward the negative pole of the generator, just as in a silver-plating battery the silver is carried away from a silver plate toward the article to be plated which is attached to the negative pole of the battery.