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36. Secondary Conductors:

Must be installed according to the rules for " Low-potential Systems."

The construction of the primary circuit must be in accordance with the rules for high-potential circuits (Rules 10, 11, 12), except that Rule 35 (c) requires, in some places, even greater distance between wires.

All the reasons for the use of service-switches with arc-light circuits, and for the use of service-switches and main fuses with low-potential circuits, apply with even greater force to their use with primary wires in buildings, for the electrical pressure is high, and the current is not limited by the regulation of the dynamo as it is with constant-current series circuits.

Class E.

ELECTRIC RAILWAYS.

37. All rules pertaining to arc-light wires and stations shall apply (so far as possible) to street-railway power-stations and their conductors in connection with them.

38. Power-stations:

Must be equipped in each circuit as it leaves the station with an approved automatic "breaker," or other device that will immediately cut off the current in case the trolley-wires become grounded. This device must be mounted on a fireproof base, and in full view and reach of the attendant.

[Section a. Automatic circuit-breakers should be submitted for approval before being used.]

Electric railways are ordinarily operated at an electrical pressure of 500 volts, which the rules classify as a "high-potential"; the generators are designed to maintain this pressure constant and as they are usually of great power, a tremendous current will flow when a short circuit occurs. For these reasons the same construction is required as that specified for arc-light circuits.

An "automatic circuit-breaker" is a device for breaking connection between the generators and the outside lines. It performs the same service as a fuse, but operates instantly when there is an excessive current, while a fuse requires time to heat sufficiently to melt. The automatic breaker is operated by the current that flows out to the lines. The current passes through the wire on an electro-magnet and this magnet becomes stronger as a larger current passes. The breaker

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Fio. 74. — Showing Principle of Automatic Circuit-breaker. An excessive current makes the magnet attract A. A strikes B thus releasing C, which is pulled out by the spring D. A gap is then left in the circuit between E and F.

is so adjusted that when the magnet reaches a certain strength a piece of iron is pulled toward it, and this iron piece is so attached to a sort of trigger that a spring will be released and left free to force open a switch. Figure 74 shows the principle of an automatic breaker. In a practical form it must be so arranged that the arc formed upon opening the circuit cannot continue, and precaution must be taken that the parts to be moved will not stick. The arc that is formed has a great heating-power and the whole piece of apparatus should be mounted on a non-combustible base.

39. Trolley-wires:

a. Must be no smaller than No. 0 B. & S. copper or No. 4 B. & S. silicon bronze, and must readily stand the strain put upon them when in use.

6. Must be well insulated from their supports, and in case of the side or double-pole construction, the supports shall also be insulated from the poles immediately outside of the trolley-wire.

c. Must be capable of being disconnected at the powerhouse, or of being divided into sections, so that in case of fire on the railway route the current may be shut off from the particular section and not interfere with the work of the firemen. This rule also applies to feeders.

d. Must be safely protected against contact with all other conductors.

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 60 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. (Fig. 75.)

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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,

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