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100 volts, there would have to be 1000 ampères at 100 volts (1000 x 100 = 100,000). But the amount of current that is forced through a given resistance is what determines the loss. For a given loss, a smaller wire may be used for a small current at a high-pressure than for a large current at a low-pressure, although the rate of work in the two cases, as in the example cited, may be the same. (Page 34.)

When passing through a wire alternating currents under certain conditions do not follow even approximately the law that holds with direct currents. The property of induction has an important effect and on account of it sometimes a very small alternating current will flow, where with a direct current there would be a short circuit. Thus in the transformer (Fig. 73) when there are no lamps turned on and the secondary coil is consequently "open," almost no current will flow in the primary coil although it is "closed." If a few lamps be turned on so that the secondary coil is partly "closed," more current will flow in the primary, and this current will increase or diminish in almost exact proportion to the number of lamps turned on in the secondary circuit.

Transformers, or converters, become somewhat heated even under ordinary conditions, and when a short circuit occurs in the secondary circuit or when there is a defect in the transformer itself, it is possible for the

temperature to become dangerously high. When a transformer "burns-out," the smudge from the burning insulation will not infrequently cause considerable smoke damage. The primary wires may become a source of danger, because they are connected to a generator of high electrical pressure so designed that it will furnish a heavy current at a constant pressure. There is some likelihood that lightning discharges will break through the insulation of the primary wires in the transformer, and if this were to occur it would be better to have the transformer out of doors and mounted so that overheating could do no damage. For the foregoing reasons the underwriters have thought it best to exclude transformers from buildings wherever possible.

35. Primary Conductors:

a. Must each be heavily insulated with a coating of moisture-proof material from the point of entrance to the transformer, and, in addition, must be so covered and protected that mechanical injury to them, or contact with them, shall be practically impossible.

b. Must each be furnished, if within a building, with a switch and a fusible cut-out where the wires enter the building, or where they leave the main line, on the pole or in the conduit. These switches should be enclosed in secure and fireproof boxes, preferably outside the building.

c. Must be kept apart at least ten inches, and at the same distance from all other conducting bodies when inside a building.

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 sub

mitted 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

M

MAGNET

DYNAMO

EARTH

LINE

JB

C

FIG. 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.

b. 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.

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