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allowed on gas fixtures since an arc to the grounded gas pipe would be very severe and would be liable to burn through the pipe and ignite the escaping gas.

Constant-Potential Systems. The character of these systems has been explained on page 12. Almost all systems for light and power in buildings of all sorts are of this type. The most common are 110-volt two-wire direct-current systems; three-wire systems with 220 volts between outside wires and 110 volts between the neutral and either outer wire; 500- to 600-volt d. c. street and elevated railroad systems with "ground return"; 440- to 600-volt a.c. systems for motors. In addition there are 1,100-, 2,200-, and 3,300-volt a. c. power circuits and the so-called a. c. transmission lines at all voltages from 1,000 up to 80,000 or 100,000 volts. While occasionally a. c. motors are made for direct operation at 1,100 or 2,200 volts, in general for voltages above 600 volts, alternating-current transmission lines are employed which are connected to transformers at the factories or mills where the power is used and which "step-down" the voltage to 440 volts or some other voltage which can conveniently be used in the motors and for lighting purposes. Both the primary and secondary circuits in this case are constant potential systems.

In street railway work large machines cailed rotary converters are employed which are driven by the high-voltage alternating current from transformers connected to the transmission lines and which deliver direct current at about 600 volts to the trolley system. Such rotary converters are usually placed in substations so located as to conveniently and economically supply the different sections of a city. A similar practice is followed in cities where direct current is to be furnished for general lighting and power, and the original generating station is more or less remote. Such substations come under the same rules as generating or dynamo stations.

All so-called isolated plants, that is, plants in individual factories or large buildings, are constant-potential systems also. In the underwriters' rules constant-potential systems are subdivided into low potential systems up to 550 volts; high-potential systems 550 to 3,500 volts; and extra-high-potential systems over 3,500 volts. Of these the low-potential systems are of most importance since they include the very great majority of equipments for using electricity in buildings for light, heat, and power.

GENERAL INSTALLATION RULES FOR CONTROLLING AND
PROTECTING DEVICES

Switches, fuses, and circuit breakers may all be described as arcing devices, that is, their operation always produces an arc. This arc may be small or large but it is impossible to break a circuitcarrying current without some arc even if it is so small that it is a mere spark. The duration and intensity of an arc depends upon the strength and voltage of the current, the rapidity with which the gap in the circuit is widened, and the design and condition of the arcing device, switch or fuse. Dust or inflammable gases may be ignited by an arc of sufficient intensity. No arcing device, therefore, should be placed near easily ignitible stuff, or exposed to inflammable gases, or dust, or flyings of any combustible material. When so exposed, as in flour mills, textile mills, etc., all switches and fuses should be enclosed in dust-tight boxes or cabinets. Open-link fuses are especially liable to flash violently and throw out molten metal and they must, therefore, be given special attention, and should never be installed outside of proper cabinets except on switchboards in fireproof rooms, such as engine rooms, generating stations, or where they will be under constant and expert supervision. Even in ordinary rooms, houses, stores, or factories, where there is no dust or combustible flyings in the air, it is much better to have all knife switches and all fuses placed in cabinets to prevent accidental shortcircuits, caused by laying a metal object across the exposed parts.

Switches immersed in oil are in common use for large currents and are quite safe as regards arcing, though the oil involves a certain hazard since it is combustible.

It should be remembered that any switch or circuit breaker which is automatic, that is, which is not operated by hand by a person at the actual device itself, requires better protection since in case of failure the arcing may be severe and no one may be at hand to take the needful steps to prevent its starting a fire. Such automatic current-breaking devices including time-switches worked by clocks, sign flashers, and the like, should always be enclosed in very substantial non-combustible cases or cabinets of ample size and so arranged that they are not liable to be left open.

Switches. The general requirements for service switches have already been discussed. While it is true that the service switch

and a switch for every motor are the only ones that are absolutely required by the rules, still convenience and economy of operation naturally call for a number of switches in practically every installation, and the correct placing of them becomes, therefore, a matter of importance. The description of some of the very numerous types of switches will be given later in this book, but we consider here the general rules for installing all types.

As a general principle, switches must always be placed in dry, accessible places and it is well to group them together so far as possible for the reason that this will often reduce the amount of wiring and also render it easier to use them in case of need.

Knife Switches. Knife switches consist of copper blades, one for each pole, hinged at one end to copper clips or posts and closing at the other into other clips. Where such a switch is made to close

[graphic][graphic]

Fig. 60. Single, Double, and Triple Pole Knife Switches

into clips at only one side of the hinge end it is called a single-throw switch and where the blades can be thrown into clips at either side of the hinge it is called a double-throw switch. Single-throw switches must always be installed so that gravity will tend to open rather than close them since otherwise they might fall and, by only partly closing, cause arcs and burning. Double-throw switches may be installed so that the throw is either vertical or horizontal as preferred. Fig. 60 shows a double-pole single-throw switch and a triple-pole double-throw switch correctly placed and a single-pole single-throw switch wrongly placed.

Whenever practicable, knife switches should be so wired that the blades will be dead when the switch is open as this leaves less exposed live metal and also makes it easier and safer to make any repairs or adjustments of the switch blades and hinges. In Fig. 61 if the supply wires (from the service or dynamo) enter at the top

and the lamps are connected from the bottom of the switch, the blades will be dead when the switch is open. If the arrangement is reversed, the switch blades will be connected to live wires all the time, whether the switch is open or closed. The illustration also shows an excellent type of cast-iron cabinet for such a switch combination.

[graphic]

Fig. 61. Approved Metal Switch Box

[blocks in formation]

It is not possible to fasten them very securely to a lath-and-plaster wall unless some block is provided for the screws to be driven into. For this reason, wherever possible, at all switch or fixture outlets, a 3-inch block must be fastened between studs or floor timbers flush with the back of lathing to hold tubes, and to support switches or fixtures. When this cannot be done, wood base blocks, not less than inch in thickness, securely screwed to lathing, must be provided for switches, and also for fixtures which are not attached to gas pipes or conduit. Figs. 62 and 63 show these blocks with the wires brought through them and through the lath and plaster in short lengths of flexible tubing. The switches can thus be firmly screwed to the blocks and the wires connected to them.

[blocks in formation]

Proper Arrangement of Wires Passing through Lath-and-Plaster Partition

If snap switches are used with exposed wiring on cleats, there must be a porcelain sub-base under each switch so made that the wires will be kept inch from the surface wired over. A similar

sub-base must be used where such a switch is used with wood molding, but in this case it may be of hard wood instead of porcelain. Figs. 64 and 65 show how this is done.

Flush Switches. These are made to be inserted into walls so that only the operating push buttons or handle will extend out beyond the surface, and are now in very general use. Inasmuch as their operating parts are concealed in the wall they should invariably be set into small steel boxes through the back of which the wires may enter either through lengths of flexible tubing or through iron conduit. The same requirement applies to all small fittings such as receptacles from which flexible cords are run to heaters and other portable devices. Fig. 66 is a sketch of such a switch box set into a lath-and-plaster wall. The sketch shows the box as it would appear from the back of the wall.

WIRE IN.
FROM WALL

SUB-BASE
PORCELAIN

Wiring on Cleats

Where it is desired to control the same electric lamps from either of two switches at different places, Fig. 64. Exposed what are called three-way switches are installed. These are chiefly used in residences, as for the control of hall lights from either upstairs or downstairs. Under the rules these are classed as single-pole switches and are preferably wired so that only one main of the circuit is carried to either switch. Three-way switches are usually of the common round-surface porcelain-base type or push-button wall variety. Fig. 67 gives a diagram of the way to connect them.

Fig. 65.

MOULDING

WOODEN
SUB-BASE

==

Wires in Wood
Molding

These may

Fuses and Circuit Breakers. be compared with "safety valves" on steam boilers, that is to say, they are primarily designed to act in case of an improper condition of affairs and prevent by their automatic action any serious trouble resulting. Although this is the purpose of fuses and overload circuit breakers, it is altogether too common for users of electric current to misuse them, and so de

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