« НазадПродовжити »
without shutting off the current from any other lamp. The switch must meet the requirements for the serviceswitch. Loose connections and imperfect switches cause heating and arcing, and it is necessary that the hanger-board be non-combustible so that nothing will be set on fire in case of defects.
These troubles may be avoided by hanging the lamp direct from insulated supports and bringing the wires to the lamp without going first to a board, but in this case there is no switch to cut off the current from the lamp, and lamps cannot be taken down for repairs with the same facility.
The greatest danger from the arc lamp is just such danger as would come from any centre of great heat, and it will be noted that the rules have for their object the complete isolation of the source of heat. Inflammable material hanging near will, of course, be in danger of catching fire, and the hot particles thrown off by the arc will be apt to burn material beneath if precautions are not taken. Tight globes, closed at the bottom, will catch most of the sparks, but many small ones are carried up by the heated air, and the sparkarresters are necessary to prevent any of these from escaping. Broken globes are an approach to no globes at all, and may allow large sparks to drop from the lamp. The wire netting thus helps to keep the arc entirely enclosed.
14. Incandescent Lamps in Series Circuits having a Maximum Potential of 300 Volts or over:
a. Must be governed by the same rules as for arc-lights, and each series lamp provided with an approved hand springswitch and automatic cut-out.
6. Must have each lamp suspended from a hanger-board by means of a rigid tube.
c. No electro-magnetic device for switches and no system of multiple-series or series-multiple lighting will be approved.
d. Under no circumstances can series lamps be attached to gas-fixtures.
Incandescent lamps that are made to operate with a comparatively large current are sometimes connected in series circuits. Except that they do not throw off sparks as arc lamps do, there are the same dangers as with other series circuits and the same precautions are to be taken.
The "hand spring-switch" is virtually the same thing as the arc service-switch. It must act in the same way since it is used on a series circuit; that is, it must be made first to form a by-path round the lamp and then to break the connection to the lamp.
The term " automatic cut-out," when used in this connection, means a switch of some kind that will by itself shunt the current round through a by-path when the lamp becomes defective.
The " hanger-board" used with the series incandescent lamp is much the same as that used for arc-lamps, but it is required that the lamp be hung by means of a rigid tube, because incandescent lamps are likely to be moved about and the insulating covering on the conductors would become abraded. This is a more serious matter with the high-potential series circuits than it is with low-potential multiple circuits, not only because of the higher potential, but because if the conductor in a series circuit were to break, a long and destructive arc would be formed.
"Electro-magnetic devices " for switches are devices that depend for their action upon magnets that become stronger or weaker according to the amount of current that passes round the iron core. The magnetic force works a mechanism that operates the switch. They are objected to because a slight sticking of the movable parts prevents satisfactory working.
A "multiple-series system" is one in which lamps are in different series and these series connected in multiple. (Fig. 45.) There is a constant pressure between the wires A and B, so that if at c, for instance, a short circuit occurs, that is, if a by-path is formed across the terminals of the lamp, there is less resistance between A and B through this particular series and a larger current will flow. This is likely to burn out the other lamps, c', c", etc., one after the other, with increasing speed and with liability of excessive heating and consequent danger.
A "series-multiple system" is one in which groups of lamps in multiple are connected in series. (Fig. 46.)
The same current must go through each group since all groups are connected in series, and while all is in good order this current will divide properly between the different lamps. If, however, one lamp, say e, burns out and thus interrupts the current going through it, the other lamps, c' and c", will have to take more than their share and are likely to burn out with the same attendant dangers as with the multiple-series system.
Gas-pipes are always connected with the ground and if series lamps are attached to the fixtures, the high potential causes great danger of leakage to the pipes, however carefully the insulation is looked after.
The use of incandescent lamps on series circuits is becoming infrequent on account of the dangers and the inconvenience. The system is not suitable to the high degree of subdivision that is the great advantage of the incandescent lamp, for the -greater precaution necessary to make the series circuit safe, makes the devices clumsy and rigid, and the portability that adds so much to the usefulness of the incandescent lamp on multiple systems, is thus not possible.
LOW-POTENTIAL SYSTEMS. — 300 VOLTS OB LESS.
15. Outside Overhead Conductors:
o. Must be erected in accordance with the rules for high-potential conductors.
6. Must be separated not less than twelve inches, and be provided with an approved fusible cut-out that will cut off the entire current as near as possible to the entrance to the building and inside the walls.
[Section 6. An approved fusible cut-out must comply with the sections of Rules 23 and 24 describing fuses and cut-outs. The cut-out required by this section must be placed so as to protect the switch required by Rule 17.]