CHAPTER XIV.

CLASS C., LOW POTENTIAL SYSTEMS.

PART VIII.

TEXT OF THE Code Covered by this CHAPTER. 26. SWITCHES:— a. Must be mounted on moisture-proof and non-combustible bases such as slate or porcelain. b. Must be double pole when the circuits which they control supply more than six 16-candlepower lamps or their equivalent. c. Must have a firm and secure contact; must make and break rapidly, and not stop when motion has once been imparted by the handle. d. Must have carrying capacity sufficient to prevent heating. e. Must be placed in dry, accessible places and be grouped as far as possible, being mounted, when practicable, upon slate or equally non-combustible back boards. Jackknife switches, whether provided with friction or spring stops, must be so placed that gravity will tend to open rather than close the switch.

FIXTURE WORK:-a. In all cases where conductors are concealed within or attached to gas fixtures, the latter must be insulated from the gas pipe system of the building by means of approved joints. The insulating material used in such joints must be of a substance not affected by gas, and that will not shrink or crack by variation in temperature. Insulating joints, with soft rubber in their construction, will not be approved. (See Definition.) b. Supply conductors and especially the splices to fixture wires, must be kept clear of the grounded part of gas pipes, and where shells are used the latter must be constructed in a manner affording sufficient area to allow this requirement. c. When fixtures are wired outside, the conductors must be so secured as not to be cut or abraded by the pressure of the fastenings or motion of the fixture. d. All conductors for fixture work must have a water-proof insulation that is durable and not easily

nd must not in any case be smaller than No. 18 B. & B. W. G., No. 2 E. S. G. e. All burrs or fins must be fore the conductors are drawn into a fixture. f. The condensation within the pipes should be guarded sealing the upper end of the fixture. g. No combinain which the conductors are concealed in a space less urth inch between the inside pipe and the outside be approved. h. Each fixture must be treated for between conductors and fixtures, for "short circuits" und connections before the fixture is connected to its uctors. i. Ceiling blocks of fixtures should be made g material; if not, the wires in passing through the e surrounded with hard rubber tubing.

LIGHTS ON LOW POTENTIAL CIRCUITS:-a. Must be branch conductors not smaller than No. 12 B. & S. Iust be connected with main conductors only through cut-outs.

ON. RULE 27. FIXTURE WORK:-Section a. Insuto be approved must be entirely made of material st the action of illuminating gases, and will not give n under the heat of an ordinary gas flame. They arranged that a deposit of moisture will not destroy g effect, and shall have an insulating resistance of s between the gas pipe attachments, and be suffito resist the strain they will be liable to in attach

hapter and the one which follows it, we will remains of the code, upon the subject of al systems. This portion of the code deals. etails of electrical construction and electrical While the principles governing construction have been covered in the preceding chapters , still there are certain mistakes which have peatedly and persistently made year after

rules upon these particular points are neces

sary. In electrical, as in mechanical construction, "The strength of any structure is the strength of its weakest part." The points referred to in this part of the code are those which experience has shown to be weak points in our electrical structure. It will be noted that the rules at the head of this chapter, and those immediately following, state specifically that certain things shall and certain things shall not be done. It is impossible in our allotted space to describe the details of electrical construction or electrical appliances. do this so as to give an intelligent idea of an electrical device, to one who has not seen the device, would require the use of illustrations or diagrams and would consume much space. We would suggest that those of our readers who are not familiar with the appearance of the appliances referred to in the code, take this occasion to inspect the things themselves, either at an electrical supply store or in some electric plant, as a knowledge of what the most common electrical devices look like cannot fail to be interesting and will aid one more than anything else to understand their uses and requirements. We make this suggestion, as the names of things electrical are often misleading; for example: a switch might be expected to be a device for switching a current from one path to another, while, as a matter of fact, a switch is the name used in the code for a device to interrupt or open a circuit.

Switches.-We have seen that a branch circuit, to a lamp or to a group of lamps or to a motor, consists of two wires, one a positive wire, by which the current is led from the positive wire of the main circuit to the

to in se

motor, and the other or negative wire, by he current returns to the negative wire of our rcuit. When a switch is so constructed as to th of these wires at the same time, it is called a pole switch." Such switches are required by for all circuits carrying more than six lights, say for any circuit carrying a current of over beres. This is desirable for two reasons: First, sary to have a double pole switch in order to ly disconnect a circuit from the rest of the as to test it. Second, when we use a double ch, we make a break in two places at the same at the arc, which is momentarily formed on ng of a circuit carrying current, is divided and er of burning the switch is thereby greatly . A "firm contact" is necessary as every cons a resistance to the flow of current, and the e contact the greater the resistance. Good maintained by good mechanical construction ving a sufficiently large surface of contact. is formed on breaking a current, the quicker the better for the switch. In order that the st be quick, the switch must be so designed noving part is thrown by a spring which, as acts on the switch, throws it wide open. It irable to have a switch close quickly, and this complished by the use of a spring. When a opened and closed by a spring, it is called a witch. This style of switch is what is referred on "c" Rule 26. Snap switches are required cuits, except circuits carrying large currents.

For large currents the switches are of the "jackknife type and are placed on a switchboard or in a cut-out cabinet. Section "d" refers to the thickness of con

ducting metal. A switch should not heat enough so that the heat can be noticed upon feeling of it with the bare hand. The material of which most switches are made is not more than half as good a conductor as copper, and sometimes it is a very poor conductor. The result is that most switches are too small for the currents which they are intended to carry. Heating is always a sure indication of poor material, insufficient surface or poor workmanship; or a combination of these defects. A "jackknife" switch is the name applied to the form of switch which is almost universally used where snap switches are not required. The form is suggested by the name, the switch being designed so that the blade or metal strip which closes the circuit, shuts into the contacts in the same manner that the blade of a jackknife switch shuts into the handle. These switches may be, but usually are not, equipped with springs to throw the blades. It will be readily seen that a switch of this kind placed vertically upon a wall will be so arranged that the blade, when open, will have a tendency to fall, and this will tend to close the switch, if the contacts are below the blade. If the arrangement is reversed the weight of the blade will tend to keep the switch open. Sometimes a spring is used to keep the switch handle straight out from the wall and sometimes the friction on the joint in which the blade turns is sufficient to hold the blade in any position in which it is placed, but since springs get