This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
The Rules of the National Electric Code require that conduits entering junction-boxes, outlet-boxes, or cut-out cabinets shall be provided with approved bushings, fitted to protect the wire from abrasion.
Fig. 51 shows a typical form of conduit bushing. This bushing is screwed on the end of the conduit after the latter has been introduced into the outlet-box, cut-out cabinet, etc., thereby forming an insulated orifice to protect the wire at the point where it leaves the conduits , and to prevent abrasion, grounds, short circuits, etc. A lock-nut (Fig. 52.) is screwed on the threaded end of the conduit before the conduit is placed in the outlet-box or cut-out cabinet, and this lock-nut and bushing clamp the conduit securely in position. Fig.
53 shows a terminal bushing for panel-boxes used for flexible steel conduit or armored cable.
The Rules of the National Electric Code require that the metal of conduits shall be permanently and effectually grounded, so as to insure a positive connection for grounds or leaking currents, and in order to provide a path of least resistance to prevent the current from finding a path through any source which might cause a fire. At outlet-boxes, the conduits and gaspipes must be fastened in such a manner as to insure good electrical connection; and at centers of distribution, the conduits should be joined by suitable bond wires, preferably of copper, the said bond wires being connected to the metal structure of the building, or, in case of a building not having an iron or steel structure, being grounded in a permanent manner to water or gas piping. Fuse-Boxes, Cut-Out Panels, etc. From the very outset, the necessity was apparent of having a protective device in circuit with the conductor to protect it from overload, short circuits, etc. For this purpose, a fusible metal having a low melting point was employed. The form of this fuse has varied greatly. Fig. 54 shows a characteristic form of what is known as the link fuse with copper terminals, on which are stamped the capacity of the fuse.
The form of fuse used probably to a greater extent than any other, although it is now being superseded by other more modern forms, is that known as the Edison fuse-plug, shown in Fig. 55. A porcelain cut-out block used with the Edison fuse is shown in Fig. 56.
Fig. 51. Conduit Bushing.
Fig. 52. Lock-Nut.
Fig. 53. Panel-Box Terminal Bushing. Courtesy of Sprague Electric Co., New York, N. Y.
Within the last four or five years, a new form of fuse, known as the enclosed fuse, has been introduced and used to a considerable extent. A fuse of this type is shown in Fig. 57. Fig. 58 gives a sectional view of this fuse, showing the porous filling surrounding the fuse-strips, and also the device for indicating when the fuse has blown. This form of fuse is made with various kinds of terminals; it can be used with spring clips in small sizes, and with a post screw contact in larger sizes. For ordinary low potentials this fuse is desirable for currents up to 25 amperes; but it is a debatable question whether it is desirable to use an enclosed fuse for heavier currents. Fig. 59 shows a cut-out box with Edison plug fuse-blocks used with knob and tube wiring. It will be seen that there is no connection compartment in this fuse-box, as the circuits enter directly opposite the terminals with which they connect.
Fig. 54. Copper Tipped Fuse Link.
Fig. 55. Edison Fuse-Plug. Courtesy of General Electric Co., Schenectady, N. Y.
Fig. 60 shows a cut-out panel adapted for enclosed fuses, and installed in a cabinet having a connection compartment. As will be seen from the cut, the tablet itself is surrounded on the four sides by slate, which is secured in the comers by angle-irons. The outer box may be of wood lined with sheet iron, or it may be of iron. Fig. 61 shows a door and trim for a cabinet of this type. It will be seen that the door opens only on the center panel, and that the trim covers and conceals the connection compartment. The inner side of the door should be lined with slate, and the inner side of the trim should be lined with sheet iron. Fig. 62 shows a sectional view of the cabinet and panel. In this type of cabinet, the conduits may enter at any point, the wires being run to the proper connectors in the connection compartment.
Fit. 56. Porcelain Cutout Block.
Fig. 57. Enclosed or Cartridge Fuse.
Fig. 58. Section of Enclosed Fuse.
Figs. 63 and 64 illustrate a type of panel-board and cabinet having a push-button switch connected with each branch circuit and so arranged that the cutout panel itself may be enclosed by locked doors, and access to the switches may be obtained through two separate doors provided with latches only. This type of panel was arranged and designed by the author of this instruction paper.