This section is from the book "A Treatise On Architecture And Building Construction Vol3: Stair Building, Ornamental Ironwork, Roofing, Sheet-Metal Work, Electric-Light Wiring And Bellwork", by The Colliery Engineer Co.. Also available from Amazon: A Treatise On Architecture And Building Construction.
24. Safety cut-outs are devices that break a circuit before a wire becomes sufficiently heated by the passage of a current to cause danger of fire. They are also used to protect lamps. Fig. 15 shows an incandescent cut-out for small currents. It is called a rosette cut-out, and is principally used on ceilings where a lamp drops from the supply wires. The figure shows the inside view of the two halves. They are both composed of porcelain, upon which metallic connection pieces are screwed. The half B is screwed to the ceiling through the holes h and h1. The ceiling or supply wires are respectively connected to the binding posts p and p1, which are themselves connected to the two projecting elastic plates of metal c and d.
The half A has two projecting metallic pieces m and n, which hook in under c and d, and make the connections when the two halves are put together. The side view of m or n is given at f. Upon each of these pieces at the end that rests against the porcelain is a binding screw s or x. Two small metallic plates, each carrying a pair of binding screws t and v, or z and y are screwed upon the porcelain at diametrically opposite points, and the lamp conductors, entering at hole o, are connected respectively to v and z. If flexible cord is used, it should be knotted as already explained, Art. 21, to sustain the weight of the lamp. Between the two binding screws t and s, as well as between x and y, are respectively connected two strips of a fusible alloy. This alloy melts and breaks the circuit when the current increases above a given value.
The current starts from one supply wire and flows through d, m, and the alloy or fuse wire x y to z. Then it flows through the lamp to v, through the fuse wire t s to c, and out to the other supply wire. The two halves are connected by a screwing motion which rubs the contact pieces together.
25. A convenient form of fuse is shown in Fig. 16, called a plug cut-out, which is intended to be screwed into a socket similar to that used for lamps, and may be employed on branch circuits which do not carry a heavy current. The screw sheathing s and plug end p are insulated from each other, except when joined by the fuse wire f, which is soldered between them. A brass cap is screwed on the larger end of the plug at c to protect the fuse.
26. The ordinary form of detachable fuse is shown in Fig. 17. The contact pieces a and b are made of sheet copper, and are intended to be clamped by screws to the cable tips or terminals at the distributing boxes or closets where the different circuits branch off. A strip c of fusible lead alloy is soldered to each contact piece, its cross-section being proportional to the maximum current to be carried, which is stamped on the copper ends.
27. A good form of connection between the mains and a branch circuit is illustrated in Fig. 18. This is known as a branch block. The mains may be connected at m, m'; the wires pass under the projecting ledges l, l'; and the branch wires are secured at e, e'. The fuses are held between the screws a and b, c and d. To prevent damage when a fuse "blows," or melts, a porcelain cover is fitted over the face of the block.
28. For the large currents of feeders or mains a double porcelain fuse block, as in Fig. 19, is used, the wires from the point of supply being inserted at one end, as at m, m, and the line continued from the terminals m', m', at the other end. The fuses are inserted between the screws a and b, and between c and d. The two sides of the circuit are separated by the partition p, so that all danger of short circuit is eliminated.
29. As a guide to the carrying capacity of fuses, the following table may be consulted, but it is to be pointed out that the fusing current depends upon the particular proportion of the metals used in the alloy, and their selection; also on the length of fuse and the character of the terminals.
B. & S. Gauge (Approximate).