entire circuit. The use of film-disk cutouts (fig. 6-17) in the lamp socket prevents lamp failure from interrupting the circuit. The cutouts consist of two metal disks separated by a thin film of insulating material. The insulating film is held in place by the spring pressure of the contact disks. When the filament of the lamp burns out, the entire circuit voltage appears across the film disk. This is more than sufficient to puncture the film and close the circuit between the two metallic disks, thereby bypassing the burned-out filament. In later series circuits, an isolation transformer is used to eliminate the need for the film disk fixture. The primary winding of the isolation transformer is connected in series with the power source and the secondary winding provides power to the light bulb. Since the primary winding is isolated from the secondary winding, a burned out bulb will not interrupt the continuity of the lighting circuit.
A series circuit is installed using only one wire, as shown in figure 6-18(a). Some of the lamps are connected in the outgoing wire, and the rest are connected in the return wire. This is called an "open- loop" series circuit. An open-loop circuit is less expensive initially, but troubleshooting is difficult, time consuming, and costly.
To make it possible to locate a fault like an open circuit or a ground, it is desirable to bring the outgoing and return conductors close together in numerous places so that the circuit can be easily short-circuited. Such a circuit is called a "closed-loop" circuit, as shown in figure 6-18(b). Sometimes the circuit is arranged to combine the open- and closed-circuit features, as shown
Figure 6-17. - Series lamp, socket, and film-disk cutout.
Figure 6-18. - Diagrams of (a) open loop, (b) closed bop, and (c) combined open and closed loop series circuits.
in figure 6-18(c). The use of the closed-loop or the combination circuit makes troubleshooting easier.
Installing the series circuit on the same crossarm as the primary-distribution conductor is usually the most economical. When two primary crossarms are used, the streetlight wires should be carried on the lower arm in the end-pin position. When two separate single- conductor street circuits are on the same crossarm, they should not be placed in adjacent pin positions because of confusion in troubleshooting.
Insulator sizes should be based on the open-circuit voltage of the largest regulator used and are usually the same size as those used for primary distribution. White insulators should be used on a series street circuit to distinguish them from the primary distribution insulators and to assist in identifying the circuits for operating and maintenance work. Small strain insulators should be used for cutting in individual lamps or loops of five lamps or fewer. Equivalent voltage insulators with automatic line splices may also be used. If the loop consists of more than five lamps, a primary disk insulator is used. The insulator is usually cut in after the conductors have been strung.
The conductor size should be No. 6 medium hard- drawn copper or its mechanical equivalent. Although No. 8 hard-drawn copper is usually too weak for longer spans, the use of copperweld or similar conductors ofContinue Reading