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
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 of