farther and farther apart, thereby lengthening the arc
until it finally breaks.
Air-break switches are usually mounted on
substation structures or on poles and are operated from
the ground level (fig. 4-35). In a three-phase circuit all
three switches. one in each phase. are opened and
closed together as a gang, as the system is called. The
switches can be operated by a handle connected to the
rod extending from the switch to the base of the pole.
Remember, the opening of a three-phase gang switch is
a two lineman project. One lineman opens the switch;
the other lineman is for safetyhe watches to ensure
that all phases open. Many of the air rods extending
from the air-break switches are operated by
mechanized equipment connected to the switch-
operating rod from a remote location. The automation
ofdistribution circuits has resulted in the installation of
many mechanized operators for key break switches so
that they can be controlled from a central operations
center. Switches for underground distribution circuits
usually are installed in pad-mounted switchgear. The
switches are operated with the cabinet doors closed to
provide protection for the lineman or cableman.
Disconnect Switch.A disconnect switch is an
air-break switch not equipped with arcing horns or
other load-break devices. The disconnect switch
cannot be opened until the circuit in which it is
connected is interrupted by. some other means, such as
a portable load break tool attached to a hot-line tool. If
a disconnect switch is opened while current is flowing
in the line. an arc is likely to be drawn between the
blade and the stationary contact where the arc might
easily, jump across to the other conductor or to some
grounded metal and cause a short circuit. The hot arc
also could melt part of the metal. thereby, damaging
Figure 1-35.Gang-operated three-pole air-break switch with
arc interrupters (in the open position).
Disconnect switches are used to complete a
connection to or isolate the following:
Two energized transmission or distribution
Transmission or distribution lines from
A distribution feeder circuit and a branchcircuit
Disconnect switches are frequently used to isolate
a line or an apparatus, such as a transformer. to
complete maintenance work. In most circumstances, it
is necessary to test the equipment for high voltage and,
if proved de-energized, to ground it before the
maintenance work is performed.
OIL SWITCH.An oil switch is a high-voltage
switch whose contacts are opened and closed in oil.
The switch is actually immersed in an oil bath and
contained in a steel tank. The reason for placing high-
voltage switches in oil is that the oil will break the
circuit when the switch is opened. With high voltages.
a separation of the switch contacts does not always
break the current flow because an electric arc forms
between the contacts. If the contacts are opened in oil.
the oil will quench the arc. Furthermore. if an arc
should form in the oil. it will evaporate part of the oil
because of the high temperature and will partially fill
the interrupters surrounding the switch contacts with
vaporized oil. This vapor develops a pressure in the
interrupters which assists in quenching or breaking the
arc by elongating the arc.
The three lines of a three-phase circuit can be
opened and closed by a single oil switch. If the voltage
is not extremely high. the three poles of the switch are
generally in the same tank (fig. 3-36): but if the voltage
of the line is high. the three poles of the switch are
placed in separate oil tanks. The poles are placed in
separate tanks to make it impossible for an arc to form
between any two phases when the switch is opened or
closed. An arc between phases would be a short circuit
across the line and would probably blow up the tank.
When an oil switch is to open the circuit
automatically because of an overload or short circuit. it
is provided with a trip coil. This trip coil consists of a
coil of wire and a movable plunger. In low-voltage
circuits carrying small currents. this coil is connected
in series with the line. When the current exceeds its
permissible value. the coil pulls up its plunger. The
plunger trips the mechanism, and a spring opens the