An air-circuit breaker switch can have both blade
and stationary contacts equipped with arcing horns (fig.
4-36). These horns are pieces of metal between which
the arc forms when a circuit-carrying current is opened.
These arc horns are drawn further and further apart until
the arc finally breaks. Air-break switches are usually
mounted on substation structures or on poles and are
operated manually from the ground. In a three-phase
circuit all three switchesone for each phaseare
opened and closed together.
An air-disconnect switch is not equipped with
arcing horns or other load-break devices. It therefore
cannot be opened while current is flowing (fig. 4-37). If
the disconnect switch should be opened while current is
flowing in the line, an arc would likely be drawn
between the blade and its stationary contacts. The hot
arc would melt part of the metal, thereby damaging the
switch. The purpose of a disconnect switch is to isolate
a line or a piece of equipment for the purpose of making
the disconnected line or equipment dead electrically,
thus making it safe for repairs, tests, or inspections.
Figure 4-36.Gang-operated air-circuit breaker.
Figure 4-37.Air-disconnect switch.
An oil switch is a high-voltage switch whose
contacts are opened and closed in oil. Oil switches may
be used as disconnect, circuit breakers, or reclosers.
The switch is actually immersed in an oil bath,
contained in a steel tank, as shown in figure 4-38. The
reason for placing high-voltage switches in oil is that the
oil may help to 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, however, the oil helps to
quench the arc. Oil is an insulator and, therefore, helps
to quench the arc between the contacts. 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. But if the voltage of the line is high, the three poles
of the switch are placed in separate containers.
The circuit reclosers most commonly used in power
distribution are electronic reclosers, oil reclosers, or
vacuum reclosers. These reclosers basically operate in
the same manner.
Reclosers come in single- or three-phase models
and can either be pole-mounted or installed in a
substation. These reclosers are for overload protection
and are designed to open a circuit in an overload
condition and then automatically reclose the circuit. If
the fault on the system has cleared, the recloser remains
closed. If the fault has not cleared, the recloser trips