Power Distribution Equipment 241
High-voltage Fuses
Since power lines are frequently short circuited, various protective
equipment is used to prevent damage to both the power lines and the
equipment. This protective equipment must be designed to handle high
voltages and currents. Either fuses or cuit breakers cir may be used to pro-
tect high-voltage power lines. High-voltage fuses (those used for over 600
volts) are made in several ways. An expulsion-type fuse has an element that
will melt and vaporize when it is overloaded, causing the power line it is
connected in series with to open. Liquid fuses have a liquid-filled metal en-
closure that contains the fuse element. The liquid acts as an arc-suppress-
ing medium. When the fuse element melts from an excessive current in
a power line, the element is immersed in the liquid to extinguish the arc.
This type of fuse reduces the problem of high-voltage arcing. A solid-mate-
rial fuse is similar to a liquid fuse, except that the arc is extinguished in a
chamber filled with solid material.
Ordinarily, high-voltage fuses at substations are mounted adjacent to
air-break disconnect switches. These switches provide a means of switching
power lines and disconnecting them for repair. The fuse and switch enclo-
sure is usually mounted near the overhead power lines at a substation.High-voltage Circuit Breakers
Circuit breakers that control high voltages are also located at electrical
substations. In this type of circuit breaker, the contacts are immersed in an
insulating oil contained in a metal enclosure. Another type of high-volt-
age circuit breaker is the magnetic air breaker in which the contacts separate,
in the air, when the power line is overloaded. Magnetic blowout coils are
used to develop a magnetic field that causes the arc (which is produced
when the contacts break) to be concentrated into arc chutes where it is
extinguished. A modification of this type of circuit breaker is the com-
pressed-air circuit breaker. In this case, a stream of compressed air is con-
centrated on the contacts when the power line is opened. The compressed
air aids in extinguishing the arc that is developed when the contacts open.
It should be pointed out that large arcs are present whenever a high-volt-
age circuit is interrupted. This problem is not encountered to any great
extent in low-voltage protective equipment. There are two major types of
high-voltage circuit breakers—oil filled and oilless.
These circuit breakers are designed to operate on voltages of 1000
volts to over 500,000 volts. Oil-filled circuit breakers are used primarily for
outdoor substations, except for very high voltages in the range of 500,000