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Selection of Proper Fuses
glass cylinder. Some fiber and plastic fuse cylinders
are filled with nonconducting powder. The smaller
Individual fuses are provided on the IC
fuses are used in circuits up to 60 amperes and are
switchboards for each associated circuit. A separate
made in the ferrule, or round-end cap, type. Large
fuse in each line of each circuit has the effect of
sizes with short flat blades attached to the end caps are
considerably increasing the maximum short-circuit
rated from 65 to 200 amperes. These blades fit tightly
current that the fuses can safely interrupt. It also
into clips on the fuse block similar to knife-switch
provides greater protection to the remaining circuits
energized from the same bus in case of a possible
Cartridge fuses are made in capacities of 1
defect in one fuse.
through 1000 amperes for voltages of 125, 250, 500,
600, and 1000 volts. Fuses intended for 600- and
In general, fuse ratings should be approximately
1000-volt service are longer and do not fit the same
10 percent above the maximum continuous connected
fuse holders intended for lower volt service. Fuses of
load. In circuits, such as call bell systems and alarm
different ampere capacity are also designed for
systems where only a small portion of the circuit is
different sizes of holders. For example, fuses of 1
likely to be operated at any one time, the fuse rating
through 30 amperes fit one size of holder, and fuses
should be 10 percent greater than the load of one
with capacities of 35 through 60 amperes fit a
associated group of signals operated, or 15 percent of
different size holder.
the total connected load, whichever is greater. Where
the circuit incorporates branch fuses, such as those
Cartridge fuses in IC equipment are of various
associated with the fire-control (FC) switchboards,
sizes, such as the miniature FO2 or FO3 (1 1/4- by
the rating of the fuses on the IC switchboard should
1/4-inch) fuse rated from 0.1 to 30 amperes at 125
be 20 percent above the maximum connected load to
volts and the midget FO9 (1 1/2- by 13/32-inch) fuse
provide sufficient margin so branch fuses will always
rated for 0.1 through 30 amperes at 125 volts. The
blow before the main fuses. In no case should the fuse
standard 2- by 9/16-inch fuse is rated from 1 to 30
rating be greater than 2 1/2 times the rated capacity of
amperes, 500 volts for ac service and 250 volts for dc
the smallest cable in the circuit. If too large a fuse
service. Fuses above 60-ampere capacity have
were used, a fire hazard would exist.
knife-blade contacts and increase in diameter and
length as the capacity increases.
Fuse Holders
Before fuses of greater than 10-ampere capacity
The type EL-1 fuse holder consists of a base and
are pulled, the switch for the circuit should be opened.
a plug, as shown in figure 2-31. The base extends
Whenever possible, this precaution should be taken
behind the panel, and the plug containing the fuse is
before any size fuse is pulled or replaced. Approved
screwed into the base. Behind a hole in the plug cap is
fuse pullers must be used for removing fuses. Fuses
a small neon lamp that serves as a blown-fuse
should never be short-circuited or replaced with fuses
indicator, lighting when the energized circuit through
of larger current capacity.
the holder is interrupted by the blowing of a fuse.
Series resistors of different values are used with the
Time-Delay Fuses
lamp on 125- and 250-volt circuits, except for the
midget holder, which is rated for 125 volts only.
Time-delay fuses are used in motor supply
c i r c u i t s , for example, where overloads and
The types FHL10U, FHL11U, and FHL12U (fig.
motor-starting surges of short duration exist. A
2-32) consist of a fuse holder body and a fuse carrier.
conventional fuse of much higher rating would be
The body is mounted on the panel, and the carrier with
required to prevent blowing of the fuse during surges.
the fuse placed in the clips is inserted into the body in
Because of its high rating, this fuse could not provide
a manner similar to inserting a bayonet-type lamp into
necessary protection for the normal steady-state
a socket. Removal of the fuse is accomplished by
current of the circuit.
p u s h i n g and turning the fuse carrier in a
counterclockwise direction, again similar to the
Time-delay fuses are rated as to their time lag
removal of a bayonet-base lamp. The types FHL10G
characteristics with a minimum blowing time at some
and FHL11G accommodate 1 1/4- by 1/4-inch fuses.
overload current. A typical rating is 12 seconds
The type FHL10G will hold two fuses and can
minimum blowing time at 200 percent rated current.

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