Time-Delay Fuses

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

clips.

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.

2-20