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 4.5.3.2 Plug-Type Fuses. Plug-type fuses have a standard screw base and are rated at 125
V with current ratings not exceeding 30 A.
4.5.4 High-Voltage Fuses. High-voltage fuses are rated above 600 V and are divided into
three classifications: (1) power fuses, (2) distribution fuses, and (3) oil fuses.
4.5.4.1 Power Fuses. A power fuse is a fuse consisting of a fuse support and a fuse unit or
a fuse holder that includes the refill unit or fuse link. Power fuses are divided into two main
categories, expulsion and current-limiting.
(a) There are two main types of expulsion power fuses in common usage, the fiber-tube
fuses and the solid material fuses. The fiber-tube fuse usually has a renewable-type link in a
vented tube holder. Gases and pressure produced by the arc and the fiber lining of the tube,
sometimes aided by a spring, extinguish the arc. These fuses are limited to outdoor use in a
location away from personnel.
Solid member fuses have a solid fusible member located in the center of a hollow cylinder. The
hollow cylinder generally consists of dry compressed boric acid and is under spring tension prior
to the operation of the fuse. When the fusible member opens the spring withdraws one end of
the member upward through the boric acid chamber lengthening the arc path. The intense heat
produced by the arc decomposes some of the compressed boric acid, resulting in the formation of
water vapor and inert boric oxide. The interruption of the arc is achieved by the deionizing
action of the steam and the high particle turbulence of boric oxide. This causes the rate of
deionization to exceed the rate of ionization of the arc. The boric acid fuse is inherently fast and
the instant of fault occurrence. Because the expelled gases are nontoxic, these fuses can be used
indoors within an enclosure provided a discharge filter or snuffer is used to contain the
explosion.
(b) Used alone or in combination with interrupter switches or circuit breakers,
current-limiting fuses provide high interrupting capability at relatively low cost. These fuses
are designed to limit the flow of fault current by opening the circuit within one-fourth cycle
provided the threshold magnitude is exceeded. A current-limiting fuse, however, will react to a
low magnitude fault current like any other fuse. Current-limiting fuses use silver-sand
construction. The current-limiting action is achieved through the melting of the sand by the fault
current into a high resistance glass-like compound which in turn chokes off the fault current
before peak value is reached. The fast operation limits the fault experienced by series system
components. Because the applied voltage is critical, a current-limiting fuse must not be used for
voltage other than design voltage. A current-limiting fuse with a rating of 200,000 A.I.C. can be
used for applications with available fault current of 200,000 A, but the fuse will not withstand a
current of 200,000 A. Because the current-limiting fuse interrupts the fault current so rapidly, a
voltage surge is often generated which may damage upstream lightning arresters.
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