TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
the arc produces a gas. A blown fuse may be indi-
a. Nonload-break devices. A nonload-break fused
cated by dropping of the fuse, dropping of the door
device energizing a circuit following fuse replace-
on an enclosed cutout, or by loss of an expendable
ment should not be reopened unless the fuse has
again blown or the circuit has been de-energized.
vent cap, which yields to relieve the internal pres-
b. Load-break devices. A load-break fused device
energizing a circuit following fuse replacement
(2) Power fuses. Power fuses can be fixed or
should not be reopened immediately. The time delay
dropout expulsion noncurrent-limiting type (solid
before reopening must allow the fuse to blow if
material boric acid) or nonexpulsion current-
there is an existing fault current beyond the load-
limiting type (silver element with high-purity silica
break rating of the device. For fuse links of more
sand). The reduced expulsive emissions of boric acid
than 100 amperes, this time delay could be as long
fuses permit their use in enclosures.
as 10 minutes.
(3) Selection by location. The selection by loca-
c. Open fuse holder. Outdoor fuses should be
tion is based on installing fuses which liberate gases
closed as soon as possible. Fuses left hanging for
(that is, some expulsion fuses) in outside locations,
extended periods can undergo water damage and
where protective enclosures such as switchgear are
warpage, making reclosing of an energized circuit
not required. Current-limiting fuses are expensive
as compared to expulsion fuses.
(a) For use within confined spaces. Power
8-7. Fuse replacement.
fuses of the nonexpulsion current-limiting (silver-
Make certain that fuses, whether new or replace-
sand) type and expulsion solid material (boric acid)
ments, are of the proper type and rating. Never
type comprise the majority of fuses suitable for ap-
replace one type of fuse arbitrarily with a different
plication within buildings, vaults, or enclosures.
type of fuse of the same physical size, or with a fuse
(b) For outdoor applications. Distribution
having a different current rating, without specific
fuse cutouts have a mechanical construction
engineering direction. Noncurrent-limiting fuses
adapted to pole or crossarm mounting. Power fuses
should not be used to replace current-limiting fuses.
can also be used.
a. Spare fuse units and replaceable parts. Mark
(4) Other types of medium-voltage fuses. Other
and store parts for re-energizing, after locating and
types of fuses used on medium-voltage lines are
correcting the situation that caused the fuse to blow.
current-limiting protectors, electronic fuses, liquid-
A potential hazard may exist, if the circuit is re-
type power fuses, and oil fuse cutouts.
energized with the fault condition still present.
(a) High-current or high-speed interruption.
(1) Marking of spare fuses and parts. Spare
A current-limiting protector can carry high currents
fuse units should be suitably marked, coded, or in-
and yet limit let-through short circuit currents.
dexed to show the mounting, circuits, or equipment
Electronic fuses provide high-speed interruption of
with which they are to be used; especially if several
fault currents. Both are more expensive than distri-
types and ratings are used in a given location. This
bution fuse cutouts or power fuses, and their use is
minimizes the possibility of improper application.
limited for most facility applications.
(2) Storing spare fuse units and parts. Store
(b) Liquid fuses. Liquid-type noncurrent-
spare fuse units and replaceable parts of fuse units
limiting power fuses have lower continuous and in-
so they will not be damaged, and will be readily
terrupting current capacities than other fuses.
available when needed.
Liquid-type fuses have been used for wood-pole
mounted applications in high-risk fire areas. The
b. Fuses subject to partial melting or deteriora-
arc-quenching liquid is a petroleum product, which
tion. Fuses can be partially melted or damaged by
is not considered environmentally advisable so they
fault currents of insufficient magnitude and melting
should not be replaced. A new fuse holder will also
time to cause complete melting. Observe the follow-
be required, since the liquid-type fuse holder will
not accept other types of fuses.
(1) In two- or three-phase applications, replace
(c) Oil fuse cutouts. Oil fuse cutouts are not
fuses in, all phases, when fuses in one or more
used as frequently as in the past, because of both
phases are blown.
environmental concerns and their lower interrupt-
(2) In applications where fuses are used in se-
ries with other fuses or interrupting devices in the
same phase, in such a manner that their melting or
8-6. Fuse operating safety considerations.
clearing curves, or both, cross one another, it is
When operating fused devices, the following consid-
advisable to consider that the blowing of one makes
the other unsuitable for continued service.