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 TM 5-684/NAVFAC MO-200/AFJMAN 32-1082
CHAPTER 8
OVERCURRENT PROTECTIVE AND SWITCHING DEVICES
Section I-CONSIDERATIONS
8-2. location of protective and switching de-
8-1. Circuit interrupting devices.
vices.
This chapter describes circuit interrupting devices
that can make (close), break (open), or modify the
These devices may be self-contained units, or in-
connections of an electrical system either under nor-
stalled in assemblies with other devices. The units
mal or (as a protective device) abnormal conditions
or assemblies may be installed indoors or outdoors
or both. Included are fuses, switches, circuit break-
in enclosures suitable for protected or exposed in-
ers, circuit switchers, and reclosers.
stallations respectively. Devices will normally be in-
a. Purpose of devices. Each of the devices pro-
sulated for medium-voltage and high-voltage levels,
vides one or more of the functions listed below:
except for secondary switchgear.
(1) Switching-opening and closing of ener-
gized or de-energized circuits.
8-3. Protective and switching device instruc-
(2) Overcurrent protection-circuit interrup-
tion manuals.
tion under excessive or fault current conditions.
Neither adjustments nor replacements of parts of
(3) Automat ic reclosing after overcurrent open-
switching apparatus should be attempted without
ing.
first consulting the manufacturer's instruction
b. Devices may be classified by their insulating
manuals. If the manuals furnished with switching
medium, such as:
apparatus, especially any power-operated switching
(1) Air
devices and any circuit breakers, are unavailable,
(2) Vacuum
every effort should be made to obtain copies from
(3) Sulfur hexifluoride (SF,)
the nearest office of the applicable manufacturer.
(4) Oil
(5) A combination of the above
8-4. Protective and switching device records.
c. Devices may be operated in the following man-
ner:
Service operating records should be maintained on
all switching apparatus. These records should in-
(1) Internal action
clude a history of all ampere ratings or settings,
(2) Manually operated external action
operations, and maintenance and inspections.
(3) Automat ically operated external action
Section II-FUSES
8-5. Fuse usage.
ing the circuit. The fuse tube also provides the
means of making contact with the rest of the elec-
Fuses provide relatively inexpensive protection by
trical circuit.
opening an electric line when a short circuit or over-
b. Types. Only medium- and high-voltage fuses
load occurs on the load side of the fuse. Always
are covered in this manual. Most fuses commonly
remember that a fuse is a single-phase device.
used on facility electrical distribution systems are
a. Construction. A fuse is designed to be an
distribution fuse cutouts and power fuses. The ap-
intentionally-weakened link in an electric circuit
plicable industry standards differentiate between
and to be the first point of failure.
the two categories on the basis of their dielectric
(1) Fuse link. A fuse link uses a metal such as
withstand or basic insulation level (BIL) either at
silver, tin, lead, copper or any alloy, which will melt
distribution or power levels respectively. Fuse selec-
when a predetermined current is maintained for a
tions are also influenced by their installation appli-
predetermined time period. The fuse's melting cur-
cation.
rent (rating) is selected to permit severing the cir-
(1) Distribution fuse cutouts. A distribution
cuit before the same current could damage the elec-
cutout provides a mounting for the fuse element.
trical system.
Some cutouts have arc chutes designed for load-
(2) Fuse tube. A fuse tube is provided to pre-
break operation. Fuse cutouts are not usually pro-
vided for switching circuits, but for protection of
vent damage from the melting fuse link, which oth-
overhead equipment and sectionalizing of lines.
erwise might start a fire from possible flying metal,
Cutouts normally use an expulsion fuse, wherein
and to aid in quenching the arc developed by sever-
8-1
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