TM
5-811-1/AFJMAN
32-1080
cutouts will be of the loadbreak type. When the
is necessary for operation or maintenance and the
application does not require a switchgear. They
underground feeder supplies only one transformer
can be used for opening a de-energized circuit
installation in the immediate vicinity of the riser
during maintenance for fault isolation at a much
pole, nonloadbreak fused cutouts may be accept-
lower cost than a fused or non-fused disconnect
able. Where the capacity of the line is more than
switch. They can also be used to open a de-
the maximum ampere rating of fuse cutouts,
energized radial feeder for maintenance, again at
power fuse units will be installed. Installation of
less cost than a disconnect switch. If provided in a
lines having an ampere rating above that of power
project design, the contract specifications shall
fuses will be avoided, since more expensive protec-
include provision of standoff bushings for phase
tion devices such as those covered in chapter 4,
isolation ("parking") of each conductor, and in-
will be necessary. Group-operated loadbreak
struction signs on the enclosure which state that
switches
sectionalizing
used
for
aerial-to-
the connectors are not to be disconnected/con-
underground connections can be either integrally
netted while energized.
fused or nonfused devices. Overcurrent protection
5-8. Joint Electrical/Communication lines for
is necessary to protect underground systems.
Air Force Installations.
Group-operated loadbreak switches at transition
a. Electrical. Where overhead construction is
poles are not justifiable solely on the basis of
required, electrical distribution shall be located
preventing possible ferroresonance, since such
along street or roads to avoid the use of separate
switches at ground-mounted transformers will
poles for street lights.
eliminate most causes of transform ferroresonance.
b. Poles, duckbanks, and manholes:
5-7. Pad Mounted Sectionalizing Equipment.
(1) For economy, telephone lines and televi-
sion cables shall be carried on the same poles or in
a. Pad-mounted, metal-enclosed switchgear. Pad-
the same ductbank as electrical power lines.
mounted, metal-enclosed switchgear (air-, oil-, gas-,
(2) Joint use of manholes is permitted if the
or vacuum-insulated) may be utilized to provide
power cables and communications cables are sepa-
group-operated, load-break switch operation, section-
rated from each other by a masonry wall and a
alizing points for large underground distribution
separate manhole cover is installed in each sec-
systems, and over-current protection (fuses) for
tion.
lateral feeders from a main feeder. Additionally,
c. Additional factors. Underground construction
they may be employed to provide group-operated,
for either electrical distribution or communications
load-break switch operation for large three-phase
systems shall meet the provisions of this manual
loads requiring interruption of all phases simulta-
and technical requirements such as airfield clear-
neously. Switchgear is also available to provide
ance, proximity to radio and radar installations,
manual switching between alternate primary
and security control of closed circuits, unless spe-
sources. Application of switchgear will be based on
cifically authorized by a waiver of these criteria.
operational needs, protective devices coordination
d. Overhead cable limits. No more than two
requirements, size of the primary system and
telephone cables shall normally be installed per
loads, and cost comparisons among alternatives.
pole. Quantity of twisted pairs and conductor sizes
b. Pad-mounted sectionalizing terminals. These
(AWG) for cables exceeding diameter and weight
devices are dead-front, cable-terminating enclo-
limitations begin with:
sures which employ separable, load-break connec-
tors and load-break, feed-through bushings. They
2-inch diameter
5 lb/ft
do not provide over-current protection. They pro-
Quantity-AWG
Quantity-AWG
vide a means to splice cables above ground and
303-19
303-19
provide lateral feeder taps from a main feeder,
606-22
508-22
while maintaining the ability to sectionalize the
909-24
909-24
underground cables at that point for operation and
e. Routing of telephone cable. Telephone cables
maintenance. These devices will not be substituted
for pad-mounted, metal-enclosed switchgear and
shall not be installed on poles until arrangements
should be used only where isolation of lateral feeds
have been made for adequate guying.
5-7
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