TM5-811-1/AFJMAN
32-1080
"Normal" and which is "Backup"; alarm in case of
static wires and aerial terminals installed above
transfer; alarm in case of loss of a source; and time
and on poles or structures to provide the necessary
delay on retransfer will be specified depending
"cone of protection." Ground conductors will be
upon the application.
grounded to the station ground grid and will be
e. Other equipment and personnel protection.
protected against physical damage and corrosion to
(1) Surge protection and grounding. Ground-
terminations for ground conductors.
ing, and surge protection against lightning and
(3) Foundations. Foundations will be designed
switching surges, are discussed in chapter 9.
to support static, dynamic, and seismic loads of
(2) Station enclosure. A station fence, with
station elements. The designer will formulate
three strands of barbed wire above a seven-foot
foundations details based on the maximum loading
high fence fabric, is the minimum requirement.
on foundations by the equipment specified. A
Other station enclosure materials, or heights, may
maximum soil bearing pressure of 4,000 pounds
be required to provide equipment masking, sound
per square foot will be used as a basis of design.
attenuation, or protection against sabotage. A
However, since sandy or soft clay soils can have
minimum lo-foot wide vehicle gate, a 3-foot wide
soil bearing pressure of as low as 2,000 pounds per
personnel gate, and a sufficient access space for
square foot, a knowledge of the actual site condi-
removal and replacement of station elements is
tions may be necessary. When necessary to deter-
required to permit maintenance or modifications to
mine the actual soil type and bearing pressures,
the station without interruption to the electrical
soil borings will be made and the resulting analy-
service. Fencing will be grounded in accordance
sis will be used in the design. The guidelines
with IEEE Std 80 and the NESC.
contained in NEMA SG 6 will be used during
f. Station protection and structures.
designs, and will include a minimum safety factor
(1) Station line structures. The standard de-
of 1.5 for overturning loads.
sign of the manufacturers of aluminum or galva-
(4) Station and substation insulators. Suspen-
nized steel structures will be used to avoid the
sion insulators will be used to dead-end incoming
greater costs associated with specially designed
line conductors, and apparatus post insulators will
structures. Structures will be designed to with-
be used where conductors terminate on apparatus.
stand all dynamic, static, or seismic forces that are
Table 4-5 lists ratings of primary insulators by
likely to be imposed on structures during a 25-year
class. When specifying suspension insulators, se-
life of the station, without damage or failure. A
lect the appropriate ANSI Class dependent upon
minimum of 1,000 pounds of tensile force will be
whether ball and socket (Class 52-3) or clevis eye
assumed for stranded conductors to be terminated
(Class 52-4) suspension insulators are required or
on station structures when conductors originate
whether the choice can be a Contractor's option.
and terminate within the station, or when the
The number of suspension insulators in tandem
station is supplied by incoming line conductors
and the choice of the NEMA Technical Reference
installed slack between the last pole or structure
Number (TR) for apparatus post insulators are
and the dead-end pole or structure within the
dependent upon the voltage level and the degree of
station. Figure 4-5 is an example of a substation
atmospheric contamination. Lower ratings and
with incoming line structures for incoming lines
fewer numbers of units than those shown in table
rated 46 kV or above. Switching stations or sub-
4-5 will not be used. Use the lower TR number in
stations with primary protective devices, and un-
areas where the atmosphere is dry or where fog
derground connections to the utility line, are all
occurs only to a limited degree and there is no
that is necessary for an incoming line voltage of
more than a moderate contamination from indus-
35kV or less. Figure 4-6 is an example of a
trial type of activities. Use the higher TR number
switching station that is suitable when the incom-
in areas where the atmosphere can be damp
ing line voltage is 35 kV or less. Fenced outdoor
because medium to heavy fog is a common occur-
switchgear will be less costly than indoor switch-
rence and there is a medium contamination.
(5) Station lighting. Lighting levels will be in
gear installed in a concrete-block structure with-
out fencing. Aesthetic features and requirements
accordance with the levels listed in the NESC.
may determine whether an indoor or outdoor
4-9. Substation Equipment at Air Force Instal-
installation is specified.
lations.
(2) Protection. The main electric supply sta-
a. Switchgear. Requirements are as follows:
tions will be protected against "lightning strikes"
(1) Incoming supply section:
and the effects of lightning on incoming aerial
(a) Switches. Loadbreak/load interrupting,
lines. Protection of stations against lightning
strikes to the station elements will be provided by
current limiting integrally or separately fused,
4-13
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