TM
5-811-1/AFJMAN
32-1080
(4) Secondary class. Secondary arresters will
be used only for low-voltage services at munitions
and connected as close as practicable to the trans-
areas and buildings which house computer and
former to be protected, in accordance with IEEE
sensitive electronics equipment. Arresters will be
Std C62.2. In regions of high lightning incidence,
located as close to the electrical service entrance
surge arresters will be mounted on each of the
as possible and a separate ground conductor from
incoming aerial line structures and directly on
the secondary service entrance will be bonded to
each of the main supply transformers.
the building ground ring. Range of voltage ratings
is 0.175 kV to 0.650 kV. Secondary class arresters
at transition points between aerial and under-
are required on Air Force facilities.
ground lines, surge protection will be necessary
d. Location. Arresters will be located as close to
within a generator plant. Surge arresters in paral-
the equipment protected as is practicable, in ac-
lel with surge protective capacitors will be in-
cordance with IEEE Std C62.2. Arresters will be
stalled either at the terminals of the generator
connected to line conductors ahead of any overcur-
switchgear bus for overall machinery protection or
rent protective devices to prevent the lightning
at the terminals of each generator, dependent
discharge from passing through the device. For Air
upon the degree of protection required. Surge
Force installations, additional arresters will also
protective capacitors reduce steep wave fronts,
be installed at corner poles, deadends, on both
which if imposed on rotating machinery could
sides of switches, riser poles, and on every tenth
result in stresses exceeding a machine's insulation
pole in long, straight distribution line runs.
impulse strength.
(1) Underground connections. Procedures for
e. Overhead ground (or shield) wires. Overhead
estimating magnitudes of surge voltages at dis-
ground wires are run parallel to and above electri-
tances remote from the transition arrester location
cal lines, in order to shield lines from a direct
are very complex. IEEE Committee Report, Surge
lightning stroke.
Protection of Cable-Connected Equipment on
(1) Transmission and distribution lines. Over-
Higher-Voltage Distribution Systems, recommends
head ground wires are used for protection of
doubling both the sparkover and the discharge
transmission lines, but rarely is such an installa-
plus lead voltage for the arrester and then requir-
tion economical for distribution lines. Overhead
ing a 15 percent margin over the equipment
ground wires will not be installed to protect
insulation. This recommendation will be followed
distribution lines, unless such an installation is
in areas with numerous lightning storms and may
necessary to be consistent with local usage.
require intermediate arresters at transition poles,
(2) Main electric supply stations. Shielding
arresters at transformer stations, or both provi-
will be provided because of the cost and impor-
sions for adequate protection.
tance of such stations. Transformer stations with
(2) Main electric supply station connections.
incoming aerial lines above 15 kV will be shielded
On main electric supply stations, the incoming
because of equipment cost. Such shielding reduces
aerial line switching devices and the transformer
possible surge voltages. This shielding may take
primary terminals are the main elements requir-
the form of lightning masts located on top of the
ing surge protection.
station metal structure to provide the required
(a) Incoming lines. Arresters will be located
cone of protection for apparatus and circuits
on the line side of any incoming line fuse to
within the station area. The incoming line will be
prevent the lightning discharge from passing
shielded for at least 1/2 mile from the station to
through the fuse. Arresters need not be installed
provide sufficient line impedance between the non-
on the line side of group-operated disconnect
shielded line and the station, otherwise high dis-
switches. However, arresters will be connected
charge currents could occur resulting in excessive
close enough to protect the switch adequately
arrester discharge voltages. When the incoming
when the switch is closed. Line entrance gaps may
lines belong to the serving utility and shielding
be used on the line side of any switch for protec-
cannot be provided, design calculations must as-
tion when the switch is open. Where two-column
sure adequate surge arrester protection or other
structures are provided as in figure 4-6, arresters
methods of limiting magnitudes of traveling
will be mounted on the load side as this structure
waves.
(a) Zone of protection. The zone of protection
configuration does not lend itself to line side
connection. Other structures, such as a double
of a shielding system is the volume of space inside
square bay structure, have a configuration which
which equipment is considered to be shielded. The
makes location of the arrester on the line side of
shaded areas on figure 9-1 illustrate the zones of
the switch the most practicable arrangement.
protection for both single and double mast or wire
9-3
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