TM
5-811-1/AFJMAN
32-1080
Single mast or
Two masts or
shield wire
shield wires
US Army Corps of Engineers
Figure 9-1. Zones of Protection for Masts and Shield Wires.
systems plus the range of angles often used and
and duration of over-voltages, thereby reducing
the maximum angles recommended.
the probability of insulation failure and equipment
(b) Strength of wire. Breakage of shield
damage. Neutrals for each voltage level will be
wires could result in outage and damage to equip-
grounded independently at each electric power
ment. To minimize possible damage, ground wires
source; that is, at transformer secondaries and at
will be at least 7/16-inch, high-strength, zinc-
(a) Transformer neutral grounding. Trans-
coated steel (ASTM Std A 475) with a minimum
breaking strength of 14,500 pound-force (lbf) and
formers which have wye-connected secondaries will
maximum design tension will be limited to 2,000
be solidly grounded. Solid grounding is the least
pounds per conductor.
expensive method of limiting transient over-
f. Grounding. For safety reasons, electric power
voltages while obtaining enough ground fault cur-
systems and equipment are intentionally grounded,
rent for fast selective fault isolation. Other meth-
so that insulation failure results in operation of
ods of grounding are resistance grounding and
protective devices to deenergize circuits, thus re-
reactance grounding, but in most cases, reactance
ducing risk to personnel. The word "grounding" is
grounding of transformers provides no advantages
commonly used in electric power system work to
over solid grounding. A disadvantage of systems
cover both "system grounding" and "equipment
grounded through resistors is that surge arrestors
grounding"; however, the distinction between sys-
must be sized as if used on ungrounded systems,
tem and equipment grounding must be recognized.
that is, with a voltage rating at least equal to the
A system ground is a connection to ground from
line-to-line voltage. Systems with voltage above 15
one of the conductors of an electric circuit, nor-
kV. will be solidly grounded because of the prohib-
mally the neutral conductor. An equipment ground
itive cost of grounding equipment and the in-
creased surge arrester cost. For voltages from 2.4
is a connection to ground from non-current carry-
ing metallic parts of the installation such as
to 15 kV., solid grounding is preferable because
distribution systems which supply transformers
conduit and equipment cases of apparatus con-
nected to an electric circuit. IEEE Std. 142 and
protected by primary fuses require enough fault
ANSI C2 grounding practices will be used for all
current to melt primary fuses on a ground fault.
However, in some cases, low-resistance grounding
power generating and delivery systems.
(1) System grounding. System grounding dis-
will be needed to limit ground fault currents to
values less than withstand ratings of equipment
cussed in IEEE Std. 142 includes ungrounded
when such equipment is designed for direct con-
(3-wire); single-grounded (3-wire, source grounded);
and multiple-grounded (4-wire) systems. The pre-
nection to voltages of the 2.4 to 15 kV level.
Systems rated 600 V or less will be solidly-
f e r r e d system for new Army projects is the
multiple-grounded four wire system. New Air
grounded except for applications such as continu-
ous processes for industrial systems where shut-
Force projects will incorporate only multiple-
down would create a hazard, loss of materials, or
grounded (4-wire) systems. See IEEE Std. 142
equipment damage. For those applications, the
discussion of grounding and supply availability.
d e s i g n e r will evaluate the use of a solidly-
Wye-connected electric distribution systems will be
grounded wye-system with a back-up power sup-
provided with a grounded neutral connection. Such
ply, or a high-resistance-grounded wye-system. Use
intentional grounding minimizes the magnitude
9-4
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