Quantcast Solidly grounded system

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TM 5-683/NAVFAC M-116/AFJMAN 32-1083
countered areas pertaining to the grounding. The
(1) Ungrounded system. An ungrounded sys-
tem is one in which there is no intentional connec-
grounding grid and grounding body (earth under
tion between the neutral or any phase and ground.
the building) with the ground rods (electrodes and
Ungrounded system implies that the system is
the water pipe system) are shown. The second part
capacitively coupled to ground. The neutral poten-
is the conductors associated with the equipment
tial of an ungrounded system under reasonably bal-
ground. Part of the equipment ground is also
anced load conditions will be close to ground poten-
formed by the switchgear ground bus.
tial because of the capacitance between each phase
c. System grounds. A system ground refers to the
conductor and ground. When a line-to-ground fault
condition of having one wire or point of an electrical
occurs on an ungrounded system, the total ground
circuit connected to earth. This connection point is
fault current is relatively small, but the voltages to
usually made at the electrical neutral although not
ground potential on the unfaulted phases will be
always. The purpose of a system ground is to protect
high. If the fault is sustained, the normal line-to-
the equipment. This ensures longer insulation life
neutral voltage on the unfaulted phases is increased
of motors, transformers and other system compo-
to the system line-to-line voltage (i.e. square root of
nent A system ground also provides a low imped-
three (3) times the normal line-to-neutral value).
ance path for fault currents improving ground fault
This, over a period of time, breaks down the line-to-
relaying selectivity. In a properly grounded system
neutral insulation and hence results in insulation
the secondary neutral of a power transformer sup-
failure. Ungrounded system operation is not recom-
plying a building or facility is connected to a trans-
mended because of the high probability of failures
former grounding electrode. The transformer neu-
due to transient overvoltages caused by restriking
tral is a part of the service entrance point which
ground faults. The remaining various grounding
bonds to the grounding electrode system of the
methods can be applied on system grounding pro-
building. According to the National Electrical Code
tection depending on technical and economic fac-
(NEC) articles 250-81 and 250-83, metal under-
tors. The one advantage of an ungrounded system
ground waterpipes, metal building frames, encased
that needs to be mentioned is that it generally can
electrodes, rods and plates are among the items
continue to operate under a single line-to-ground
that can make up the grounding electrode system of
fault without an interruption of power to the loads.
a building. The NEC article 250-S3 requires that
(2) Solidly grounded system. A s o l i d l y
the size of the grounding electrode iron or steel rod
grounded system is one in which the neutral (or
must be at least 5/8 inches in diameter and driven
occasionally one phase) is connected to ground with-
eight feet deep. The resistance of the electrode to
out an intentional intervening impedance (fig 8-4).
ground cannot exceed 25 ohms (NEC 250-84). Oth-
On a solidly grounded system in contrast to an un-
erwise a second electrode should be added and the
grounded system, a ground fault on one phase will
distance between the two electrodes must be at
result in a large magnitude of ground current to
flow but there will be no increase in voltage on the
least six feet. However, in some systems the 25
unfaulted phase. Solid grounding is commonly used
ohms resistance value cannot achieve the goals of
in low voltage distribution systems. Solid grounding
grounding. They require ground resistance values
has the lowest initial cost of all Wounding methods.
below ten ohms. According to MIL-STD-l88-12A
It is usually recommended for overhead distribution
ten ohms ground resistance is acceptable. If the
systems supplying transformers protected by pri-
main building load is composed of computers or
mary fuses. However, it is not the preferred scheme
sensitive electronic equipment, the earth ground re-
for most industrial and commercial systems, again
sistance should not exceed five ohms. There are
because of the severe damage potential of high mag-
many methods of system grounding used in indus-
nitude ground fault currents. The NEC Article
trial and commercial power systems, the major ones
250-5 (1990) requires that the following classes of
being ungrounded, solid grounding, and low and
high resistance grounding (fig 8-3). Technically,
systems be solidly grounded:
there is no general acceptance to use any one par-
(a) Where the system can be so grounded
ticular method. Each type of system grounding has
that the maximum voltage to ground on the un-
advantages and disadvantages. Factors which influ-
grounded conductors does not exceed 150 volts.
ence the choice of selection include voltage level of
(b) Where the system is 3-phase, 4-wire,
the power system, transient overvoltage possibili-
wye-connected in which the neutral is used as a
ties, types of equipment on the system, cost of
circuit conductor.
equipment, required continuity of service, quality of
(c) Where the system is 3-phase, 4-wire
system operating personnel and safety consider-
delta-connected in which the midpoint of one phase
ation including fire hazards.
is used as a circuit conductor.
8-2





 


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