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 TM 5-685/NAVFAC MO-912
(2) Overvoltage limitation is particularly im-
Manufacturers Association 1-78 places a require-
portant in systems over 1 Kv, because equipment in
ment on the design of synchronous generators that
their windings shall be braced to withstand the
these voltage classes are designed with less margin
mechanical forces resulting from a bolted 3-phase
between 50/60 Hz test and operating voltages than
short circuit at the machine terminals. The current
low voltage equipment. The remaining various
created by a phase-to-ground fault occurring close
grounding methods can be applied on system
grounding protection depending on technical and
to the generator will usually exceed the 3-phase
bolted fault current. Due to the high cost of genera-
economic factors. The one advantage of an un-
grounded system that needs to be mentioned is that
tors, the long lead time for replacement, and system
it generally can continue to operate under a single
impedance characteristics, a solidly grounded neu-
tral is not recommended for generators rated be-
line-to-ground fault without significant damage to
electrical equipment and without an interruption of
tween 2.4 Kv and 15 Kv.
(4) Limiting the available ground fault current
power to the loads.
by resistance grounding is an excellent way to re-
g. A solidly grounded system refers to a system in
which the neutral, or occasionally one phase, is con-
duce damage to equipment during ground fault con-
ditions, and to eliminate personal hazards and elec-
nected to ground without an intentional intervening
trical fire dangers. It also limits transient
impedance. On a solidly grounded system, in con-
trast to an ungrounded system, a ground fault on
overvoltages during ground fault conditions. The
one phase will result in a large magnitude of ground
resistor can limit the ground fault current to a de-
sired level based on relaying needs.
current flow but there will be no increase in voltage
on the unfaulted phase.
-
(1) On low-voltage systems (1 Kv and below),
which the neutral is grounded through a consider-
the National Electrical Code (NEC) Handbook, ar-
ably smaller resistance than used for high-
ticle 250-5(b) requires that the following class of
resistance grounding. The resistor limits ground
fault current magnitudes to reduce the damage dur-
systems be solidly grounded:
(a) Where the system can be so grounded
ing ground faults. The magnitude of the grounding
that the maximum voltage to ground on the un-
resistance is selected to detect and clear the faulted
circuit. Low-resistance grounding is used mainly on
grounded conductors does not exceed 150 volts.
medium voltage systems (i.e., 2.4 Kv to 15 Kv),
(b) Where the system is 3 phase, 4 wire wye
especially those which have directly connected ro-
connected in which the neutral is used as a circuit
tating apparatus. Low-resistance grounding is not
conductor.
used on low-voltage systems, because the limited
(c) Where the system is 3 phase, 4 wire delta
available ground fault current is insufficient to posi-
connected in which the midpoint of one phase wind-
tively operate series trip units.
ing is used as a circuit conductor.
(1) Low-resistance grounding normally limits
(d) Where a grounded service conductor is
the ground fault currents to approximately 100 to
uninsulated in accordance with the exceptions to
600 amps (A). The amount of current necessary for
NEC articles 230-22, 230-30, and 230-41.
selective relaying determines the value of resistance
(2) Solid grounding is mainly used in low-
to be used.
voltage distribution systems (less than 1000 volt (V)
(2) At the o ccurrence of a line-to-ground fault
system) and high-voltage transmission systems
(over 15 Kv). It is seldom used in medium-voltage
on a resistance-grounded system, a voltage appears
systems (1 Kv to 15 Kv). Solid grounding has the
across the resistor which nearly equals the normal
lowest initial cost of all grounding methods. It is
line-to-neutral voltage. of the system. The resistor
usually recomrrended for overhead distribution sys-
current is essentially equal to the current in the
tems supplying transformers protected by primary
fault. Therefore, the current is practically equal to
fuses. However, it is not the preferred scheme for
the line-to-neutral voltage divided by the number of
most industrial and commercial systems, again be-
ohms of resistance used.
i. High-resistance grounding is a system in which
cause of the severe damage potential of high-
the neutral is grounded through a predominantly
magnitude ground fault currents.
resistive impedance whose resistance is selected to
(3) In most generators, solid grounding may
permit the maximum ground fault current from the
allow a ground fault current through the resistor
generator to exceed the maximum 3-phase fault cur-
equal to or slightly more than the capacitive charg-
rent which the generator can deliver and for which
ing current (i.e.,
>
of the system. The resis-
its windings are braced. This situation occurs when
tor can be connected either directly from neutral to
the reactance of the generator is large in compari-
ground for wye type systems where a system neu-
son to the system reactance. National Electrical
tral point exists, or in the secondary circuit of a
2-6
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