Eaton Magnum DS Technical Data Manual - Cutler-Hammer
Technical Data
Effective: May 1998
Page 15
TD.44A.01.T.E
Cutler-Hammer
Figure A1. Three-Wire Distribution, Delta Source (Ungrounded)
Figure A2. Three-Wire Distribution, Wye Source (Ungrounded)
Figure A3. Four-Wire Distribution, Solidly Grounded System
Figure A4. Three-Wire Distribution, Solidly Grounded System
Figure A5.
Blown Fuse
Ground Current Still
Flows Through Load
From Other Fuses
Ground Fault Ground Fault
Application
Distribution Systems
The power distribution in three-phase
low-voltage systems can be three- or
four-wire distribution. The three-wire
distribution can be served from either
delta or wye sources, but the four-wire
distribution is obtained from wye sol-
idly grounded source only. Fig. A1
shows three-wire distribution with
delta source and Fig. A2 shows three-
wire distribution with wye source. It is
significant on Fig. A2, that the wye con-
nection of a transformer secondary
does not necessarily mean four-wire
distribution in switchgear. This is
worthwhile to note because four-wire
distribution is quite frequently
assumed when the transformer sec-
ondary is wye connected. The low-volt-
age system is three-phase four-wire
distribution only if a fourth wire is car-
ried through the switchgear, the trans-
former neutral is solidly grounded, and
single phase loads are connected to
feeder breakers. This fourth wire is the
neutral bus. The neutral bus is con-
nected to the neutral of the wye con-
nected transformer secondary as
shown on Fig. A3.
Three- or four-wire sources can be
grounded or ungrounded in service.
Generally, where the source is delta
connected it is ungrounded, but in
some very rare cases it is grounded at
one corner of the delta, or at some
other point. When the source is wye
connected it can be grounded or
ungrounded, and when grounded, the
grounding is at the neutral. When low-
voltage systems are grounded they are
generally solidly grounded; however,
occasionally the grounding is through
a resistor. Three- and four-wire solidly
grounded systems are shown on Fig.
A3 and A4. Most installations are sol-
idly grounded. Solidly grounded sys-
tems have the advantage of being the
easiest to maintain, yet have the poten-
tial for producing extremely high fault
levels.
When feeding critical facilities, or
continuous industrial processes, it
is sometimes preferable to allow the
system to continue operating when a
phase conductor goes to ground.
There are two methods of accommo-
dating this application; the source
transformer may either be left
ungrounded or high resistance
grounded. If the correct system condi-
tions of inductance and capacitance
manifests themselves, arcing ground
on ungrounded systems can produce
escalating line-to-ground voltages,
which in turn can lead to insulation
breakdown in other devices. This con-
dition is known as ferro-resonance.
The high resistance grounded system
does not suffer from this potential phe-
nomenon. Regardless of which system
is selected, both require the application
of an appropriate UL recognized
ground detection method. Upon
grounding of one of the phase conduc-
tors, the detection device alerts opera-
tors of the condition. Personnel trained
to locate these grounds can do so and
remove the ground when the process
permits, and before a second ground
occurs on another phase.
Magnum DS
Metal-Enclosed
Low-Voltage Switchgear
Application
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