Technical Data
Effective: May 1998
Page
16
TD.44A.01.T.E
Cutler-Hammer
Magnum DS
Metal-Enclosed
Low-Voltage Switchgear
Since ungrounded and resistance
grounded systems produce minimal
ground current, no damage occurs to
the grounded equipment. These
ground currents are also too low for
detection by integral trip unit ground
elements, therefore serve no ground
fault tripping function if applied on
these systems. Ground fault elements
on these types of systems can, how-
ever, provide supplemental protec-
tion. If a second ground occurs on
another phase, and exceeds the
ground element pickup setting, the
ground element can serve as a more
sensitive short delay trip.
Ungrounded or resistance grounded
systems can not be applied as 4-wire
networks. Even if supplied from a 4-
wire source, no line-to-neutral loads
may be served. These applications are
limited to 3-wire distribution systems
only.
Need For Ground Fault Protection
If the magnitude of all ground currents
would be large enough to operate the
short delay or instantaneous elements
of the phase overcurrent trip devices,
there would be no need for separate
ground fault protection on solidly
grounded systems. Unfortunately,
because low magnitude ground cur-
rents are quite common, this is not the
case. Low level ground currents can
exist if the ground is in the winding of
a motor or a transformer, or if it is a
high impedance ground. Low level
ground currents may also be due to an
arcing type ground. The arcing type
grounds are the source of the most
severe damages to electrical equip-
ment. The lower limit of the arcing
ground currents is unpredictable and
the magnitude may be considerably
below the setting of the breaker phase
overcurrent trip devices. It is for this
reason that the National Electric Code,
and UL, require ground fault protection
for all service disconnect breakers
rated 1000 amperes and greater,
applied on systems with greater than
150 volts line-to-ground.
Since the breaker phase overcurrent
trip devices cannot provide sensitive
enough protection against low magni-
tude ground faults, there is a need for
an additional protective device. This
additional device is not to operate on
normal overloads and it is to be sensi-
tive and fast enough to protect against
low magnitude grounds. It is also
important that this additional ground
protecting device be simple and reli-
able. If the Magnum DS breaker solid-
state tripping system including an
optional “ground element” is selected,
good ground fault protection will be
assured.
The Ground Element
The ground element of the solid-state
trip unit is in addition to the usual
phase protection. The ground element
has adjustable pickup with calibrated
marks as shown in Tables R8 and R9
and adjustable time delay. The input
current to the trip unit can be provided
by:
(a)Residual connection of phase sen-
sors, with the residual circuit con-
nected to the ground element
terminals. This is the Magnum DS
Low-Voltage Switchgear standard
ground protection system for 3-wire
systems. On 4-wire systems, stan-
dard ground fault protection
includes a fourth “neutral sensor.” It
is connected to vectorally subtract
from the residual current of the
phase sensors. Its only function is to
sense neutral currents. It does not
sense ground current. These sys-
tems produce pickup values as
shown in Table R8.
(b)External ground sensing current
transformers connected to the
ground element terminals. This
means that this external ground sen-
sor will trip the breaker whenever its
secondary output current exceeds
the values shown in Table R8.
Tripping is independent of phase
currents. The lower the CT ratio,
the more sensitive the ground fault
protection.
Ground Fault Protection Application
and Coordination
In all power systems, continuity of ser-
vice is very important. For reliable ser-
vice continuity, selective tripping is
applied between main, tie, and feeder
breakers, and downstream protecting
devices, for phase-to-phase faults.
Similar selective tripping is desirable
when breakers trip on grounds. The
application of ground protection only
to main breakers may assure good
ground protection. However, it will not
provide good service continuity
because the main breaker will trip on
grounds that should have been cleared
by feeder breakers. For proper protec-
tion and for good service continuity,
main, tie and feeder breakers all
should be equipped with ground fault
protection.
In view of the above, it is evident that
properly applied ground protection
requires ground elements as far down
the system to the loads as practical. For
best results, downstream molded case
breakers should have individual ground
protection. This would result in excel-
lent ground protection because ground
elements of Magnum DS and down-
stream breakers having similar tripping
characteristics can be coordinated.
Depending on the sensitivity of the
ground fault protection method
applied, coordination between Mag-
num DS Breaker ground elements and
downstream branch circuit fuses is
sometimes impractical. This is due to
the basic fact that the blowing of one
phase fuse will not clear a ground on a
three-phase system. The other two-
phase fuses will let the load “single-
phase,” and also continue to feed the
ground through the load, as shown in
Figure A5.
Application
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