Technical Data
Effective: May 1998
Page
20
TD.44A.01.T.E
Cutler-Hammer
Magnum DS
Metal-Enclosed
Low-Voltage Switchgear
The Following Provides Guidelines for Ground Fault Protection.
System
Advantages
Disadvantages
Equipment Available for Protection
Main Breaker
Tie Breaker
Fdr. Breaker
Notes
Ungrounded
(3-Wire)
Minimum distur-
bance to service
continuity. Currents
for the majority of
grounds will be lim-
ited to capacitance
charging current of
the system. Can
operate with the first
ground until it is
removed during a
regular shutdown.
Low cost. Supple-
mental protection
for an ungrounded
system utilizing trip
unit ground element.
When ground detector
shows that a ground exists,
corrective action must be
taken at the earliest possi-
ble shutdown. However,
experience indicates that
this attention is not always
possible. Therefore, these
systems tend to operate
with one phase grounded
through the first uncleared
ground. A high impedance
ground on another part of
the system would result
in low values of current,
which would not operate
a breaker phase trip, and
could produce fire damage.
High voltages from arcing
grounds are possible.
Lamp type
ground detector
or ground
detecting volt-
meters with or
without voltage
transformers. If
voltage trans-
formers are
used, a ground
alarm relay can
be added for
remote or local
alarming. 3-wire
residual protec-
tion, minimum
pickup.
0.50 sec. time
delay. See SK
No. 1, No. 4,
and No. 6.
3-wire residual
protection, mini-
mum pickup.
0.35 sec. time
delay. See
sketch No. 4
and No. 6.
3-wire protec-
tion, minimum
pickup.
0.22 sec. time
delay. See SK
No. 1, No. 4,
and No. 6.
20
Solid
Grounded
20
20
Standard resid-
ual ground pro-
tection for
single source
systems, and
source ground,
per SK 5, for
multiple ground
sources. Mini-
mum pickup.
0.50 sec. time
delay.
Ground 3-wire
or 4-wire (as
required) fault
protection. Mini-
mum pickup.
0.35 sec. time
delay. See SK
No. 4 or 5.
Ground 3-wire
or 4-wire (as
required) fault
protection. Min-
imum pickup.
0.22 sec. time
delay or zero
sequence cur-
rent transformer
feeding Into trip
unit. See SK No.
1, No. 2, and
No. 6.
This is the most
common system
in use today. As
long as it is not
necessary to
coordinate with
phase devices
down the line, it
will give very
good main bus
and feeder pro-
tection.
High
Resistance
Grounded
(3-Wire)
Ground fault current
is limited. Unground-
ing can result in
high voltages during
arcing grounds, and
this is corrected by
high resistance
grounding. Can oper-
ate with the first
ground until it is
removed during a
regular shutdown.
Very sensitive detection is
required to detect the lim-
ited fault current. When
the ground detector shows
that a ground exists, cor-
rective action must be
taken at the earliest possi-
ble shutdown. However,
experience indicates that
this attention is not always
possible, therefore, these
systems tend to operate
with one phase grounded
through the first uncleared
ground. A high impedance
ground on another part of
the system would result in
low values of current,
which would not operate a
breaker phase trip, and
could produce fire damage.
Higher cost than
ungrounded.
Same as for
ungrounded
except ground
voltage alarm
relay is con-
nected across
grounding resis-
tor, or current
relay between
resistor and
ground.
Same as for
ungrounded.
Same as for
ungrounded.
Same as for
ungrounded.
This system is
most effective
when supplied
with a pulsing
option.
Application
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