Technical Data
Effective: May 1998
Page
17
TD.44A.01.T.E
Cutler-Hammer
High-Resistance Grounding
Where continuity of service is a high
priority, high-resistance grounding
can add the safety of a grounded sys-
tem while minimizing the risk of ser-
vice interruptions due to grounds. The
concept is a simple one: provide a path
for ground current via a resistance that
limits the current magnitude, and mon-
itor to determine when an abnormal
condition exists.
The ground current path is provided at
the point where the service begins, by
placing resistance in the connection
from system neutral to ground. Control
equipment continuously measures
ground current. A relay detects when
the current exceeds a predetermined
level. An alarm alerts building person-
nel that a ground exists. The system
has built-in fault tracing means to
assist in finding the source of the
ground. An integral transformer pro-
vides control power from the primary
source.
600 Volt (Maximum) Delta Systems
To add high-resistance grounding to
an ungrounded delta-connected sys-
tem, a neutral point must be created.
Three single-phase transformers can
be interconnected in a zigzag or wye-
broken delta configuration to provide
such a neutral point. The transformers
and grounding resistors are chosen to
limit the ground current to a maximum
value of 5 amperes.
Application note
–
The neutral point may not be used to
serve phase-to-neutral loads. Also,
this technique may be applied on wye-
connected sources when the neutral
point is not conveniently accessible
from the service entrance location.
600/347 Volt (Maximum) Wye
Systems
To add high-resistance grounding to a
wye-connected system, resistors are
placed in series with the neutral-to-
ground connection of the power
source. The resistors are chosen to
limit the current to a maximum value
of 5 amperes.
Application note
– Per
1993 NEC 250-5b, exception no. 5 line-
to-neutral loads may not be connected
to a system where the neutral is
resistance-grounded.
Ground Current Detection
Any time a system is energized, a small
ground current called the “capacitive
charging current” will be observed. For
low-voltage (600V and below) systems,
this naturally-occurring current is typi-
cally 1 ampere or less.
When one phase becomes grounded,
additional current above the charging
level will flow. As all ground current
must flow through the grounding resis-
tor/grounding transformer assembly,
an ammeter in this circuit will read the
total amount of ground current. By
placing a current-sensing relay in
series with the ammeter, the current
relay can be adjusted to pick up at a
level in excess of the capacitive charg-
ing current, thus indicating the abnor-
mal condition.
Alternatively, an optional voltmeter-
relay can be connected across the
grounding resistors. The voltage
across the resistors is proportional to
the amount of ground current. The
voltmeter-relay’s pickup adjustment is
set above the capacitive charging cur-
rent, to the desired detection level.
In both current and voltage detection
methods, the ground current ammeter
provides a direct reading of the total,
actual ac ground current present in the
system at that time. It will be helpful to
periodically note the ammeter’s read-
ing; a trend towards higher values may
indicate the need for equipment main-
tenance, and hence reduce the occur-
rence of unplanned shutdowns.
Indication and Alarm Circuits
When a fault is detected, an adjustable
time delay is provided to override tran-
sients. When the time delay has been
exceeded, the green “normal” light
will turn off, the red “ground fault”
light will turn on, and the ground alarm
contacts will transfer. If equipped with
the optional alarm horn, it will sound.
When the fault is cleared, the current/
voltage relay will reset. If the reset con-
trol is set on “auto,” the lights will
return to “normal” on, “ground fault”
off, and the ground alarm contacts will
re-transfer. If the reset control is set on
“manual,” the lights and relay contacts
will remain latched until the operator
turns the reset control to “reset.” The
lights and ground alarm contacts will
then return to normal. The system can
be reset only if the fault has been
cleared.
During a fault, the optional alarm horn
can be silenced at any time by using
the “alarm silence” pushbutton. It will
not re-sound until either the system is
reset, or the re-alarm timer expires.
The re-alarm timer is activated by the
“alarm silence” control. If the horn has
been silenced but the fault has not
been cleared, the timer will run. It has a
range of 2–48 hours. When the timer
times out, the horn will re-sound, alert-
ing maintenance personnel that the
fault has not been cleared.
Test Circuit
A test circuit is provided to allow the
user to quickly determine that the sys-
tem is working properly. The test cir-
cuit will operate only under normal
conditions — it will not allow testing if
the system is sensing a fault. A sepa-
rate grounding resistor is provided,
connected to a relay operated by the
“test” position of the mode selector
switch. The relay’s contact grounds
phase B through the test resistor, caus-
ing ground current to flow. The system
then reacts as it would under actual
system ground conditions: lights trans-
fer, alarm contacts transfer and the
(optional) horn sounds.
Pulser Circuit
The pulser circuit offers a convenient
means to locate the faulted feeder and
trace the fault to its origin. The pulser is
available any time a fault has been
detected. An adjustable recycle timer
controls the pulse intervals. The “pulse”
light flashes on and off, corresponding
to the on-off cycles of the pulser contac-
tor. The pulser contactor switches a
bank of resistors on and off, thus allow-
ing a momentary increase in the ground
current (approximately a 5 ampere cur-
rent pulse above the ground current).
Locating a Ground Fault
The current pulses can be noted with a
clamp-on ammeter when the ammeter
is placed around the cables or conduit
feeding the fault. The operator tests
each conduit or set of cables until the
pulsing current is noted. By moving the
ammeter along the conduit, or check-
ing the conduit periodically along its
length, the fault can be traced to its ori-
gin. The fault may be located at the
point where the pulsing current drops
off or stops.
If little or no change in the pulsing cur-
rent is noted along the entire length of
a cable, then the fault may be in the
connected load. If the load is a panel-
board, distribution switchboard or
motor control center, repeat the pro-
cess of checking all outgoing cable
groups to find the faulted feeder. If the
fault is not found in an outgoing
feeder, the fault may be internal to that
equipment.
Magnum DS
Metal-Enclosed
Low-Voltage Switchgear
Application
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