Page 2
SE-330
Neutral-Grounding-Resistor
Monitor
Rev.
9-A-112913
Additional features include LED trip indication, trip
memory, front-panel and remote reset, 4–20-mA analog
output, RS-232 local communications, data logging, and
optional network communications.
The SE-330 provides additional features and improved
performance over the SE-325 NGR Monitor:
SE-330 dc rejection is sufficient for reliable operation
in overhead-line applications.
Digital filtering of voltage and current signals
minimizes false trips due to harmonics.
Resistance measurement is calibrated to the NGR to
achieve a lower trip resistance.
Broader setting ranges.
Independent ground-fault and resistor-fault relays can
be used for indication and control.
Universal power supply.
Wider CT-selection range.
Network communications options provide
information to a distributed control system.
4–20 mA analog output.
UNIT HEALTHY output contact.
Power-up trip memory.
Pulsing capability can be used on low- and medium-
voltage systems to assist in locating ground faults.
RS-232 local communications port for firmware
upgrades and access to SE-330 measured parameters.
2. O
PERATION
2.1 S
ETTINGS
2.1.1 GF T
RIP
T
IME
GF TRIP TIME (definite time) is adjustable from 0.1 to
10.0 seconds. Time-coordinated ground-fault protection
requires this setting to be longer than the trip times of
downstream ground-fault devices.
A trip-time accumulator provides a ground-fault memory
function for detection of intermittent faults. The
accumulated time increases when a ground fault is detected
and decreases when a ground fault is not detected. A trip
will eventually occur when the time for fault current above
the trip level is greater than the time for fault current below
the trip level.
2.1.2 GF T
RIP
L
EVEL
The SE-330 uses a Discrete-Fourier Transform (DFT)
algorithm to measure the fundamental component of NGR
current.
Choose an NGR let-through current and a ground-fault
trip level according to the guidelines in Section 1.1. Set the
ground-fault trip level as a percentage (2, 4, 6, 8, 10, 15, 20,
40, 60, 80, or 100) of the CT-primary rating. Inputs are
provided for 5-, 1-, and 0.05-A-secondary CT’s. Typical
values for 5-, 15-, and 25-A tripping systems are shown in
Table 1. Ground-fault trip levels for selected CT’s are
shown in Table 2. For other systems, refer to the NGR
Monitor Set-Point Assistant at
www.littelfuse.com/protectionrelays.
2.1.3 V
N
T
RIP
L
EVEL
The SE-330 uses a DFT algorithm to measure the
fundamental component of neutral voltage.
Calculate the voltage across the NGR when NGR current
is equal to the pick-up current of the ground-fault circuit.
Set the V
N
TRIP LEVEL at the next largest value. The V
N
TRIP LEVEL range is 20 to 2,000 V with switch S5 in the
20-k
(Vx1) position, and the range is 100 to 10,000 V
with switch S5 in the 100-k
(Vx5) position. See Fig. 1
and Section 2.1.5.5.
If neutral voltage is greater than the V
N
TRIP LEVEL
setting for 12 seconds and ground-fault current is less than
5% of the CT rating, the SE-330 will trip on resistor fault. If
the resistor-fault circuit is tripped and the neutral voltage
exceeds the V
N
TRIP LEVEL setting for an interval greater
than the GF TRIP TIME setting, the ground-fault circuit
will also trip.
Typical values for 5-, 15-, and 25-A tripping systems are
shown in Table 1. For an NGR resistance greater than 2
k
, use a 100-k
sensing resistor. For other systems, refer
to the NGR Monitor Set-Point Assistant at
www.littelfuse.com/protectionrelays.
N
OTE
:
A resistor-fault trip is held off if the ground-fault
current is above 5% of the CT rating.
