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SE-330
Neutral-Grounding-Resistor
Monitor
Rev.
9-A-112913
3.3 G
ROUND
-F
AULT
CT
Select and install a ground-fault CT that will provide
the desired trip level. Typically, the CT-primary rating
should approximately equal the NGR let-through-current
rating. This provides an appropriate GF TRIP LEVEL
setting range and analog-output scaling. See
Section 2.1.2 and 2.9.
Outline and mounting details for the sensitive EFCT-
and SE-CS30-series current sensors are shown in Figs.
16, 17, and 18. Ground-fault-CT connections and the
typical ground-fault-CT location are shown in Fig. 3.
For SE-325 replacement applications, the existing
CT200 current transformer will typically have to be
replaced. However, where replacement is not necessary
or possible, the CT200 can be connected to either the 1-
or 5-A input. This CT has a 200:5 current ratio. If
connected to the 1-A input, the ground-fault trip level
will be a percentage of 40 A. See Section 2.1.2.
The accuracy of a typical current transformer,
including the CT200, decreases below 5% of its current
rating. CT-primary current injection testing is
recommended to verify trip levels below 5% of the
CT-primary rating. See Section 9.4. Littelfuse Startco
sensitive current sensors are designed for use at low
levels and respond linearly to 2% current rating.
N
OTE
:
The current-transformer insulation class is of no
consequence if its secondary is grounded and the
conductors through its window are insulated for the
system voltage. Medium-voltage systems may require a
bushing-type CT.
