CHAPTER 2: INSTALLATION ELECTRICAL INSTALLATION
350 FEEDER PROTECTION SYSTEM – INSTRUCTION MANUAL 2–11
Current inputs
The 350 relay has four (4) channels for AC current inputs, each with an isolating
transformer. There are no internal ground connections on the current inputs. Current
transformers with 1 to 6000 A primaries may be used.
CAUTION:
Verify that the relay’s nominal input current of 1 A or 5 A matches the secondary rating
of the connected CTs. Unmatched CTs may result in equipment damage or inadequate
protection.
CAUTION:
IMPORTANT: The phase and ground current inputs will correctly measure up to 20
times the current input’s nominal rating. Time overcurrent curves become horizontal
lines for currents above the 20 × CT rating. This becomes apparent if the pickup level is
set above the nominal CT rating.
CAUTION:
Before working on CTs, they MUST be short circuited.
Ground and sensitive ground CT inputs
One dedicated ground input is referred to throughout this manual as the Ground Current
or Sensitive Ground Current input. Before making ground connections, consider that the
relay automatically calculates the neutral (residual) current from the sum of the three
phase current phasors. The following figures show three possible ground connections (or
three possible sensitive ground connections).
The ground input (Terminals D8 and E8) is used in conjunction with a Zero Sequence CT as
source, or in the neutral of wye-connected source CTs. The ground current input can be
used to polarize the neutral directional element. When using the residual connection set
the GROUND CT PRIMARY setpoint to a value equal to the PHASE CT PRIMARY setpoint.
In cases where the relay is equipped with sensitive ground CT (terminals D8 and E8) the
sensitive ground current input is intended for use either with a CT in a source neutral of a
high-impedance grounded system, or on ungrounded systems. On ungrounded systems it
is connected residually with the phase current inputs. In this case, the SENSTV GND CT
PRIMARY setpoint should be programmed to a value equal to the PHASE CT PRIMARY
setpoint. The sensitive ground current input can be connected to a Zero Sequence CT for
increased sensitivity and accuracy when physically possible in the system.
NOTE:
The Sensitive Ground input must only be used on systems where the maximum ground
current does not exceed 100 times the rated current for 1 second.
Courtesy of NationalSwitchgear.com
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