Current Inputs; Ground And Cbct Inputs - GE 339 Instruction Manual

CHAPTER 2: INSTALLATION

Current inputs

CAUTION:
CAUTION:

Ground and CBCT inputs

339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
The 339 relay has three (3) channels for phase current inputs, each with an isolating
transformer. There are no internal ground connections on the current inputs. Current
transformers with 10 to 1500 A primaries may be used.
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.
Before working on CTs, they MUST be short circuited.
The 339 has two isolating transformers with separate terminals for the 1A/5A secondary
and the CBCT (50:0.025). Only one ground terminal type can be used at a time. There are
no internal ground connections on the ground current inputs.
The maximum ground CT primary for the 1 A and 5 A taps is 1500 A. Alternatively the
sensitive ground input, 50:0.025, can be used to detect ground current on high resistance
grounded systems.
The ground CT connection can either be a zero sequence (core balance) installation or a
residual connection. Note that only 1 A and 5 A secondary CTs may be used for the
residual connection. A typical residual connection is illustrated below. The zero-sequence
connection is shown in the typical wiring diagram. The zero-sequence connection is
recommended. Unequal saturation of CTs, CT mismatch, size and location of motor,
resistance of the power system, motor core saturation density, etc. may cause false
readings in the residually connected ground fault circuit.
Figure 2-30: Residual ground CT connection
A
B
C
1A/5A
PHASE A CT
PHASE B CT
PHASE C CT
E5 D5 E6 D6 E7 D7
COM 1A/5A COM 1A/5A COM
PHASE A PHASE B PHASE C
CURRENT INPUTS
ELECTRICAL INSTALLATION
E8 D8
1A/5A COM
GROUND
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