GE Multilin MM300, Instruction Manual

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2–10 MM300 MOTOR MANAGEMENT SYSTEM – INSTRUCTION MANUAL
ELECTRICAL INSTALLATION CHAPTER 2: INSTALLATION
CPU module
The main CPU module and optional communications board is contained in slot B. This
module provides a Modbus RTU RS485 port, a thermistor input, and a 50:0.025 CBCT input.
The optional communications board provides Fieldbus and Ethernet ports.
Ground current input
The MM300 CPU module has a sensitive ground input suitable for a 50:0.025 CT. There is
also a residual ground input on the CT module (IO_A) with the same rating as the phase
current inputs. Only one ground CT input (either CBCT described here, or a residual ground
connection) can be used at any time. Refer to IO-A installation,
section 2.2.3.1 , for more
information on residual ground connection.
Figure 2-12: CBCT connection
CAUTION:
The zero-sequence connection is recommended. Unequal saturation of CTs, size and
location of motor, resistance of power system, motor core saturation density, and
other factors, may cause false readings in the residually connected ground fault
circuit.
NOTE
NOTE:
Only one ground input should be wired; the other input should be unconnected.
The exact placement of a zero-sequence CT to detect only ground fault current is shown
below. If the core balance CT is placed over shielded cable, capacitive coupling of phase
current into the cable shield during motor starts may be detected as ground current unless
the shield wire is also passed through the CT window. Twisted-pair cabling on the zero-
sequence CT is recommended.
A
C
B
Contactor
– +
– C – +
CPU module
MM300
Motor Management System
To switchgear
ground bus
853711A1.CDR
R I SG
RS485 Thermistor CBCT
Control power
L N +
SG = Surge Ground = Functional Ground