GE 469 Instruction Manual page 160

5–18
input should be used for either zero-sequence or residual ground sensing. If the
connection is residual, the Ground CT secondary and primary values should be the same
as the phase CT. If however, the connection is zero-sequence, the Ground CT secondary
and primary values must be entered. The Ground CT primary should be selected such that
potential fault current does not exceed 20 times the primary rating. When relaying class
CTs are purchased, this precaution will ensure that the Ground CT does not saturate under
fault conditions.
The
PHASE DIFFERENTIAL CT PRIMARY
used. If two CTs are used per phase in a vectorial summation configuration, the CTs should
be chosen to ensure there is no saturation during motor starting. If however, a core
balance CT is used for the differential protection in each phase, a low CT rating of 50 or 100
A allows for very sensitive differential protection.
When the two-speed motor feature is used, a value for a second set of Phase CTs and
motor FLA must be entered here for Speed 2. If the Phase CTs are the same as the speed 1
phase CTs, simply enter the same value here as well.
Example 1:
Consider a 469 with a 5 A Phase CT secondary and Ground Fault Detection set to Residual
and a motor with the following specifications:
Motor Nameplate FLA: 87 A; Low Resistance Grounded; Maximum Fault: 400 A
The following settings are required:
"100"
PHASE CT PRIMARY:
MOTOR FULL LOAD AMPS:
5 A Secondary"
GROUND CT: "
GROUND CT PRIMARY:
Example 2:
Consider a 469 with a 5 A Phase CT secondary and Ground Fault Detection set to Residual
and a motor with the following specifications:
Motor Nameplate FLA: 255 A; Solidly Grounded; Maximum Fault: 10000 A;
Zero Sequence Ground CT: (10000/20) 500:1
The following settings are required:
"300"
PHASE CT PRIMARY:
MOTOR FULL LOAD AMPS: "
"5 A Secondary"
GROUND CT:
GROUND CT PRIMARY:
Example 3:
Again, consider a 469 with a 5 A Phase CT secondary and Ground Fault Detection set to
Residual and a motor with the following specifications:
Motor Nameplate FLA: 330 A; High Resistance Grounded; Maximum Fault: 5 A
The following settings are required:
must be entered if the differential feature is to be
"87"
"100"
255"
"500"
469 MOTOR MANAGEMENT RELAY – INSTRUCTION MANUAL
CHAPTER 5: SETTINGS