Ground Fault - GE 339 Instruction Manual

CHAPTER 6: SETPOINTS

Ground fault

NOTE:
NOTE
339 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
When motor stator windings become wet or otherwise suffer insulation deterioration, low
magnitude leakage currents often precede complete failure and resultant destructive fault
currents. Ground fault protection provides early detection of such leakage current, so that
the motor can be taken off line in time to limit motor damage.
Depending on the setting
S2 SYSTEM SETUP > CURRENT SENSING > GROUND CT TYPE
current measured by this element is either the Core Balance CT (CBCT) current, the fourth
CT input current, or the sum of the first three CT inputs' currents. For high resistance
grounded systems, sensitive ground current detection is possible if the CBCT 50:0.025 input
is used. To use the CBCT 50:0.025 input, select "50:0.025" for the Ground CT Type. On solidly
grounded systems where fault currents may be quite large, the 1A or 5A secondary ground
CT 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 Fault protection alarms or trips when the ground current magnitude exceeds
the set pickup for the set time.
A ground fault trip is a serious event, and therefore results in a lockout that cannot be reset
unless an Emergency Restart or a Lockout Reset is issued.
Various situations (e.g. contactor bounce) may cause transient ground currents during
motor starting that may exceed the Ground Fault pickup levels for a very short period of
time. The delay can be fine tuned to an application such that it still responds very quickly,
but rides through normal operational disturbances. Normally, the Ground Fault time
delays are set as short as possible, that is, 0.00 seconds. Time to trip may have to be
increased if nuisance tripping occurs.
Special care must be taken when the ground input is wired to the phase CTs in a residual
connection. When a motor starts, the starting current (typically 6 × FLA for an induction
motor) has an asymmetrical or DC component. This momentary DC component will cause
each of the phase CTs to react differently, and cause a net current into the ground input of
the relay. A 20 ms block of the ground fault elements when the motor starts normally
enables the relay to ride through this momentary ground current signal.
The settings GND ALARM PKP and GND TRIP PKP are entered in units of 'xCT' if the setting
GROUND CT TYPE is programmed as "1A Secondary" or "5A Secondary," or in units of 'A' if
the setting GROUND CT TYPE is programmed as "50:0.025".
PATH:
SETPOINTS > S3 PROTECTION > GROUND FAULT
GND ALARM FUNC
Range: Disabled, Enabled
Default: Disabled
This setting enables the Ground Fault Alarm functionality.
GND ALARM PKP
Ground CT Type = 1A Secondary, 5A Secondary:
Range: 0.03 to 1.00xCT in steps of 0.01xCT
Default: 0.10xCT
Ground CT Type = 50:0.025:
Range: 0.50 to 15.00 A in steps of 0.01 A
Default: 10.00 A
This setting specifies the Pickup threshold for the Alarm stage.
S3 PROTECTION
, the
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