Ground Fault (50G/Sg) - GE 339 Instruction Manual

CHAPTER 6: SETPOINTS

Ground fault (50G/SG)

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.
The 339 has one Ground/Sensitive Ground Fault protection. Depending on the setting
SYSTEM SETUP > CURRENT SENSING > GROUND CT TYPE
element is either the Core Balance CT (CBCT) current used for detection of small by
magnitude ground fault currents, or it is the fourth CT input current with either 1A or 5A
secondary current rating.
The Ground Fault protection alarms or trips when the ground current magnitude exceeds
the set pickup for the set time.
Ground Fault (50G) protection with 1A/5A Ground CT
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.
Sensitive Ground Fault (50SG) protection with Core-Balance CT (50:0.025)
Usually the CBCT is used for sensitive ground fault detection including ground faults due to
resistive leakage that depends on the cable insulation and motor insulation, capacitive
charging currents caused by the capacitance of the motor windings and motor circuit
conductors, or harmonic currents. The CBCT has ratio of 50:0.025 which makes it ideal for
detection of ground fault primary currents even in the range of 0.5A. To use the CBCT
50:0.025 input, select "50:0.025" for the Ground CT Type.
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
For details see Common setpoints.
S3 PROTECTION
, the current measured by this
S2
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