4–156 869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
PROTECTION CHAPTER 4: SETPOINTS
Ground Fault
When motor stator windings become wet or otherwise suffer insulation deterioration, low
magnitude leakage currents often precede complete failure and resultant destructive fault
currents. This ground fault protection provides early detection of such leakage current, so
that the motor can be tripped in time to limit motor damage. However, if a high magnitude
ground fault occurs that is beyond the capability of the contactor to interrupt, it is
desirable to program the function of Ground Fault as Control to signal an upstream device
or wait for the fuses to do the interruption. Various situations (e.g. contactor bounce) can
cause transient ground currents during motor starting that can exceed the Pickup level for
a very short period of time. The Trip Pickup Start Delay setting 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. However, time to trip might 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 causes
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 element when the motor starts normally
enables the relay to ride through this momentary ground current signal.
Path:
Setpoints > Protection > Group 1(6) > Motor > Ground Fault
TRIP FUNCTION
Range: Disabled, Trip, Configurable
Default: Disabled
This setting enables the Ground Fault Trip functionality.
GROUND CT TYPE
Range: 1/5A, 50:0.025A
Default: 1/5A
For high resistance grounded systems, sensitive ground current detection is possible if
the 50:0.025A input is used. To use the 50:0.025A input, select “50:0.025A” for the Ground
CT Type. On solidly grounded systems where fault currents can be quite large, the 1/5A
(1A or 5A depending on the order code) secondary ground CT input must be used for
either zero-sequence or residual ground sensing. If the connection is residual, the
Ground CT secondary and primary values must be the same as the phase CT. If however,
the connection is zero-sequence, the Ground CT secondary and primary values must be
entered.
NOTE:
The Ground CT type setting is only applicable when 50:0.025A is selected in the order code.
Otherwise, this setting is hidden and 1/5A secondary Ground CT Type is used as the base
value.
TRIP PICKUP
For 1/5A Ground CT Type:
Range: 0.01 to 10.00 x CT in steps of 0.01 x CT
Default: 0.2 x CT
For 50:0.025 Ground CT Type:
Range: 0.50 to 15.00 A in steps of 0.01 A
Default: 10.00 A
This setting specifies a pickup threshold for the trip function.
TRIP PICKUP START DELAY
Range: 0.00 to 180.00 s in steps of 0.01 s
Default: 1.00 s
This setting specifies the amount of time motor ground current must exceed pickup to
generate a trip when the motor is in a starting condition.
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