Overcurrent Protection Configuration
28
To set the mode, press Enter
until the I
2
T and the FIXED
checkbox is highlighted. Press the Up
or Down
Arrow
key so that a check appears in the desired selection box.
To program the delay value, press Enter
until the Delay
parameter is selected. Press the Up
or Down
Arrow key
until the scale shows the desired value. In the example shown,
the delay is set at 400 milliseconds at 0.5 times rated current.
When selected, press Enter
to move to the mode parameter
or press Escape
then Enter
to accept these settings.
5.7
Ground Fault Sensing Scheme
Brief descriptions follow for the ground fault sensing schemes
as they relate to the trip unit. Detailed technical and application
information of the ground fault sensing schemes is contained in
NEMA Standard No. PB 2.2 “Application Guide for Ground
Fault Protective Devices for Equipment.”
Set the switch on the side of the trip unit to either Ground
Return or Residual mode.
5.7.1
Ground Return
In this scheme, also called Source Ground Return, the phase
currents are not used to detect and process ground faults. The
trip unit executes the ground fault protection function based on
data from a ground current sensor. This sensor is located on the
neutral connection to ground at the service entrance, and is con-
nected to the neutral transformer input terminals on the trip unit.
Figure 5.2
Ground Return Sensing Scheme
5.7.2
Residual (3-Phase, 3-Wire)
Under normal system conditions (without a ground fault condi-
tion), the vector sum of the phase currents being monitored by
the trip unit is zero. This is also true for overcurrent, phase-to-
phase fault, and phase-unbalance conditions. When a phase-
to-ground fault occurs, the vector sum of the phase currents is
directly proportional to the magnitude of the fault. The trip unit’s
microprocessor uses this vector sum data from the internal
breaker current transformers in the execution of the ground
fault protection function.
5.7.3
Residual (3-Phase, 4-Wire)
In the 3-Phase, 4-Wire Residual scheme a fourth current trans-
former is connected in the neutral conductor to sense normal
neutral currents. Under normal system conditions the vector
sum of the currents in all phases and neutral equals zero. This
is also true under the condition of an overcurrent phase-to-
phase fault and phase-unbalance condition. When a phase-to-
ground fault occurs, the fault current returns via a path other
than the neutral. Therefore, the vector sum of the currents no
longer equals zero. This current differential is detected by the
trip unit and used in the execution of the ground fault protec-
tion function.
Figure 5.3
Residual Sensing Scheme
Ground Fault Mode Switch
Residual
Ground
Return
Neutral
Trip
Unit
Main
Source
Neutural
Disconnect
Link
Ground Fault Sensor
Main Bonding
Jumper
4-Wire
Loads
3-Wire
Loads
Feeders
Equipment Ground Bus
Equipment
Grounding
Conductor
Grounding
Electrode
Conductor
Neutral
Trip
Unit
Main
Source
Neutural
Disconnect
Link
Neutral Current Sensor
Main Bonding
Jumper
4-Wire
Loads
3-Wire
Loads
Feeders
Equipment Ground Bus
Equipment
Grounding
Conductor
Grounding
Electrode
Conductor
