Operations - Ground Fault Protection
33
Adjustable Ground Fault Pickup and Adjustable Ground Fault Delay
Adjustable Ground Fault Pickup
The Ground Fault Pickup adjustment is used to set the level
of ground current at which circuit interruption will be initiated.
Together with the Ground Fault Delay, this adjustment allows
selective tripping between main and feeder or other down-
stream breakers. The available ground fault pickup settings,
as a percent of the SB breaker frame ampere rating In are
given in the table below. In compliance with the National
Electric Code (NEC230-95), no trip point setting exceeds
1200 amperes.
Frame Ampere
Rating In
Available Setting % In
400A
800A
20 25 30 40 50 60 70 80 90 l00
20 25 30 40 50 60 70 80 90 l00
1200A
1600A
20 25 30 40 50 60 70 80 90 100
20 26 32 38 44 50 56 62 68 75
2000A
20 23 27 30 35 40 45 50 55 60
Adjustable Ground Fault Delay
The Ground Fault Delay adjustment is used to set the time
interval the breaker will wait before responding once the
ground fault pickup level has been reached . There are two
modes of operation of this adjustment for Siemens Electronic
Trip Units; one is a fixed delay and the other is an inverse I
2
t
ramp delay. In the fixed delay mode, the Ground Fault Delay
may be set to .1, . 2, .3, .4, or .5 seconds. In the inverse I
2
t
ramp delay mode, the delay may be set to a calibrated value
of .1 .2, .3, .4, or .5 seconds at a current equal to 0.5 times
the frame ampere rating In. The inverse I
2
t ramp delay reverts
to a fixed delay of the same value when the ground current
(I
g
) exceeds 50 percent of the frame rating (I
n
)
Ground Fault Memory Circuit
All Siemens Electronic Trip Units with ground fault protection
come equipped with a ground fault memory circuit. This
circuit effectively integrates ground fault currents with time.
This provides an added protection by preventing the ground
fault delay circuits from being reset to zero when the ground
fault currents are intermittent and erratic. The time constants
for the current integration are preset within the trip unit as a
function of the Ground Fault Delay.
Ground Fault Sensing Schemes
The trip unit can be configured to accommodate the
following ground fault sensing schemes.
• 3-Phase, 3-Wire Residual
• 3-Phase, 4-Wire Residual
• Source Ground
• Zero Sequence
All that is required by the user to configure the trip unit to
support these protection schemes is to set the ground fault
selection switch to the desired configuration. The selection
switch is on the left side of the trip unit and must be set prior
to the trip unit being installed in the SB breaker.
Ground Fault Delay
Summary of Contents for SBA 1200
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