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Op erations- Ground Fault Protection
Adjustable Ground Fault Pickup and Adjustable Ground Fault Delay
23
24
26
Pickup
30
28
� !�� n d
.
5
@.Sin
Delay
Sees
·1
.
4
12t
Adjustable Ground Fault Pickup
The G round Fault Pickup adjustment is used to set the level
of ground current at which circuit interruption will be initi
ated. Together with the G round Fault Delay, this adjustment
allows selective tripping between main and feeder or other
downstream breakers. The available ground fault pickup
settings, as a percent of the SB breaker frame ampere
rating I
are g iven in the table below. In compliance with the
n
National Electric Code (NEC 230-95), no trip point setting
exceeds 1 200 amperes.
Frame A mpere
Rating In
A vailable Setting % In
400A
20
25
30
800A
20
25
30
1200A
20
25
30
1600A
20
26
32
2000A
20
23
27
2500A
20
23
26
3200A
20
21
23
4000A
20
21
22
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 Elec
tronic Trip Uni t s; one is a fixed delay and the other is an inverse
12t 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 l2t
ramp delay mode, the delay may be set toa calibrated value
of . 1 , .2, .3, .4, or .5 seconds at a current equal to 0 .5 times
the frame ampere rating ln. The inverse 12t ramp delay
reverts to a fixed delay of the same value when the ground
(lg)
current
exceeds 50 percent of the frame rating (In)·
Ground Fault Memory Circuit
All Siemens Electronic Trip Units with ground fault protec
tion come equipped with a ground fault memory circuit . This
circuit effectively i ntegrates ground fault currents with time.
This provides an added protection by preventing the ground
fault delay circuits from bei ng 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 G round Fault Delay.
32
40
50
60
70
80
90
100
40
50
60
70
80
90
100
40
50
60
70
80
90
100
38
44
50
56
62
68
75
30
35
40
45
50
55
60
29
32
35
38
41
44
48
25
27
29
31
33
35
37
24
23
25
26
27
28
30
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 G round
•
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.
lg =%In
10
100
lg
%In
100
10
.o::
Ground Fault Delay
1000
Ground Fault
Fixed Delay
F1 x ; t5
1
Available Sw1tch Settings
(All Frame Sizes)
Adjustable Ground
Fault Delay
1000
Ground Fault
12t Delay
1
Sees
@51,
2
.5
.4
l2t
Available Switch Settings
(All Frame Sizes)
Adjustable Ground
Fault Delay
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