GE AKR Manual page 18

Ground Fault Protection
A ground fault is uninten -
tional current flowing from a
circuit through a conducting
path
to
ground
.
It is usually
l *
t
Function
An
l
2
t
function is available
with MicroVersaTrip
.
® This
provides a ramp function
( l
2
t
=
constant )
in the short -
time and ground fault char
-
of
a low
magnitude
{ less
acteristic ( See Fig
.
16.1 )
that allows maximum
coor -
value ) and
not
sensed by the
dination
with downstream
thermal
- magnetic protective modern electronic trip
devices that follow a curve
instead of a square path
.
The l
2
t
function also allows
tighter motor protection by
accommodating the tran
-
sient inrush while the short
-
time pickup remains just
slightly higher than the
steady state starting current
.
In other words , the path fol
-
plex collection of external
lows
the hypotenuse in
-
stead of the
right angle
.
When the square short
-
time ground fault and signal a
path might intersect with
a downstream fuse or
breaker
,
the l
2
t function per
-
delay bands with l
2
t
in and
mits the short
-
time delay
path to veer in
front of the
time
current
curve
for the
8
8
2
S
4
$ $
7
| *
10 H
.
1 - t
.
7 M I I
»
H 51 B ) 0 HN
:
ltd
7
Ml
toe
/
700
MO
r
/
MO GROUND FAULT
/
PICKUP
IDO
y ' A
than the long
-
time pickup
200
protective device
.
Using
/
100
«
0
'
•
0 devices , protection against
ground faults can be built
into a distribution
protection
system
with ground fault
pickup and delay functions
added
to
the circuit break
-
er
' s fault interrupting capa -
bility
.
Before electronic trips
came on the scene ,
a
com
-
70
«
0
so
40
'
' lauiLr 30
'
-
/
20
/
X
V
GROUND FAULT DELAY
!
*
T IN l
*
T OUT
10
7
I
7
MAX MAX /
'
/
6
1 /
7
in
5
relays and current sensors
were needed
to
sense a
a V
z
V
,
o
«
/
/
IA
/
z
/
u
breaker
to
trip
.
Eight ground
fault pickup levels and three
m
% ^
m
in
in
IN
our
i
KB
9
out to
allow full selectivity
,
insuring that only the
breaker closest
to
the
downstream
device
.
Without ground fault will trip
.
Addi
-
this function , coordination
and selectivity would be dif
-
locking for the ground fault
ficult to
attain
.
SIS
OUT
SIS
1
Y
A
SS
SIS !
KZ
res
g
5 f
5
*
r
<
tVPCB
MC
a
.
icce
Mai 4
55
cx
*
L
tion of zone selective inter
-
>
C
LVPC 8
MC
S
ICCB
i
x
V
,
\
N A
r XX
4
,
J
ivpce
MC
.
& ICCB
suss
Hi
BSSSSSSSS *
function further enhances
1
v
\
\
\ this protection by permitting
" timed " tripping and closer
back
-
up
protection
.
x .
\
i
0
»
Dt
01
M
.
05
0
»
.
0
)
.
02
.
01
I 4
S
•
7
•
I 10 20 30 40 10 ID 701 DM
s
i 7 i
a
Fig 17.1 Time Current Curve Definition Ground Fault
17