GE AKR-30S Manual page 18

Ft
Function
An Ft function is available
with MicroVersaTrip.® This
provides a ramp function
Wt =constant) in the short­
time and ground fault char­
acteristic (See Fig. 1 6 .1)
that allows maximum coor ­
dination with downstream
thermal-magnetic protective
devices that follow a curve
instead of a square path.
The 12t 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
lows the hypotenuse in­
stead of the right angle.
When the square short-time
path might intersect with
a downstream fuse or
breaker, the l2t function per­
mits the short-time delay
path to veer in front of the
time current curve for the
downstream device. Without
this function, coordination
and selectivity would be dif­
ficult to attain.
Ground Fault Protection
A ground fault is uninten­
tional current flowing from a
circuit through a conducting
path to ground. It is usually
of a low magnitude (less
than the long-time pickup
value) and not sensed by the
protective device. Using
modern electronic trip
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
bil ity. Before electronic trips
came on the scene, a com­
plex collection of external
relays and current sensors
were needed to sense a
ground fault and signal a
breaker to trip. Eight ground
fault pickup levels and three
delay bands with l2t in and
out to allow full selectivity,
insuring that only the
breaker closest to the
ground fault will trip. Addi­
tion of zone selective inter­
locking for the ground fault
function further enhances
this protection by permitting
"timed" tripping and closer
back-up protection.
1000
'"
'"
"'
"' f-+-++-I'H-- -+- --t--- 1
I' I
OlfT
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17.1
Fig Time Current Curve Definition Ground Fault
g
GROUND FAULT
PICKUP
�
35 .355
255 .45
�
25 455
OFF 55
.65
GROUND FAULT DELAY
1 2 T IN
1 2 T OUT
MAX MAX
LVPCB
Ma�
MC & ICCB
LVPCB
LVPCB
MC & !CCB
Moo
�
17