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GEI-86134, Power Circuit Breqkers
TYPE
SST
OVERCURRENT TRIP DEVICE
The
SST
is a
solid-state, direct-acting, self-
powered
trip
device
system. Referring to
the block
diagram of
Fig.
16
and
the
photograph
of Fig.
15,
it
comprises the following individual
components:
Programmer
Unit
-
provides the
comparison
bas-is-T6T-6T6?6frrGnt detection and
delivers
the
energ'y
necessary
to
trip
the
breaker.
Contains the
electronic
circuitry for the various
trip
elements.
Their
associated pickup and
timedelay
adjustments
(set-points) are located on
the
lace
plate.
Depend-
ing
on the application, programmer units may
be
equipped
rvith various combinations
of
Long Time,
Short Time,
Instantaneous
and Ground
Fault trip
elements. See Table
4 for
available
ratings,
settings
and
trip
characteristics.
The
SST programmer
units
can be optionally
equipped
with
trip
indicators
(targets).
These are
pop-out, mechanically-resettable plungers
Iocated
across the top
of
the programmer's
front.
Units
with
a
ground
fault
element employ three targets:
from left to right,
the
first is
for
overload,
the
second
for short circuit
(actuated
by the
short time
andinstantaneous
elements) andthe
third
for
ground
fault.
The
latter is
omitted
on
units without
qround
fault.
Each
target
pops
out when
its
associated
trip
element operates
to
trip
the
breaker. After
a
trip,
the popped
target must
be
reset
by
hand.
However,
neglecting
to reset
does
not aJfectnormal operation
of
any
trip
element
or
prevent the breaker from
being
reclosed.
Current
Sensors
-
each
pole mounts
a
single
senE5i-66II(ef)-EETch supplies the power
and
sig-
nal
inputs
necessa-ry
to
operate the
trip
system.
Each sensor
has
four
taps which provide
field
ad-
justment of the
trip
device's continuous
ampere
rating.
See
Fig.
18.
The
SST
Ground
Fault
trip
element operates
on
the
principle that the
instantaneous vaLues
of cur-
rent in the three conductors (four on
4-wire
svs-
tems)
add
to
zero unless ground
current
exists.
On
SST's
equipped
with
Ground
Fault, the
ground
trip
signal
is
developed
by
connecting each phase
sensor
in series with
a companion
primary
winding
on a ground
differentia-ltransformer
mounted
in
the
programmer
unit.
Its
secondary
output
is
zero
so
Iong
as
there
is
no ground
current.
l4
Fig. 15.
SST
Programmer unit
-1
r
I
-
I
([$F$ )
Fig.
16.
SST
Block diagrant
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