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Power Circuit Breakers, GEI'86134
FALSE
TRIPPING
-
BREAKERS EQUIPPED
WITH
GROUND FAULI
When nuisance
tripping occurs on
breakers
equipped
with the
Ground
Fault
trip
element,
a
p"onalte
cause
is
the existence of
a
false
"ground"
slgnaf
.
As indicated by the cabling diagram of Fig.
26, each phase sensor
is
connected
in series with
a
primary
winding on
the
Ground
Fault differential
lransformer.
Under
no-fault conditions on 3-wire
Ioad
circuits,
the
currents in these three
windings
add
to zero
and
no
ground signal
is
developed.
This current
sum
will
be zero only i-t
all
three
sensors
have the same electricaL characteristics.
If
one
sensor di-ffers
from
the
others (i.
e.,
differ-
ent rating
or
wrong tap setting), the dilferential
transformer
can
produce output
sufficient to
trip
the
breaker. Similarly,
discontinuity between
any
sensor
and
the
programmer unit can cause
a
false
trip
signal.
If
nuisanc
e
tripping is
encountered on any breal<er
whose
SST
components
have
previously demonstrated
satisfactory performance
via
the
TAK-TS1 Test
Set, the
sensors and
their
connections should
be
closely scrutinized.
After
disconnecting
the
breaker from
all
power sources,
a)
Check
that
all
phase
sensors
are the
same
type (ampere range).
b) Ensure that the tap
settings on
all
3-phase
sensors
are identical.
c) Verify that the
harness
connections
to
the
sensors meet the polarity constraints indi-
cated
by the
cabling diagram,
i.e.,
white
wire
to COMMON, black
wire
to TAP.
d)
On
Ground
Fault breakers serving
4-wire
loads,
check
that
the
neutral sensor is prop-
erly
connected
(see cabling
diagramFig.
2?).
In particular,
(1) Verify
that the
neutral sensor has
the
same
rating
and
tap
setting as
the
phase
sensors.
(2)
Check continuity
between
tlle
neutral
sensor
ald its
equipment-mounted se-
condary disconnect
block.
Also
check
for
continuity from the breaker-mounted
neutral
secondary discormect
block
through
to
the
female harness
connector
(terminals
L
and
N).
If
the breal<er's lower studs
connect to
the supply
source, then the neutral sen-
sor must
have
its
LOAD end
connected
to
the source.
Ensure
that the neutral
eonductor is
carrying only that neutral current
asso-
ciated
with the
breaker's load current
(neutral not shared with other loads).
e)
If
the
precedingstepsfail toidenti-ty the prob-
Iem,
then
the
sensor resistances should
be
measured.
Since
the phase
and neutral sen-
sors are
electrically identical, their tap-to-
tap
resistances should closely agree.
REPTACEMENT
OF
CURRENT
SENSORS
Referring to
Fig.
17, remova! of individuat
SST
current
sensors
is
accomplished as
follows:
a)
Disconnect the
breaker harness
from
the tap
terminal
board
(5),
removing cable
ties
as
necessary.
Unfastenthe
terminal
board
from
the
breaker
base.
b) At
the
rear of
the breaker, remove the
two
Allen
head screws (2)
to
separate the
stud
connector (3)
from
the contact
pivot block.
c)
Loosen the clamping
bolt
(7) and
remove the
stud connector.
Lift
out the
sensor and
its
tap
terminal
board.
NOTE:
To
prouide more
working clearance
to
adjacentaccessories,
i,t
may sometimes
be
necessary to remoae the sensor stud
(8)
also.
Do this by
remouing
its foar
holdi,ng
bolts, accessible
from
the
rear of
the
breaker
(see
Fig.
20).
(3)
(4)
19
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