GE AKR-30 Series Maintenance Manual page 231

1 0.4.3 FALSE TRIPPI NG-B R EAKERS
EQUIPPED WITH G RO U N D FAU LT
When nuisance tripping occurs on breakers equi pped
with the G round Fault trip element, a probable cause is the
existence of a false "ground" sig nal . As indicated by the
cabling diagram of Fig. 69, each phase sensor is con­
nected in a series with a primary winding on the G round
Fault differential transformer. Under no-fault conditions on
3-wire load circuits, the cu rrents in these three windings
add to zero and no ground signal is developed. This current
sum will be zero only if all three sensors have the same
electrical characteristics. If one sensor differs from the
others ( i . e . , different rating or wrong tap setti ng ) , the dif­
ferential transformer can produce output suff icient to trip
the breaker. Similarly, discontinu ity between any sensor
and the programmer unit can cause a false trip signal.
If nuisance tripping is encountered on any breaker
whose SST components have previously demonstrated
satisfactory performance via the TAK-TS 1 Test Set, the
s e n so rs and t h e i r c o n n e c t i o n s s h o u l d be c l o s e ly
scrutin ized. After disconnecting the breaker from all power
sources,
a) Check that all phase sensors are the same type (am­
pere range).
b) Ensure that the tap settings on all 3-phase sensors
are identical.
1 0. 5 SST CABLING DIAG RAMS
:�� � � ; �
B
LEFT POLE
CURRENT
SENSOR
FIG. 68 - CABLING DIAG RAM - SST WITHOUT G RO U N D FAULT
+ A + B + C
A A A
FLUX SHIFT
1 -t- t- 1
TRIP DEVICE
�
r
E
)===)= ==)=
I
LOAD
c) Verify that the harness connections to the sensors
meet the polarity constraints i ndicated by the cabling dia­
gram, i . e . , white wire to COMMON, black wire to TAP.
d) On G round Fau lt bre akers serving 4-wire loads,
check that the neutral se nsor is properly connected (see
cabling diagram Fig. 70) . In particular,
( 1 ) Verify that the neutral sensor has the same rating
and tap setting as the phase sensors.
( 2) Check continu ity between the neutral sensor and its
equ ipment-mou nted secondary disco nnect block. Also
check for conti nuity from the breaker-mounted neutral
secondary discon nect block through to the female harness
con nector (terminals L and N ) .
( 3 ) I f the breaker's lower studs connect t o the supply
source , then the neutral sensor must have its LOAD encl
connected to the source.
(4) Ensure that the neutral conductor is carrying only
that neutral current associated with the breaker's load cur­
rent (neutral not shared with other loads) .
e ) I f the preceding steps fai l to identify the proble m, then
the sensor resistances should be measured . Since th13
phase and ne utral sensors are electrically identical, their
tap-to-tap resistance should closely agree. See Table 8 .
PROG RAMMER
UNIT
�- - - - - -
de)
(4BV.
(TO SCR
ANODE)
WHITE
BLACK
WHITE
BLACK
WHITE
BLACK
HARNESS
PR O GR A MM ER
C ON NE C TO R
CONNECTOR
201 298- 1 )
(AMP
201 297-1 )
(AMP
53