0.4.3 False Tripping-Br Eakers; Equ I Pped With G Round Fault; 0.5 Sst Cabling Diag Rams - GE AKR-30 Series Maintenance Manual

1 0.4.3 FALSE TRIPPING-BR EAKERS
EQUIPPED WITH G RO U N D FAU LT
When nuisance tripping occurs on breakers equipped
with the G round Fault trip element, a probable cause is the
existence of a false "ground" signal. 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. U nder no-fault conditions on
3-wire load circuits, the currents 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 sufficient to trip
the breaker. Similarly, disconti nuity between any sensor
and the programmer un it can cause a false trip signal.
If n u isance tripping is e ncou ntered on any breaker
whose SST components have previously demonstrated
satisfactory performance via the T AK-TS 1 Test Set, the
s e n s o rs a n d t h e i r c o n n e ct i o n s s h o u l d be c l o s e l y
scruti nized. After disconnecting t h e 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
:������
e
LEFT POLE
CURRENT
SENSOR
FIG. 68 - CABLING DIAG RAM - SST WITHOUT G RO U N D FAULT
+ A + B + C
;__ ;__ ;__
FLUX SHIFT
TRIP DEVICE
11 -1-t- 1
E
� )===}= ==)=
I
I
LOAD
c) Verify that the harness connections to the sensors
meet the polarity constraints i ndicated by the cabling dia­
gram, i . e . , wh ite wire to COMMON, black wire to TAP .
d) On G round Fault breakers serving 4-wire loads,
check that the neutral sensor is properly connected (see
cabling diagram Fig. 70). I n particular,
( 1 ) Verify that the neutral sensor has the same rating
and tap setting as the phase sensors.
(2) Check continuity between the neutral sensor and its
equipment-mounted secondary disconnect block. Also
check for continuity from the breaker-mounted neutral
secondary discon nect block through to the female harness
con nector (terminals L and N ) .
( 3 ) If t h e breaker's lower studs con nect t o the supply
source, then the neutral sensor must have its LOAD encl
connected to the source.
(4) Ensure that the neutral cond uctor is carrying onl�'
that neutral current associated with the breaker's load cur­
rent (neutral not shared with other loads).
e) If the preceding steps fail to identify the problem, then
the sensor resistances should be measured. Since th13
phase and neutral sensors are electrically identical , their
tap-to-tap resistance should closely agree. See Table 8.
PROGRAMMER
UNIT
�-- - - - -
48V. de)
(
(TO SCR
ANODE)
WHITE
BLACK
WHITE
BLACK
WHITE
BLACK
HARNESS
PR O GR AM M ER
CO NN E CT OR
CONNECTOR
201 298-1 )
(AMP
201 297-1 )
(AMP
53