False Tripping (Ground Fault); Replacement Of Current Sensors - GE AKR-4-30 Maintenance Manual Supplement

FALSE TRIPPING - BREAKERS EQUIPPED WITH
GROUND FAULT
When nuisance tripping occurs on breakers
equipped with the Ground Fault trip element, a
probable cause is the existence of a false "ground"
signal. 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
transformer. Under 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
3ensors have the same electrical characteristics.
If one sensor differs from the others (i.e., differ­
ent rating or wrong tap setting), the differential
transformer can produce output sufficient to trip the
breaker. Similarly, discontinuity between any
sensor and the programmer unit can cause a false
trip signal.
nuisance tripping is encountered on any breaker
If
whose SST components have previously demonstrated
satisfactory performance via the TAK-TS1 Test
Set, the sensors and their connections should be
closely scrutinized.
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 diagram Fig. 27).
In 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 se-
After disconnecting the
Power Circuit Breakers, GEI-86134
condary disconnect block.
for continuity from the breaker-mounted
neutral secondary
through to the female harness connector
(terminals L and N).
(3) the breaker's lower studs connect to
If
the supply source, then the neutral sen­
sor must have its LOAD end connected
to the source.
(4) Ensure that the neutral conductor is
carrying only that neutral current asso­
ciated with the breaker's load current
(neutral not shared with other loads).
e) the preceding steps fail to identify the prob­
If
lem, 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.

REPLACEMENT OF CURRENT SENSORS

Referring to Fig. 17, removal of individual SST
current sensors is accomplished as follows:
a) Disconnect the breaker harness from the tap
terminal board (5 ), removing cable ties as
necessary. Unfasten the 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 provide more working clearance
to adjacent accessories, it may sometimes
be necessary to remove the sensor stud
(8) also. Do this by removing its four
holding bolts, accessible from the rear of
the breaker (see Fig. 20).
Also check
disconnect block
19