6.4-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 exis
tence of a false "ground" signal. As indicated by the cabling
diagram of Fig. 6-2, each phase sensor is connected to sum
ming circuitry in the programmer. Under no-fault conditions
on 3-wire load circuits, the currents in this circuitry add to
zero and no ground signal is developed. This current sum will
be zero only if all three sensors have the same electrical char
acteristics. If one sensor differs from the others (i.e., different
rating or wrong tap setting), the circuitry can produce output
sufficient to trip the breaker. Similarly, discontinuity between
any sensor and the programmer unit can cause a false trip
signal.
If nuisance tripping is encountered on any breaker whose
MicroVersaTriprM components have previously demonstrated
satisfactory performance via the TVTS1 Test Set, the sensors
and their connections should be closely scrutinized. After
disconnecting the breaker from all power sources, perform
the following steps.
1. Check that all phase sensors are the same type
(ampere range).
2. Ensure that the tap settings on all three-phase
sensors are identical.
3. Verify that the harness connections to the sensors
meet the polarity constraints indicated by the cabling
diagram.
q,
A
q,
B
q,
c
AAA
BREAKER
;t-t-t-1
BACK FRAME"!
)===)=.==)=-
!
4. On Ground Fault breakers serving four-wire loads,
check that the neutral sensor is properly connected. See
cabling diagram Fig. 6-3. In particular, the following:
A. Verify that the neutral sensor has the same rating
and tap setting as the phase sensors.
B. Check continuity between the neutral sensor and
its equipment-mounted secondary disconnect
block. Also check for continuity from the breaker
mounted neutral secondary disconnect block
through to the female harness connector.
C. If the breaker's lower studs connect to the supply
source, then the neutral sensor must have its
LOAD end connected to the source. See Fig. 6-4.
D. Ensure that the neutral conductor is carrying only
that neutral current associated with the breaker's
load current (neutral not shared with other loads).
5. If the preceding steps fail to identify the problem, then
the sensor resistances should be measured. Since the phase
and neutral sensors are electrically identical, their tap-to-tap
resistance should closely agree. See Table 6-1.
6.5-MicroVersaTrip™ Cabling
Diagrams
FLUX SHIFT
TRIP DEVICE
PROGRAMMER
UNIT
----
--,
I
I
CURRENT
SENSOR
LOAD
HARNESS
CONNECTOR
(AMP 1-3503 56-9) PROGRAMMER
CONNECTOR
Fig. 6-1. Cabling diagram- MicroVersaTrip™ without ground fault
25
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