GE Industrial Solutions MicroVersaTrip Plus and MicroVersaTrip PM AK-25 and AKU-25 - Manual (Page 7)

7
SECTION 3 TESTING AND
TROUBLESHOOTING
(CONTINUED)
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. Each phase sensor is connected
to summing circuitry in the programmer. Under
no-fault conditions on 3-wire load circuits, the
currents 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 setting),
the circuitry can produce output sufficient to trip
the breaker. Similarly, discontinuity between any
sensor and the trip unit can cause a false trip
signal.
The sensors and their connections should be
closely examined if nuisance tripping is
encountered on any breaker whose
MicroVersaTrip
 Plus or MicroVersaTrip  PM
components have previously demonstrated
satisfactory performance. After disconnecting the
breaker from all power sources, perform the
following steps:
Step 1.
Check that all phase sensors are the same type
(ampere range).
Step 2.
Make sure that the tap settings on all three phase
sensors are identical.
Step 3.
Verify that the harness connections to the sensors
meet the polarity constraints indicated by the
cabling diagram detailed in the installation
instructions included with the conversion kit.
Step 4.
On ground fault breakers serving four-wire loads,
check that the neutral sensor is properly
connected. Refer to the cabling diagram . In
particular, check 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 dis-
connect block. Also check for continuity from the breaker mounted neutral secondary dis-
connect block through 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.
Refer to the cabling diagram detailed in the
installation instructions included with the
conversion kit.
d. Make sure that the neutral conductor is
carrying only that neutral current associated
with the breaker’s load current (neutral not
shared with other loads).
Step 5.
If the preceding steps fail to identify the problem,
then measure the sensor resistance’s. Since the
phase and neutral sensors are electrically
identical, their resistance’s should closely agree.
Table 3-1. CT Resistance Values
Refer to the CT Resistance Values detailed
in the installation instructions included
with the MicroVersaTrip Plus   or PM
Conversion Kit