GE 239 Instruction Manual page 65

4 PROGRAMMING
• BREAKER FAILURE PICKUP: If a trip is present and the current is still flowing (breaker failed to
open) and the level of the average three phase current is equal to or greater than the setting in
the %5($.(5 )$,/85( 3,&.83 setpoint, the breaker failure feature will operate.
• BREAKER FAIL PICKUP DELAY: If all other conditions are met, the breaker failure feature will
operate after the delay programmed in this setpoint has elapsed. See Section 1.4: SPECIFICA-
TIONS on page 1–7 for BREAKER FAILURE timing specifications.
• BREAKER FAIL DROPOUT DELAY: If the breaker opens or if the average three phase current
falls below the %5($.(5 )$,/85( 3,&.83 setpoint, the breaker failure feature will not clear until the
delay programmed in this setpoint has elapsed. See Section 1.4: SPECIFICATIONS on page 1–
7 for BREAKER FAILURE timing specifications.
i) GROUND CURRENT
• GROUND TRIP: Aging and thermal cycling can eventually cause a lowering of the dielectric
strength of the winding insulation in the stator winding. This can produce a low impedance path
from the supply to ground resulting in ground currents which can be quite high in solidly
grounded systems. These could quickly cause severe structural damage to the motor stator
slots. In resistance grounded systems there is a resistance in series with the supply ground con-
nection to limit ground current and allow the system to continue operating for a short time under
fault conditions. The fault should be located and corrected as soon as possible, however, since a
second fault on another phase would result in a very high current flow between the phases
through the two ground fault paths. In addition to damaging the motor, a ground fault can place
the motor casing above ground potential thus presenting a safety hazard to personnel.
On the occurrence of a ground fault caused by insulation breakdown, a motor will usually have to
be taken out of service and rewound. However an unprotected motor could suffer mechanical
damage to the stator slots making repair impossible. The fault could also cause the power supply
bus to which the faulty motor is connected to trip in order to clear the fault resulting in unneces-
sary process shutdowns. Ground faults can occur in otherwise good motors because of environ-
mental conditions. Moisture or conductive dust, which are often present in mines, can provide an
electrical path to ground thus allowing ground current to flow. In this case, ground fault protection
should shut down the motor immediately so that it can be dried or cleaned before being restarted.
On low resistance or solidly grounded systems, sensing of the ground current is done using the
phase CTs wired in a residual connection. For more sensitive ground current detection, a sepa-
rate CT, referred to as a core balance or zero sequence CT, encircles the three motor conduc-
tors. Ground fault detection is only suitable for systems that have a path from the supply to
ground either through a resistance or by direct connection. Ungrounded systems require an arti-
ficial ground to be created through use of a device like a zig-zag transformer if ground fault pro-
tection is to be used.
In systems with several levels of ground fault detection, time co-ordination is required for satis-
factory operation. If ground fault protection is used on a bus, each motor must have a shorter
ground fault trip time delay than the bus ground fault detector or a fault in any motor will shut
down the whole bus. In a solidly grounded system, time delays as short as possible should be
used to prevent system damage unless the contactor is not capable of breaking the fault current
in which case a backup detection system of sufficient interrupting capacity should be allowed to
operate first. When contactors are used in solidly grounded systems, the ground fault trip time
should be longer than the fuse interrupt time.
GE Multilin
Courtesy of NationalSwitchgear.com
239 Motor Protection Relay
4 PROGRAMMING
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