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GE Power Management
3 SETUP AND USE
3.11 Ground Fault (Earth Leakage) Setpoints
Aging and thermal cycling can eventually cause a lowering of the dielectric strength of the insulation in the stator
winding. This can produce a low impedance path from the supply to ground resulting in ground fault currents which
can be quite high in solidly grounded systems. In resistance grounded systems there is a resistance in series with
the supply source to limit ground fault 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. 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, an unprotected motor will commonly suffer
severe structural damage and have to be replaced. The fault could also shut down the power supply bus to which
the faulty motor is connected.
Ground faults can occur in otherwise good motors because of environmental conditions. Moisture or conductive
dust, which are often present in mines, can provide an electrical path to ground thus allowing ground fault 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.
For ground fault protection by the 169 relay, all three of the motor conductors must pass through a separate ground
fault CT (see section 2.6). The CT ratio may be either 2000:1 or 50:5 up to 250:5 (in increments of 50) and is
chosen in SETPOINTS mode, page 1. Separate ground fault trip and alarm levels (entered in actual ground fault
amps), and persistence times (time delays) may also be set. The ground fault trip can be instantaneous, or up to
20.0 seconds of time delay can be chosen to allow the 169 relay to be coordinated with other protective devices and
switchgear.
The amount of current that will flow due to a fault depends on where the fault occurs in the motor winding. A high
current flow will result if a short to ground occurs near the end of the stator winding nearest the terminal voltage. A
low ground fault current will flow if a fault occurs at the neutral end of the winding since this end should be a virtual
ground. Thus a low level of ground fault pickup is desirable to protect as much of the stator winding as possible and
to prevent the motor casing from becoming a shock hazard. In resistance grounded systems the ground fault trip
level must be set below the maximum current limited by the ground resistor or else the relay will not see a large
enough ground fault current to cause a trip.
The ground fault trip level should be set as low as possible, although too sensitive a setting may cause nuisance
trips due to capacitive current flow. If nuisance trips occur with no apparent cause the trip level should be increased;
conversely if no nuisance trips occur a lower fault setpoint may be desirable.
3.12 Undercurrent Setpoints
These setpoints are found in SETPOINTS mode, page 1 and are normally used to detect a decrease in motor
current flow caused by a loss of, or decrease in, motor load. This is especially useful for indication of loss of suction
for pumps, loss of airflow for fans, or a broken belt for conveyors. When the current falls below the setpoint value for
the setpoint time, the relay assigned to this alarm function will become active.
If this feature is used for loss of load detection, the "UNDERCURRENT ALARM LEVEL" setpoint should be chosen
to be just above the motor current level for the anticipated reduced load condition. If the feature is not desired, the
alarm level should be set to "OFF". The delay time setpoint will then be ignored by the relay.
If the motor is normally operated at a current level below its rated full load current, this feature may be used for a
pre-overload warning. This is accomplished by setting the "UNDERCURRENT ALARM LEVEL" to be above the
normal operating current of the motor but below the rated full load current. In this way the undercurrent function will
cause the relay assigned to it to become inactive if the motor current increases above the Undercurrent setpoint
level. This would indicate an abnormal loading condition prior to an actual motor overload.
The output relay assigned to this function will automatically reset itself when the motor stops (i.e. when the phase
current becomes zero) unless this relay is programmed as latched (see "RELAY ALARM LATCHCODE",
SETPOINTS, page 5).
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- Quick Links:
- 169 Relay Features
- Trip/Alarm Mode
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