Ground Fault (Earth Leakage) Setpoints - GE 269Plus Instruction Manual

GROUND FAULT (EARTH LEAKAGE) SETPOINTS

Aging and thermal cycling can eventually cause a lowering of the dielectric strength of the insulation in the sta-
tor winding. This can produce a low impedance path from the supply to ground resulting in ground fault cur-
rents 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 oper-
ating 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 damag-
ing the motor, a ground fault can place the motor casing above ground potential thus presenting a safety haz-
ard to personnel.
On the occurrence of a ground fault caused by insulation breakdown, an unprotected motor will commonly suf-
fer severe structural damage and have to be replaced. The fault could also shut down the power supply bus to
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which the faulty motor is connected.
Ground faults can occur in otherwise good motors because of environmental conditions. Moisture or conduc-
tive 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 269Plus, all three of the motor conductors must pass through a separate
ground fault CT (see Section 2.6: GROUND CT INPUT on page 2–15). The CT may be either GE Power Man-
agement's 50:0.025A (2000:1 ratio) or 50:5 up to 1500:5 and is chosen in Setpoints page 1. Separate ground
fault trip and alarm levels, 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 269Plus 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 nui-
sance 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.
Care must be taken when turning on this feature. If the interrupting device (circuit breaker or
contactor) is not rated to break ground fault current (low resistance or solidly grounded sys-
tems), the trip setpoint should be set to
WARNING
and connected such that it trips an upstream device that is capable of breaking the fault cur-
rent.
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Courtesy of NationalSwitchgear.com
3.12 GROUND FAULT (EARTH LEAKAGE) SETPOINTS
. The feature may be assigned to the AUX1 relay
O F F
269Plus Motor Management Relay
3 SETUP AND USE
GE Power Management