Short Circuit - GE MI-869 Instruction Manual

CHAPTER 4: SETPOINTS

Short Circuit

IMPORTANT:
NOTE:
869 MOTOR PROTECTION SYSTEM – INSTRUCTION MANUAL
If Short Circuit is enabled, a trip or alarm occurs once the magnitude of any phase current
exceeds the setting Pickup for the time specified by the setting Pickup Delay.
Care must be taken when turning on this feature. If the interrupting device (contactor
or circuit breaker) is not rated to break the fault current, the function of this feature
must not be programmed as TRIP. Alternatively, this feature may be programmed as
ALARM or LATCHED ALARM and assigned to an auxiliary relay connected to an
upstream device which is capable of breaking the fault current.
Path:
Setpoints > Protection > Group 1(6) > Motor > Short Circuit
FUNCTION
Range: Disabled, Trip, Alarm, Latched Alarm, Configurable
Default: Disabled
The setting enables the Short Circuit functionality. The selection of Trip, Alarm or Latched
Alarm for the setting value enables the Short Circuit function.
If the operating condition is satisfied when Trip is selected as the function, the logic
operands Short Circuit PKP and Short Circuit OP are asserted, which in turn operates the
LED "TRIP" and trip output relay.
If Alarm is selected, the LED "ALARM" flashes upon the Short Circuit operation, and it
automatically resets when the activating condition clears.
If Latched Alarm is selected, the LED "ALARM" flashes upon Short Circuit operation, and
stays "ON" after the condition clears, until a reset command is initiated. The TRIP output
relay does not operate if Latched Alarm or Alarm function is selected. Any assignable
output relays can be selected to operate when the Short Circuit function is selected as
Latched Alarm, Alarm or Trip.
OVERREACH FILTER
Range: Off, On
Default: Off
When a motor starts, the starting current (typically 6 × FLA for an induction motor) has
an asymmetrical component. This asymmetrical current may cause one phase to see as
much as 1.6 times the normal RMS starting current. If the Pickup was set at 1.25 times
the symmetrical starting current, it is probable that there would be nuisance trips during
motor starting. A rule of thumb has been developed over time that short circuit
protection be at least 1.6 times the symmetrical starting current value. This allows the
motor to start without nuisance tripping. The overreach filter removes the DC
component from the asymmetrical current present at the moment of fault. This
eliminates overreach; however, the response time slows slightly (10 to 15 ms) but
remains within specification.
PICKUP
Range: 1.00 to 30.00 x CT in steps of 0.01
Default: 6.00 x CT
The setting specifies a pickup threshold for the Short Circuit element.
If 2-Speed Motor Protection functionality is employed, then the CT primary is the value of
setting "2-Speed CT Primary" that can be found under Setpoints > System > Motor.
Special care must be taken when setting Pickup for motor applications with low and high
speed windings. Pickup must be set with enough margin such that short circuit elements
do not malfunction when switching from one speed to another.
PICKUP DELAY
Range: 0.00 to 180.00 s in steps of 0.01
Default: 0.00 s
The setting specifies the pickup delay for the Short Circuit element.
PROTECTION
4–153