Schweitzer Engineering Laboratories SEL-749M - Table 4.9 Residual Overcurrent Settings; Table 4.10 Load Jam Settings; Figure 4.4 Ground Fault Protection Using Core-Balance CT

4.13
Date Code 20080918 Instruction Manual SEL-749M Relay
Protection and Logic Functions
Basic Motor Protection
Figure 4.4 Ground Fault Protection Using Core-Balance CT
When a core balance CT is not available, use the 50G residual overcurrent
elements. Set the residual overcurrent trip level (50G1P) between one-half and
one-fifth of the full load phase current and set the residual overcurrent trip
delay (50G1D) equal to 0.2 seconds. Set the warn level more sensitively, but
with a longer time delay. The long time delay allows the sensitive warn
element to ride through the false residual current that can be caused by phase
CT saturation during motor starting.
Load Jam Elements
Load jam protection is available only when the relay detects that the motor is
in the RUNNING state. During a load jam condition, the motor stalls and the
phase current rises to near the locked rotor value. When load jam tripping is
enabled and the phase current exceeds the jam trip level setting (LJTPU) for
longer than the time delay setting (LJTDLY), the relay trips (see Figure 4.5).
Set the jam trip level greater than the expected normal load current but less
than the rated locked rotor current. This setting is entered in per unit of the full
load amps (FLA) setting.
Table 4.9 Residual Overcurrent Settings
Setting Prompt Setting Range Setting Name := Factory Default
RES OC TRIP LVL Off, 0.10–1.00 xFLA 50G1P := OFF
RES OC TRIP DLAY 0.00–5.00 sec 50G1D := 0.50
RES OC WARN LVL Off, 0.10–1.00 xFLA 50G2P := OFF
RES OC WARN DLAY 0.0–120.0 sec 50G2D := 10.0
10 A
C-Ground Fault
10 A
AB
C
1 A
10:1
Window CT
Z
0
7
Z
0
8
Motor
NOTE: Phase CT ratios are typically
higher than core balance (CB) CT
ratios. For this reason, the relay
sensitivity to motor ground faults is
less when the residual overcurrent
element is used instead of the CB
element. A separate ground fault
detection method should be used if a
CB CT is not available in applications
where resistance grounding reduces
the available ground fault current.
Table 4.10 Load Jam Settings
Setting Prompt Setting Range Setting Name := Factory Default
JAM TRIP LEVEL Off, 1.00–6.00 xFLA LJTPU := OFF
JAM TRIP DELAY 0.0–120.0 sec LJTDLY := 0.5
JAM WARN LEVEL Off, 1.00–6.00 xFLA LJAPU := OFF
JAM WARN DELAY 0.0–120.0 sec LJADLY := 5.0
Courtesy of NationalSwitchgear.com