4.12
SEL-749M Relay Instruction Manual Date Code 20080918
Protection and Logic Functions
Basic Motor Protection
Figure 4.3 Overcurrent Element Logic
EXAMPLE 4.6 Ground Fault Core-Balance CT Application
A resistance-grounded transformer limits the current for motor or
cable ground faults. The resistor is sized to limit the current to 10 A
primary. The three motor leads are passed through the window of a
10:1 core balance CT. The CT secondary is connected to the SEL-749M
IN current input, as shown in Figure 4.4. Setting the Neutral OC CT
Ratio (CTRN, see Main Settings (SET Command) on page 4.3) equal to
10 and Neutral Trip Lvl (50N1P) equal to 5 A with 0.10 second time
delay ensures that the element will quickly detect and trip for motor
ground faults, but prevent misoperation resulting from unequal
breaker or contactor pole closing times.
50P1P
|IP|
IA
IB
IC
IN
PHROT
IG
Current
Magnitude
Calculations
|IP|
|IN|
|IG|
50P1T
50P2P
50P2T
50N1P
|IN|
50N1T
50N2P
50N2T
50G1P
|IG|
50G1T
50G2P
50G2T
Settings
(Maximum Phase Current Magnitude)
(Core-Balance Current Magnitude)
(Residual Current Magnitude)
50P1D
0
50P2D
0
50N1D
0
50G2D
0
50G1D
0
50N2D
0
Negative-Sequence
Current Calculation
(Negative-Sequence Current Magnitude)
|3I2|
50P1P
50P2P
50N1P
50N2P
50G1P
50G2P
Relay
Word
Bits
Courtesy of NationalSwitchgear.com
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