Schweitzer Engineering Laboratories SEL-351 - Time-Overcurrent Curves

Date Code 20001006 Setting the Relay 9-3
SEL-351 Instruction Manual
TIME-OVERCURRENT CURVES
The following information describes the curve timing for the curve and time dial settings made for
the time-overcurrent elements (see Figures 3.14 through 3.20). The time-overcurrent relay curves
in Figure 9.1 through Figure 9.10 conform to IEEE C37.112-1996 IEEE Standard Inverse-Time
Characteristic Equations for Overcurrent Relays.
tp = operating time in seconds
tr = electromechanical induction-disk emulation reset time in seconds (if electromechanical reset
setting is made)
TD = time dial setting
M = applied multiples of pickup current [for operating time (tp), M>1; for reset time (tr), M≤1].
U.S. Moderately Inverse Curve: U1 U.S. Inverse Curve: U2
tp = TD*(0.0226 + 0.0104/(M
0.02
-1)) tp = TD*(0.180 + 5.95/(M
2
-1))
tr = TD*(1.08/(1-M
2
)) tr = TD*(5.95/(1-M
2
))
U.S. Very Inverse Curve: U3 U.S. Extremely Inverse Curve: U4
tp = TD*(0.0963 + 3.88/(M
2
-1)) tp = TD*(0.0352 + 5.67/(M
2
-1))
tr = TD*(3.88/(1-M
2
)) tr = TD*(5.67/(1-M
2
))
U.S. Short-Time Inverse Curve: U5
tp = TD*(0.00262 + 0.00342/(M
0.02
-1))
tr = TD*(0.323/(1-M
2
))
I.E.C. Class A Curve (Standard Inverse): C1 I.E.C. Class B Curve (Very Inverse): C2)
tp = TD*(0.14/(M
0.02
-1)) tp = TD*(13.5/(M-1))
tr = TD*(13.5/(1-M
2
)) tr = TD*(47.3/(1-M
2
))
I.E.C. Class C Curve (Extremely Inverse): C3 I.E.C. Long-Time Inverse Curve: C4
tp = TD*(80.0/(M
2
-1)) tp = TD*(120.0/(M-1))
tr = TD*(80.0/(1-M
2
)) tr = TD*(120.0/(1-M))
I.E.C. Short-Time Inverse Curve: C5
tp = TD*(0.05/(M
0.04
-1))
tr = TD*(4.85/(1-M
2
))
Courtesy of NationalSwitchgear.com