Schweitzer Engineering Laboratories SEL-321 - Equations for IEC Curves

2-82 Specifications Date Code 20011026
SEL-321/321-1 Instruction Manual
Equations for IEC Curves
t = operating time in seconds,
TD = time dial setting,
M = multiples of pickup.
Curve C1: Standard Inverse - See Figure 2.54
ú
û
ù
ê
ë
é
−
=
1M
14.0
TDt
02.0
m
2
r
M1
5.13TD
t
−
⋅
=
Beyond 20 multiples of pickup, all characteristics become definite-time equal to the time of
M = 20.
Curve C2: Very Inverse - See Figure 2.55
ú
û
ù
ê
ë
é
−
=
1M
5.13
TDt
m
2
r
M1
3.47TD
t
−
⋅
=
Curve C3: Extremely Inverse - See Figure 2.56
ú
û
ù
ê
ë
é
−
=
1M
0.80
TDt
2
m
2
r
M1
80TD
t
−
⋅
=
Curve C4: Long Time Backup - See Figure 2.57
ú
û
ù
ê
ë
é
−
=
1M
0.120
TDt
m
M1
120TD
t
r
−
⋅
=