Schweitzer Engineering Laboratories SEL-321 - Equations for US Curves

Date Code 20011026 Specifications 2-81
SEL-321/321-1 Instruction Manual
Equations for US Curves
t
M
= curve operating time in seconds
t
r
= reset time in seconds (for time delay reset)
TD = time dial setting
M = multiples of pickup. For curve operating time (t
M
), M>1.
For reset time (t
r
), M≤1.
Curve U1: Moderately Inverse - See Figure 2.50
ú
û
ù
ê
ë
é
−
+=
1M
0104.0
0226.0TDt
02.0
m
2
r
M1
08.1TD
t
−
⋅
=
Curve U2: Inverse - See Figure 2.51
ú
û
ù
ê
ë
é
−
+=
1M
95.5
180.0TDt
2
m
2
r
M1
95.5TD
t
−
⋅
=
Curve U3: Very Inverse - See Figure 2.52
ú
û
ù
ê
ë
é
−
+=
1M
88.3
0963.0TDt
2
m
2
r
M1
88.3TD
t
−
⋅
=
Curve U4: Extremely Inverse - See Figure 2.53
ú
û
ù
ê
ë
é
−
+=
1M
67.5
0352.0TDt
2
m
2
r
M1
67.5TD
t
−
⋅
=