BECKWITH ELECTRIC M-3425A, Instruction Book

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Inverse Time Curves: Appendix– D
D–1
D
Appendix D – Inverse Time Curves
This Appendix contains two sets of Inverse Time Curve Families. The first set is used for Volts per Hertz func‑
tions (Figures D‑1 through D‑4), and the second set is for the M‑3425A functions which utilize the Inverse Time
Overcurrent curves (Figures D‑5 through D‑12).

NOTE
: Table D‑1A and D‑1B on pages D–6 and D–7 contains a list of the data that characterizes Definite
Time, Inverse Time, Very Inverse Time, and Extremely Inverse Time Overcurrent Curves.
Expression for Time Delay Setting
Operating time defined by IEC and ANSI/IEEE:
t = TD A t = TD A
M P ‑ 1 5 M P ‑ 1
+ B
IEC Equation IEEE Equation (IEEE Equation Constants are defined at TD of 5)
Where
t = Relay operating time in seconds
TD = Time dial, or time multiplier setting
I = Fault current level in secondary amps
I
P
= Tap or pickup current selected
B = Constant
p = Slope constant
A = Slope constant
M = I
I
P
Setting Time Delay on Overcurrent Relays ANSI/IEEE and IEC Constants for Overcurrent Relays
IDMT Curve Description Standard p A B
Moderately Inverse IEEE 0.02 0.0515 0.114
Very Inverse IEEE 2 19.61 0.491
Extremely Inverse IEEE 2 28.2 0.1217
Standard Inverse IEC 0.02 0.14 -
Very Inverse IEC 1.0 13.5 -
Extremely Inverse IEC 2.0 80.0 -