Ansi C37.112 Idmt Characteristic; Ri Type Characteristic - ABB REF601 Applications Manual

Section 4
Protection and Control Function
4.10.3
4.10.4
32

ANSI C37.112 IDMT characteristic

The relationship between current and time for standard moderate inverse, normal
inverse, very inverse, extremely inverse complies with the ANSI C37.112
standards and can be expressed as follows:
Where:
= + ∗
− 1
Where,
t = operate time in seconds
K = time multiplier
I = measured current value
Iset = set start current value
The slope of the time/current characteristics shall be determined by the
constants a and b and g as indicated below:
Table 20: Values of constant a, b and g
Slope of the time/current curve set
ANSI – Moderate inverse
ANSI – Normal inverse
ANSI – Very inverse
ANSI – Extremely inverse

RI type characteristic

The RI-type characteristic is a special characteristic used mainly in combination
with existing mechanical relays. The characteristic is based on the following
mathematical expression:
=
− ( )
Where,
t = operate time in seconds
K = time multiplier
I = measured current value
Iset = set start current value
α
= 0.339
β
= 0.236
1MDU07212-YN Rev. C
a b g
0.02 0.0515 0.1140
0.02 0.0086 0.0185
2.0 19.61 0.491
2.0 28.2 0.1217
REF601 / REJ601
Application Manual