DOC. MIE91093 Rev. 1.34
Page 140 of 145
APPENDIX 1 OVERCURRE NT RELAYS
There are many types of time-dependent curves. They follow
standards set by IEC, IEEE, IAC, ANSI, US, and are defined as
Standard Inverse; Very Inverse; Extremely Inverse or so.
The curve is defined by two parameters:
. The setting threshold I>;
. Another parameter, that can be either the Time Multiplier (K
or KT), or the Time Dial setting (tI>), that is the delay with
Current equal to 10 times the Pick-up Current (10I>).
From these parameters the diagram can be drawn, as follows.
A) Parameter KT
In this instance, IEC and IEEE curves correspond to the
following formula.
c
1
b
I
I
a
*
K
)
s
(
t
R
T
Constants of the formula change according to the type of
curve; they are summarised in the following table.
IEC, IEEE
a
c
b
IEC Class A Standard Inverse
0.14
0
0.02
IEC Class B Very Inverse
13.5
0
1
IEC Class C Extremely Inverse 80
0
2
IEC Long Time Inverse
120
0
1
IEC Short Time Inverse
0.05
0
0.04
IEEE - US Moderately Inverse
0.0515 0.114
0.02
IEEE - US Very Inverse
19.61
0.491
2
IEEE - US Extremely Inverse
28.2
0.1217 2
