Appendix 1 Overcurrent Relays - ISA T1000 PLUS User Manual

DOC. MIE91093
AP PE N DI X 1 O VE R C UR RE NT R EL A YS
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.

t ( s )

 

Constants of the formula change according to the type of
curve; they are summarised in the following table.
IEC, IEEE
IEC Class A Standard Inverse
IEC Class B Very Inverse
IEC Class C Extremely Inverse
IEC Long Time Inverse
IEC Short Time Inverse
IEEE - US Moderately Inverse
IEEE - US Very Inverse
IEEE - US Extremely Inverse
Rev. 1.34
K * a
T
b

I
 

1

I
R
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
c
a c
0.14 0
13.5 0
80 0
120 0
0.05 0
0.0515 0.114
19.61 0.491
28.2 0.1217
b
0.02
1
2
1
0.04
0.02
2
2