GE P24DM - European Standards; Figure 21: IEC 60255 IDMT Curves

1000.00
100.00
10.00
1.00
0.10
10
100
Operating time (seconds)
Current (multiples of I
S
)
Standard Inverse (SI)
Very Inverse (VI)
Extremely Inverse (EI)
E00600
1
Long Time LTIInverse ( )
Figure 21: IEC 60255 IDMT curves
3.1.2 EUROPEAN STANDARDS
The IEC 60255 IDMT Operate equation is:
t T
M
L C
op
=
−
+






+
β
α
1
and the IEC 60255 IDMT Reset equation is:
t T
M
r
=
−






β
α
1
where:
● t
op
is the operating time
● t
r
is the reset time
● T is the Time Multiplier setting
● M is the ratio of the measured current divided by the threshold current (I/Is)
● β is a constant, which can be chosen to satisfy the required curve characteristic
● α is a constant, which can be chosen to satisfy the required curve characteristic
● C is a constant for adding Definite Time (Definite Time adder)
● L is a constant (usually only used for ANSI/IEEE curves)
The constant values for the IEC IDMT curves are as follows:
Curve Description
b
constant a constant
L constant
IEC Standard Inverse Operate 0.14 0.02 0
IEC Standard Inverse Reset 8.2 6.45 0
P24xM Chapter 6 - Current Protection Functions
P24xM-TM-EN-2.1 87