1000.00100.0010.001.000.1010 100Operating time (seconds)Current (multiples of IS)Standard Inverse (SI)Very Inverse (VI)Extremely Inverse (EI)E006001Long Time LTIInverse ( )Figure 29: IEC 60255 IDMT curves2.1.2 EUROPEAN STANDARDSThe IEC 60255 IDMT Operate equation is:t T M L Cop = − + +βα 1and the IEC 60255 IDMT Reset equation is:t T Mr = − β α1where:● top is the operating time● tr 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 constantIEC Standard Inverse Operate 0.14 0.02 0IEC Standard Inverse Reset 8.2 6.45 0P14x Chapter 6 - Current Protection FunctionsP14xEd1-TM-EN-1 91
