Thytronic SME2-IS - Appendix; APPENDIX A - IEC Time Dependent Curves; General Formula

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SME2-IS - Manual - 06 - 2021
APPENDIX
9 APPENDIX9 APPENDIX
9.1 APPENDIX A - IEC time dependent curves
 General formula
The general formula, pursuant to IEC 60255-3/BS142, is:
[1]
Where:
• t = tripping time (in seconds).
• t
>inv
= tripping time regulation (in seconds).
• I = measured current.
• I>
inv
= tripping threshold regulation.
Constant α
• α = 0.02 for the IEC type A normal inverse curve.
• α = 1 for the IEC type B very inverse curve.
• α = 1 for the IEC type LIT long inverse time curve.
• α = 2 for the IEC type C extremely inverse curve.
Coefcient K
• K = 0.14 for the IEC type A normal inverse curve
• K = 13.5 for the IEC type B very inverse curve.
• K = 120 for the IEC type LIT long inverse time curve.
• K = 80 for the IEC type C extremely inverse curve.
The following applies to all inverse time characteristics:
• The asymptotic reference value (minimum tripping current ) is 1.1 time the set threshold.
• The minimum tripping time is 0.1 s.
• The characteristics are dened between 1.1 and 20 times the set threshold
[2][3]
Note 1 The formula uses the symbols relating to the overcurrent function. The full description of the symbols used in the time-dependent curves for
protections 50/51 and 50N/51N are given in 4.4 PROTECTION FUNCTIONS
Note 2 When the threshold regulation exceeds 2.5 In for protection 50/51 and 0.5 IEn for protection 50N/51N, the upper limit of the measurement range
is limited to 30 In and 10 IEn respectively
Note 3 when the threshold regulation exceeds 20 times the threshold, the tripping time is limited to the value corresponding to 20 times the threshold
t =
t>
inv
·
[(I/I>
inv
)
α
-1]
K