APPENDIX
118
NA10 - Manual - 04 - 2022
8 APPENDIX8 APPENDIX
8.1 APPENDIX A1 - Inverse time IEC curves
Mathematical formula
The mathematical formula, according the IEC 60255-3/BS142 standards is:
[1]
Where:
• t = operate time (in seconds)
• t
I>inv
= setting time multiplier (in seconds)
• I = input current
• I>
inv
= threshold setting
K coefficient:
• K = 0.14 for IEC-A curve (Normal inverse)
• K = 13.5 for IEC-B curve (Very inverse)
• K = 80 for IEC-C curve (Extremely inverse)
α curve shape constant:
• α = 0.02 for IEC-A curve (Normal inverse)
• α = 1 for IEC-B curve (Very inverse)
• α = 2 for IEC-C curve (Extremely inverse)
For all inverse time characteristics, following data applies:
• Asymptotic reference value (minimum pickup value): 1.1 I>
inv
• Minimum operate time: 0.1 s
• Range where the equation is valid:
[2][3]
1.1 ≤ I /I>
inv
≤ 20
• If I> pickup ≥ 2.5 I
n
, the upper limit is 50 I
n
Note 1 Symbols are concerning the overcurrent element. The comprehensive overview of the inverse time characteristics concerning the 50/51 and
50N/51N elements is dealt within the PROTECTIVE ELEMENTS section
Note 2 When the input value is more than 20 times the set point , the operate time is limited to the value corresponding to 20 times the set point
Nota 3 With setting more than 2.5 In for the 50/51 elements and 0.5 IEn for the 50N/51N elements, the upper limit of the measuring range is limited to 50
In and 10 IEn respectively.
t =
t>
inv
·
[(I/I>
inv
)
α
-1]
K
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