APPENDIX
85
NA011 - Manual - 05 - 2022
8.3 APPENDIX A3 - Inverse time I
2
t curves
Formula generale
The mathematical formula for I-squared-t (I
2
t) is:
[1]
Where:
• t = operate time (in seconds)
• t
I>inv
= setting time multiplier (in seconds)
• I = input current
• I>
inv
= threshold setting
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
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
)
2
]
16
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