ABB Switzerland Ltd REG 316*4 1MRB520049-Uen / Rev. F
3-100
Multiplier ‘k1-Setting’
The multiplier ‘k1-Setting’ enables the IDMT characteristic cho-
sen by the setting of parameter c to be shifted without changing
its shape. This is used for grading the operating times of a series
of IDMT relays along a line to achieve discrimination.
For example, in the case of the “very inverse” characteristic, the
constant c = 1 and the factor k1
£ 13.5. The operating time t is
given by the equation
t
k
I
I
B
=
-
1
1
Assuming a grading time of 0.5 s at 6 times the base current I
B
is
required, the factor k1 for each of the relays is given by
k1 = 5 t.
This produces for operating times between 0.5 and 2.5 s the fol-
lowing settings for k1:
t [s] k1 [s]
0.5 2.5
15
1.5 7.5
210
2.5 12.5
The characteristics according to BS 142 are set as follows:
“normal inverse” : k1 = 0.14 s
“very inverse” : k1 = 13.5 s
“extremely inverse” : k1 = 80 s
“long time earth fault” : k1 = 120 s.
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