REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-83
An alternative is to adjust the position of the IDMT characteristic
to match the rated load of the protected unit and set the
reference current to its rated current instead of its load current.
Enabling the characteristic ‘I-Start’
The IDMT characteristic is enabled when the current exceeds
the setting ‘I-Start’. A typical setting for ‘I-Start’ is 1.1 I
B
.
Choice of characteristic ‘c-Setting’
The constant ‘c-Setting’ determines the shape of the IDMT
characteristic.
The settings for the standard characteristics according to
B.S. 142 are:
“normal inverse” : c = 0.02
“very inverse” and “long time earth fault” : c = 1.00
“extremely inverse” : c = 2.00.
Fig. 3.5.6.1 Operating characteristic of the IDMT overcurrent
function
“c-Setting” can also be set to “RXIDG”, in which case the
function’s inverse characteristic corresponds to that of the relay
Type RXIDG:
t [s] = 5.8 – 1.35 In (I/I
B
)
The parameter “k1-Setting” has no influence in this case.
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