REG 316*4 1MRB520049-Uen / Rev. F ABB Switzerland Ltd
3-99
An alternative is to adjust the position of the IDMT characteristic
to match the rated load of the protected unit and set the base
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 char-
acteristic. 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. 3.5.8.1 Operating characteristic of the directional IDMT
overcurrent function
To Next Page
Loading...
