An alternative is to adjust the position of the 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 IStart
The characteristic is enabled when the current exceeds the setting
IStart. A typical setting for
IStart is 1.1 I
B
.
Choice of characteristic c-Setting
The constant
c-Setting determines the shape of the characteristic.
The settings for the standard characteristics according to BS 142 are:
normal inverse:
c = 0.02
very inverse and long time
earth fault:
c = 1.00
extremely inverse:
c = 2.00
t
I
I
Start
I
B
t =
k1
I
I
B
c
1
t
min
18000042-IEC19000447-1-en.vsdx
IEC19000447 V1 EN-US
Figure 43: Operating characteristic of the directional overcurrent inverse time function
Multiplier k
1
-Setting
The multiplier
k1-Setting enables the directional overcurrent inverse time characteristic to be
shifted. This is used for grading a series of relays along a line to achieve discrimination.
For example, in the case of the
very inverse characteristic, the constant c = 1 and the factor k
1
≤
13.5. The operating time t is given by the equation:
1
1
B
k
t
I
I
Section 5
1MRK 505 406-UEN B
Bay protection functions
108
Bay protection functions REB500
Technical manual
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