Step 1
M15282-123 v5
This step has directional instantaneous function. The requirement is that overreaching of the
protected line is not allowed.
IEC05000150-3-en.vsd
IN
3I
0
One- or two-phase earth-fault
or unsymmetric short circuit
without earth connection
IEC05000150 V4 EN-US
Figure 55: Step 1, first calculation
The residual current out on the line is calculated at a fault on the remote busbar (one- or two-
phase-to-earth fault). To assure selectivity it is required that step 1 shall not give a trip at this
fault. The requirement can be formulated according to equation
39.
step1 0
I 1.2 3I (remote busbar)³ ×
EQUATION1199 V3 EN-US (Equation 39)
As a consequence of the distribution of zero sequence current in the power system, the
current to the protection might be larger if one line out from the remote busbar is taken out of
service, see figure
56.
IEC05000151-en-2.vsd
IN
>
One- or two-phase-earth-fault
3I
0
IEC05000151 V2 EN-US
Figure 56: Step 1, second calculation. Remote busbar with, one line taken out of service
The requirement is now according to equation 40.
step1 0
I 1.2 3I (remote busbar with one line out)³ ×
EQUATION1200 V3 EN-US (Equation 40)
1MRK 511 358-UEN A Section 7
Current protection
125
Application manual
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