67N
One- or two-phase ground-fault
50/51N
ANSI05000154 V2 EN
Figure 49: Step 2, check of reach calculation
The residual current, out on the line, is calculated at an operational case with minimal
ground-fault current. The requirement that the whole line shall be covered by step 2 can
be formulated according to equation
41.
step2 0I 0.7 3I (at remote busbar)³ ×
EQUATION1202 V4 EN (Equation 41)
To assure selectivity the current setting must be chosen so that step 2 does not operate at
step 2 for faults on the next line from the remote substation. Consider a fault as shown in
figure
50.
One phase-ground-fault
67N
50/51N
50/51N
67N
ANSI05000155 V2 EN
Figure 50: Step 2, selectivity calculation
A second criterion for step 2 is according to equation 42.
1
1
0
step2 step1
0
3I
I .2 I
3I
³ × ×
EQUATION1203 V4 EN (Equation 42)
where:
Istep1 is the current setting for step 1 on the faulted line.
Section 6 1MRK 511 286-UUS A
Current protection
116
Application manual
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