The residual current, out on the line, is calculated at an operational case with minimal earth-
fault current. The requirement that the whole line shall be covered by step 2 can be formulated
according to equation 42.
step2 0I 0.7 3I (at remote busbar)³ ×
EQUATION1202 V4 EN-US (Equation 42)
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 59.
IN
>
One phase-to-earth
fault
3I
0
IN
>
3I
01
IEC05000155-en-2.vsd
IEC05000155 V3 EN-US
Figure 59: Step 2, selectivity calculation
A second criterion for step 2 is according to equation 43.
1
1
0
step2 step1
0
3I
I .2 I
3I
³ × ×
EQUATION1203 V4 EN-US (Equation 43)
where:
I
step1
is the current setting for step 1 on the faulted line.
Step 3
M15282-164 v6
This step has directional function and a time delay slightly larger than step 2, often 0.8 s. Step
3 shall enable selective trip of earth faults having higher fault resistance to earth, compared to
step 2. The requirement on step 3 is selectivity to other earth-fault protections in the network.
One criterion for setting is shown in Figure
60.
IEC05000156-3-en.vsd
IN
>
One phase-to-
earth fault
3I
0
IN
>
3I
02
IEC05000156 V3 EN-US
Figure 60: Step 3, Selectivity calculation
1MRK 511 358-UEN A Section 7
Current protection
127
Application manual
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