A higher value of step 1 might be necessary if a big power transformer (Y0/D) at remote bus
bar is disconnected.
A special case occurs at double circuit lines, with mutual zero-sequence impedance between
the parallel lines, see figure 57.
IN
>
One phase-to-earth fault
3I
0
IEC05000152-en-2.vsd
IEC05000152 V2 EN-US
Figure 57: Step 1, third calculation
In this case the residual current out on the line can be larger than in the case of earth fault on
the remote busbar.
EQUATION1201 V3 EN-US (Equation 41)
The current setting for step 1 is chosen as the largest of the above calculated residual
currents, measured by the protection.
Step 2
M15282-144 v6
This step has directional function and a short time delay, often about 0.4 s. Step 2 shall
securely detect all earth faults on the line, not detected by step 1.
IN
>
One- or two-phase earth-fault
3I
0
IEC05000154-en-2.vsd
IEC05000154 V2 EN-US
Figure 58: Step 2, check of reach calculation
Section 7 1MRK 511 358-UEN A
Current protection
126
Application manual
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