The requirement is now according to equation 39.
step1 0I 1.2 3I (remote busbar with one line out)³ ×
EQUATION1200 V3 EN (Equation 39)
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 48.
ANSI05000152_2_en.vsd
One phase-to-ground fault
67N
ANSI05000152 V2 EN
Figure 48: Step 1, third calculation
In this case the residual current out on the line can be larger than in the case of ground fault
on the remote busbar.
EQUATION1201 V3 EN
(Equation 40)
The current setting for step 1 is chosen as the largest of the above calculated residual
currents, measured by the protection.
Step 2
This step has directional function and a short time delay, often about 0.4 s. Step 2 shall
securely detect all ground faults on the line, not detected by step 1.
1MRK 511 286-UUS A Section 6
Current protection
115
Application manual
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