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# What kind of fault current calculation is required during ground grid design 2

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## What kind of fault current calculation is required during ground grid design

(OP)
Hi all,

i'm working on the preliminary design for a subway ground grid. I got informed the soil resistivity from the site and it is 300 ohm-m.

so I started playing with the number using IEEE 80-2000.

So my concern is , what kind of simulation is required for sizing grid grounding.

For example, should I calculate 3 phase fault, double line to ground fault and single line to ground fault or just sing line to ground fault?

### RE: What kind of fault current calculation is required during ground grid design

Whichever fault results in the most zero-sequence current. In most systems, a single line to ground fault is the most severe.

### RE: What kind of fault current calculation is required during ground grid design

Just to add to jghrist, the SLG and DLG have zero sequence and can produce GPR that result in touch and step voltages. Often the maximum SLG fault is what you will study, unless there is a long fault clearing that can actually drive the grounding design.

As with many engineering studies, there are more details to consider for grounding analysis fault data Grounding Analysis Fault Data.

### RE: What kind of fault current calculation is required during ground grid design

(OP)
Thanks DBL-E for your response.

In general, is SLG the most severe and DLG will be the second most severe value ? Also what is relationship fault clear time and SLG or DLG fault ?

### RE: What kind of fault current calculation is required during ground grid design

Hi @james64,

In general yes. However, it would be remiss to advise you to not perform a specific case analysis as part of your engineering study.

To answer to your OP, the kind of simulation you require is power systems analysis, more specifically a short-circuit study. You can reference IEEE 399, 141, 242 & 551 or IEC 60909. IEEE 80 does give good guidance on the inputs that are required.

Faults which do not flow through the ground/earth system will not cause EPR/GPR so only ground/earth faults are important here.

The relationship between fault clearing time and type of fault is a function of the protection design, available fault current, earthing resistance & location of fault. In general a larger magnitude fault equates to a shorter clearing time, but the specific system can be much more complex.

Based on your questions and your indication that this is a subway, I suggest you seek professional engineering advice or peer review. Electrified rail systems can have complex earthing system arrangements, sometimes requiring isolation between station & traction earths etc.

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