Q237 : Modelling of Building Foundations

Question
Do you typically model the rebar in MALT when analyzing a ground grid for a substation control building and associated switch yard?

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Answer
It is common practice to connect the rebar in the foundations of buildings located in a substation to the grounding grid to avoid developing large transfer potentials. It is therefore desirable to model the rebar in MALT, to get an accurate representation of the grounding system. Normally, the touch and step voltages above the foundations are not a concern since they are essentially equipotentials; the rebar should be modelled since it tends to decrease the overall ground impedance and therefore helps in reducing potentials everywhere in the substation.

You may chose not to model the foundations if you are not certain that they are connected to the main grounding grid. Even if they are connected, this would be a conservative approximation. Note also that the effects of the foundations may well be quite small. In particular, if the building is located completely inside the perimeter of the main grounding grid, the effects of the foundation will be much less important than if it is outside.

If you chose to model the foundations, remember that you don't need to model a very dense grid: only a few conductors (such as the outline of the fondation) are sometimes sufficient. You can experiment for yourself by adding more conductors later, and verify the impact on the results.

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