why aggregate is placed all around substation ground.
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Answer Posted / brandon
A thin layer of aggregate above a low-resistance substrate
(or embedded ground grid if the substrate is a
high-resistance material) provides a high resistance between
the bottom of a human's feet and the substrate. This will
lower the upward current through the body considerably in
the event of a L-G fault compared to the equipment bonds
(assuming equipment is properly bonded to the substation
ground). However, since the aggregate is thin relative to
the ground plane, it does not impede the shape of the
equipotential voltage field of the substation and thus
reduces the step potential by keeping a consistant voltage
grading.
Aggregate is not for lightening as lightening protection is
provided through overhead shield conductors bonded to the
ground grid or substrate.
If oil-filled equipment fails and and a fire is started, the
aggregate also has a quenching effect by distributing the
oil within the void spaces and creating small pockets
instead of a large surface of oil.
See IEEE 80 for the complete explanation.
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