Why are pieces of stones used to cover the ground at electric switchyards instead of just having a concrete floor ?
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Not all substations are gravelled fully (I’ve seen fly ash and concrete). Transformers are filled with oil. As such they require a containment to prevent that oil from running into navigable waters in the event of a leak. The gravel immediately under the transformer is a containment volume that collects rainwater and oil. There is a large pit (sized for oil volume plus record rainfall amounts), lined with a membrane and filled with large gravel. You could make a containment volume without that gravel but it would be much more expensive, involve large concrete curbs, and wouldn’t provide a flat surface to allow for vehicles and people to easily transit. Additionally, there is an extensive grounding grid in the substation buried beneath all that rock. Everything in the substation is grounded, even the fence, creating an equipotential zone and to eliminate induced voltages from the high voltage.
“Step potential” is the voltage difference between your feet when fault current (or any high ground current) is present, and it can be deadly. To minimize it, the ground surface needs a high resistance cover, i.e., gravel. As others have said, it drains easily, and has minimal point to point contact with the other stones. Concrete, with moisture retention, rebar, and the cost of installation/material, is not practical. financially or from a safety standpoint.
The big reason is to provide extra resistance between people and (earth) ground. The gravel is actually pretty high resistance overall because in a pile each piece only touches others at corners. Concrete on the other hand is pretty uniform, has a higher degree of ions that increase conductivity like calcium, and retains alot of moisture in it’s pores.
You don’t usually think of concrete as conductive, but it’s still conductive enough to electrocute. Gravel is about 5000x more insulating than concrete.
There’s also some side benefits for water drainage and maintenance compared to concrete.
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