Why are pieces of stones used to cover the ground at electric switchyards instead of just having a concrete floor ?


Why are pieces of stones used to cover the ground at electric switchyards instead of just having a concrete floor ?

In: 4541

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.

“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.

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.

Along with all the valid reasons listed, Ive also heard that snakes love that soothing 50Hz vibration (60Hz in the US) that emanates from HV switch gear – so much so that they’ve been known to wrap themselves around transformers causing it to short out. The jaggedly ass ballast rock acts as a deterrent since they don’t like crossing it.

I like this explanation but I haven’t had much luck in trying to verify how true it is, so it might be bullshit.

Gravel is pervious and drains water whereas concrete is not. So the latter is more expensive

Also, concrete needs a grid of rebar to keep it from collapsing under its own weight and cracking/ shifting. I can’t imagine that being a good thing at an electrical switchyard..

What hasn’t been mentioned as well is there is often asset replacement/addition going on. It’s a lot easier and cheaper to dig up gravel and dirt to lay new cables than it is to break up concrete.

Water drainage and insulation are a couple of reason.

Don’t want to be walking through muddy ground or puddles of water when you have 150K Volts above you head.

You need to get to what’s underneath for maintenance/repair.

It’s easier to dig up some rocks than it is to break up concrete.

The grave is much cheaper than a slab, as well as providing the benefits others have noted.

This is the kind of very odd and specific question and answer that makes me super suspicious

It’s cheaper than concrete, requires almost no maintenance, drains well, compacts well for moving heavy gear and trucks, and there is usually a grounding grid built underneath the whole substation which requires the whole area to be dug subgrade and backfilled, for which gravel is a good material.

My first job was at the Centralia hydro project, it goes beyond gravel but that’s 16ft of gravel. Keep in mind those *wires* are 2inch by 12inch flat copper *wires*. A massive amount of juice goes thru that switch yard. A guy got melted wearing a full arc suit but lived due to his PPE. Our safety manager had one arm, burned completely off. The lineman in every state who handle storms are completely unsung heros.