Substations - Rock vs. Grass 1

[davidbeach]

I just got back from nearly three weeks in Italy (and a bit in Switzerland) and I saw lots of substations with grass or other vegetation throughout. What I’m used to is a high resistance rock surface to limit step and touch potentials. I can’t imagine wet grass having nearly as high a resistance.

How does the vegetation fit into a substation safety plan? Some of the ANSI/IEC differences seem to be more style differences at core rather than substance differences. But this seems like different laws of physics. How is a low resistance surface made safe for anyone in the station?

 

[Mbrooke]

You saw correctly- most substations in Europe and Russia have grass growing on the floor of the substation. Why that is I will let others explain.

 

[HamburgerHelper]

In South America, they have people wear insulated boots and go cheap on the grounding grids.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[crshears]

My utility in Ontario, Canada has always used crushed rock / large gravel in switchyards - and a number of years back they stipulated that employees were henceforth to wear dielectrically resistive footwear when working in electrical environments. As an employee I of course complied with this directive - which led to a very nasty surprise the first time I tried to open bare-handed the mechanism cabinet of a circuit breaker in a 230 kV switchyard with air-insulated overhead buswork. I quickly learned to carry my keys in my left hand and always use them to bond myself to any grounded objects before touching them with my right hand...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[aussiejohn2]

Hi David,
Areas where grass grows well (and all year) often have lower resistivity, which helps somewhat counteract the lower permissible step and touch limits due to the lack of high resistance surface layer. I have tried and failed twice to talk people into considering it - opposition was religious, not factual.
There are some instances of people tinkering with it in the US, e.g.

https://ewh.ieee.org/cmte/substatio...Meeting/Sustainable_Substations_IEEE 5.14.pdf

and i think Dr Dawalibi may have had a design using it.
John.

 

[ElectricRob]

What's interesting to me is that all of the substation ground grid calculations that I've seen assume there is about 6 inches of gravel in the substation (3 inches of crusher run/fines, then 3 inches of larger gravel). In reality, its rare to find a substation with anywhere near that much gravel in place. Usually, I see a very thin layer of gravel with dirt showing though. Non-engineers see no purpose for the gravel except as a better surface to walk on than mud (and spraying herbicide is cheaper than putting thick enough gravel to prevent vegetation from growing through).
Walking into a substation full of grass would bother me - gravel makes is easier to see and avoid rattlesnakes.

Right now, the latest copy of T&D World is sitting on my desk. The cover photo shows a substation with a very thin layer of gravel and much dirt showing through. The article it refers to is about the use of drones, but they should either have picked a different substation or cropped the picture. My guess is that they are so used to seeing poor substation ground cover that nobody pays attention to it.

 

[cuky2000]

David,

Variability of footware resistance is one factor that most designer do not consider in the USA. For example, a wet leather boot can have a resistance on the order of 100 ohms while other rubber base footwares are in the 1000's Ohms. On the other hand, other countries in the IEC marketplace considering the shoe resistance such as the UK BS EN 50522 standard allow to use 4 Ohm per shoe.

The IEEE Std 80 have provision to consider the footware in the allowable Step & touch potential: [highlight #FCE94F]E[sub]touch[/sub] = (1000+1.5C[sub]s[/sub].ρ[sub]s[/sub] + R[sub]shoes[/sub]).0.116/√t[sub]s[/sub][/highlight] ...................... [sub] Where R[sub]shoes[/sub] = Optional term for the shoes and/or glove resistance.[/sub]

Should be noted that for relative small fault current into the ground and fast clearing time, the shoe resistance may not be the driving determinant factor.

For cold wather countries, the ice is considered as additional insulator particularly in permafrost areas.

See the following thread238-418053

 

[HamburgerHelper]

The rock is to prevent the currents from straying on the surface. I don't know if it is conductive enough to help. I would feel safe for step potentials while standing on metal walkway. I wouldn't like grass even if it worked because it looks bad and am afraid it would invite rodents and snakes.




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If you can't explain it to a six year old, you don't understand it yourself.

 

[RRaghunath]

'Sustainable Substations' document shared by John indicates a gravel of 12" thick above the ground mat. The grass is planted only on top layer. This way, between the ground rod and the grass, there is thick layer of gravel that ensures enough insulation to limit the step and touch potentials.
I have seen grass in some substations in India, deliberately grown in some patches to improve aesthetics.
However, I believe it is safer to have gravel as top layer all over the switchyard for the reason that snakes etc. are deterred. It is a known fact that the snakes and other rodents prefer warm environment of switchyards and switchgears.

Rompicherla Raghunath

 

[cranky108]

Another perspective I saw years ago in the US. The substation that had grass needed to be mowed at least once a week. As well it needed a shack for the mower, and a gas can.
Sound like anything you would want in a substation?

Later they removed the grass (I don't know how), replaced it with rock, and painted it green.

The reason may have had something to do with the thefts of window air conditioners.

On the other hand, at my current company, our civil engineer has spent many hair removing sessions looking at substation rock. Which includes becoming stuck in the thick rock in one substation.

Interesting enough we did pave one substation with aspulit.

 

[HamburgerHelper]

"Interesting enough we did pave one substation with aspulit."

When you decommission, I guess you just turn it into a parking lot. Paved paradise and turned it into a parking lot.

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If you can't explain it to a six year old, you don't understand it yourself.

 

[FPelec]

Yes, it is quite common in Italy and in many European countries to have green spaces inside substations.
My office is sited in a large 400 / 230 / 150 kV substation with grass and many small trees; oleanders, mulberry and olive trees grow in the spaces between HV and EHV zones, obviously where there are no overhead conductors and clearances issues.
From the technical point of view, in large substations the grounding resistance is low enough to allow for a safe operation even without adding an high resistivity layer.
Consequently only the internal roads and lanes are paved.
Note that in the old days when most substation were built, in Italy (as well as in many European countries) each substation had a permanent presidium, so it was easy to keep the grass mowed.
On the other hand nowadays, when you build a new substation, environmental constraints and visual impact often lead to plant trees and keep grass in the substations.

Si duri puer ingeni videtur,
preconem facias vel architectum.

 

[jghrist]

When I worked in Bangladesh, the older substations had grass along with cows for mowing.

Have you ever tried to drive on 6" of crushed rock? We normally specify 4" and design the grid for that. Putting in fines defeats the purpose of the rock - it lowers the resistivity when wet.

 

[Mbrooke]

I've never heard this explanation for Gravel- but the up votes say maybe I'm wrong?

https://www.youtube.com/watch?v=StbceMWrQB8

 

[bacon4life]

When specifying yard rock, we definitely consider how easy it is to drive across the rock. It is easy to drive on 6" from our current supplier, but easy to get stuck in 4" from other suppliers.

 

[waross]

We were building a sub where the civil contractor was the general and subbed out the electrical.
Anxious to maximize a progress payment the general had the crushed rock placed before the main transformer arrived on site.
It took a while to get the truck with the transformer unstuck.

Bill
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"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Have a look at this gem I just came across

https://www.youtube.com/watch?v=qr28HIsc_2c