Substation Grounding - To pave or not to pave

[signalguy]

I?m helping a Contractor produce detailed shop drawings based on the Owners substation ground grid design and spec. This calls for a perimeter 4/0 ring with 20? ground rods attached every 20?. This is attached to a 4? grid of #4 bare under the roughly 60?x40? series of substations needed. This grid is 48? down with 6? of conductive soil below and above. All seem ok so far but here?s what I think is a red flag?

The civil design under this substation then adds 18? of aggregate, then 12? of washed rock, topped off with 6? of permeable asphalt. I?m concerned this will create a poor ground mat for those working in the substation area do to a pavement surface that is poorly coupled to ground. Thus touch and step potentials be a safety issue.

Any thoughts? I?m not familiar with substations being paved. If so, it seems concrete with an integral Uffer ground system would be a better choice. Has anyone successfully used asphalt under substations?

 

[kingpb]

Permeable asphalt is more like concrete than asphalt. It essentially looks like a rice crispy treat when installed. Water will run through it just like crushed granite. However, at least one drawback is that to be installed properly, under the "asphalt" there should be a drain system that allows the sediment to be collected in a sump so that it does not build-up/clog-up the surface. It also requires maintenance to keep the surface from becoming non-permeable.

Hope that helps a little.

 

[beanland]

Conductvity of Asphalt

Conductvity of Asphalt

Check for data on the electrical conductivity of asphalt. You will find it is a poor conductor, similar to substation rock. As a result, the resistivity is high and S&T potentials may not be a problem. We know that an energized line can sit on ashalt all day just barely arcing.

 

[Hv&Lv]

I don't know that I would worry about step potential differences between gravel and asphalt. equipotential mats should be installed at any and all operatible switch locations unless the switches can be operated with a switchstick.

Seems odd to asphalt a substation though. There are always changes made that necessitate trenching around a substation for new fiberoptics, lighting wiring, SCADA controls, etc...
Also won't the mineral oil degrade the asphalt?

 

[mivey]

I?m concerned this will create a poor ground mat for those working in the substation area do to a pavement surface that is poorly coupled to ground. Thus touch and step potentials be a safety issue.

A poorly coupled surface is what you want. In fact, we add gravel in our substations for that very reason. We want to reduce the touch and step potential by increasing the contact resistance from the feet to soil. Never heard of asphalt being put in before.

 

[signalguy]

Thanks for your insights everyone.

Agreed asphalt is a pore conductor. It seemed odd to me to construct a ground grid 4' down and then to almost completely isolate the surface from it. However, I guess that is similar in concept to how transmission lines are inspected hot.