earth bed in a desert location 1

[stanleycrimper]

i have to install an earth bed in a desert site,earth spikes only go down roughly a metre in depth.the client wants soil resistance to electrode reading of 4 ohms or less.tips for getting readings down to this without bentonite or anything similar?

 

[rterickson]

What sort of site is it?

Depending on the installation, you could use:

1) Longer ground rods
2) Multiple ground rods
3) Buried ground grid/counterpoise

 

[jghrist]

The grid resistance will be limited by the area of the grid and the soil resistivity. It may not be possible to get the resistance to 4 ohms without enlarging the grid area. You may have to model a grid with unreasonably small mesh size and an unreasonable number of 1 meter rods to prove to the client that it is not possible. A safe ground grid, in terms of touch- and step-voltage, can be designed with higher resistance.

 

[niravacharya9]

The problem with ohmic requirements of Grid resistances is that it becomes practically impossible to achieve it as it depends on multiple factors like soil resistivity, moisture content at time of measurement, etc.

Also, soil resistivity measured at one location will not be same if measured at a point 5-10 mtrs away. So, if you have to achieve 4 ohms, it will be a very much debatable issue where are you measuring the resistance.

Refer IEEE 80 for specific methods to achieve proper grounding as per standards. Also, nowadays the requirements are not to specify ground resistance, but to concentrate on touch & step potentials.

 

[Kapai]

I assume that the ground is either sand or poor soil or a mixture of these, and is quite dry most of the time. Not sure why you say the earths are only 1 m deep ; is this as far as they can be driven before hitting rock?

One method is to run the earth grid to a place where the ground is damp all year like a river bed or swampy area and install at enough depth to ensure all year moisture around the grid.

Another is use concrete which is a fantastic way to earth. Connect to the reinforcing steel in building foundations and piles. Piles are really good if they are deep. Dryness might be a problem with surface foundations. Also the building slab, if concrete, and installed on grade (on the ground surface) and it is not insulated by waterproof membrane, can also work well as moisture retention under a large slab can be quite good, although if really dry, even this could fail.

Another answer might be deep rods. For instance, on the site I am at in Qatar, the earth rods pilot holes will be drilled to depths of something like 30 m to find the summer water table, and multiple rods installed to reach 1 ohm resistance. Here the ground is lime rock after about 500mm and the surface super dry. Only the deepest end of the rods really do anything useful.

Your rods need to reach something consistently wet in the undersurface and again only the deep ends really do the work. Can't help with how to install deep rods in sandy soils.

The one that I think might work the best is deep rods. This doesn't require considerations like whose land in the swamp on and is it corrosive in there, or will the structural engineers like their concrete/steel being used for earthing.

 

[stanleycrimper]

yes,the rods can only go down as far as 1m before hitting virgin rock,i have installed one rod and added salt and water,left it for a day and the reading is 50ohms.in process of adding more and see the difference.thinking best way forward may be to install earth mat/mesh with multiple rods,and keep moist with daily watering.no location on site is naturally"swampy or moist"

 

[RRaghunath]

Here in Muscat too Deepwell earth rods driven to a depth of 50mtrs is the norm.
In substations the earth grid placed in Bentonite (equivalent).