Backfill around an earth electrode?

[marko333]

Hello all!

When you install an earthing electrode for a LV M.E.N distribution system, what sort of backfill should you use around it? Does it depend on the soil makeup to the area?

I am told to use bentonite and i dont know what it is and cannot find much about it. Is there a better material?

Also, how much backfill? For the electrode to be installed, you only require to bore a 50mm diameter hole for it to fit down. Should you excavate a larger hole and have more backfill around the electrode?

Thankyou

 

[alehman]

Typical solid rods should be driven into the earth if possible to obtain the best possible contact. Backfill with ordinary soil will tend to shrink away from the rod which will cause poor contact and high resistance.

Larger diameter electrodes such as chemical filled varieties must be installed in a bored hole. These should be backfilled with a material which has good conductivity and will remain in good contact with the surrounding earth. Bentonite is one such material. Bentonite is a type of clay formed from weathering of volcanic rock.

http://www.glossary.oilfield.slb.com/Display.cfm?Term=bentonite

Some grounding product suppliers have their own variations of bentonite which they sell under different names.

http://www.lyncole.com/p.lynconite.asp

 

[mc5w]

According to one of the top electrical inspectors in northeast Ohio it takes about 20 years for backfill to compact enough to give a good ground. What he liked to do was to drive ground rods through basement floors or some other place at the bottom of the foundation. An 8 foot ground rod that is driven from the surface really only has 2 effective feet.

What you really need to do is to couple 2 ground rods with ground rod couplings and drive at least 16 feet into the ground it the soil will allow it. Alternatively you can drive your rods at a slant so that most of the rod is in undisturbed soil.

According to another source ( I forget where but it may be the IEEE Green boook ) bentonite clay actually increases ground resistance by squeezing out water.

 

[alehman]

mc: I concur that backfill is a bad idea. A ground rod driven through a basement floor will likely be in dry soil and consequently have high resistance. I'm unclear about your statement that a ground rod only has "2 effective feet". Which two feet are effective, top or bottom?

Driving rods at a slant is an option where rock or other obstructions are encountered. Typically the objective is to get the maximum depth to maximize contact with permanently moist soil.

I've never heard your concern regarding bentonite before. I would be interested to know the source.

 

[jghrist]

I don't know what an M.E.N. distribution system is, but if it does not require a particularly low resistance ground, then a driven rod may do quite well. As indicated by others, driving the rod will make for the best contact with the soil.

You need to determine how low a resistance ground you need and what the resistivity of the soil is to design the grounding electrode.

If you need a lower resistance, driving a longer rod would generally be better than using a bored hole with low resistivity material like bentonite. Bentonite or a proprietary low resistivity material (Eritech offers GEM

http://www.erico.com/products/GEM.asp)

effectively increases the diameter of a rod in high resistivity soil. To do this you need a larger diameter bore, at least 150 mm. You get a greater decrease in ground resistance by using a longer rod than by using a larger diameter, but you can do both if you need low resistance.

Bentonite, also known as drilling mud, needs to be kept moist to be effective, so it has to be in soil with good moisture.

The worst thing you could do is bore a hole only slightly larger than the ground rod. This won't allow complete compact backfill and will not give you a large diameter of low resistivity material such as bentonite.

 

[mc5w]

The statement about the bottom 2 feet of a 8 foot rod only being effective came from John Labriola who is one of the top electrical inspectors in Northeast Ohio. His email is electric at northcantonohio dot com.

There is also someone here at engineering tips who is with a Colorado electric utility who says that the portions of their system that have 17 foot ground rods ( using rod couplings ) have ZERO lightning damage.

If you know anything about basement waterproofing you know that typically in most of the U.S. the basement acts as a gigantic dug water well. Typically, the soil underneath a basement floor is saturated with water unless you live in a desert or there is a severe drought. A slab on grade building is a totally different story.

Reinforcing steel that is at the bottom of a foundation is now a mandantory grounding electrode in the U.S. if available.

According to the IEEE Green book backfilling a ground rod hole with concrete also works.

There is also a radio amateur in the American southwest desrt who has had considerable success with a mixture of gravel, gypsum, rock salt, and a SMALL amount of bentonite clay. The small amount of bentonite clay provides MODEST compaction and still allows a perforated pipe to be used to add salt water.

Bentonite clay is a valid idea just as long as you do not use 100%

 

[cuky2000]

I am not convinced that the soil around ground rods needs to be enhanced as suggested by the local top inspector. Regardless of the effectiveness of the ground rod in contact with the soil, if the resistance is corrected calculated or measure and meet the project requirement and comply with the appropriate codes (i.e. NEC, IEEE Std 80, etc),soil enhancement is not required at all.

Bentonite may works and could be effective in rare occasions. This material has several disadvantages in dry environment and cost and installation time may be significant high. Beware that there are other methods more effectives for reducing ground resistances such as adding rods, increasing rod lengths, chemical rod, Ufer ground using the concrete foundation, etc or a combination of these methods.

See the enclose info for additional details in this matter.

http://cuky2000.250free.com/GND_Electrode1.JPG