earthing scheme suitable for worst/large value of soil resistivity in order to get earth resistance value below 5 ohm?

[Navaraj]

i don't the exact value of the soil resistivity as no survey of soil sample is carried out at the site .However Copper earthing electrode 50mm diameter of 2.5 m is driven where its circumference were filled with chemical compound power to the height of rod and earth resistance was found to be 40 ohms after one day of measurement?
Is there any alternative way to reduce the value of resistance ? does the increase in length of rod significantlyreduce the value of resistance ?

 

[SparkyChick]

I think you're going to have to provide more information.

Is it a dry environment? What type of ground? What's the elevation above sea level (this may impact the water table height which could impact conductivity etc.). So many factors will impact this.

 

[Navaraj]

I think you're going to have to provide more information.

Is it a dry environment? What type of ground? What's the elevation above sea level (this may impact the water table height which could impact conductivity etc.). So many factors will impact this.

It's bit dry environment, the soil nature of the site is mostly rocky, Sandy soil. The site is located in Hilly region above 1200m from sea level and is more prone to lightning area. I want the permanent solution for earthing, mostly for sensitive equipment installed at the site and more to protect from lightning.

 

[pc1966]

There are some equations for the nice, uniform known cases of soil to compute earth rod impedance here:

Measurements and Calculations Of Earth Electrode Systems (BS 7430) | EEP

System earthing involves the provision of a connection to the general mass of earth. This connection should have a resistance not greater than that required to operate safety mechanisms to isolate the electricity supply from a fault situation.

electrical-engineering-portal.com

But generally what you find on site won't fit nicely to any simple model. Making the rod longer helps, but it is not linear (so doubling the rod won't quite half the impedance),

Multiple rods is the simplest solution, but to get good results they have to be more than a rod-depth apart and depending on the terrain and ownership boundaries that may not be practical. I guess in your case you are looking to get below 5 ohms for lightening and so probably you will need 8-10 additional rods.

In stony ground you really need something to extend the rod's contact outside of its immediate contact point, and some folk use conductive concrete as a means of doing so as it is more permanent and stable than bentonite clay filling if you don't have much precipitation or ground water to keep it moist.

However, soil conductivity is very dependent on the presence of moisture in liquid form, so going deeper towards any water table can help in those cases (and to reduce the risk of the top layer freezing and the loss of conductivity that causes).

 

[nicebutdim]

But generally what you find on site won't fit nicely to any simple model. Making the rod longer helps, but it is not linear (so doubling the rod won't quite half the impedance),

Multiple rods is the simplest solution, but to get good results they have to be more than a rod-depth apart and depending on the terrain and ownership boundaries that may not be practical. I guess in your case you are looking to get below 5 ohms for lightening and so probably you will need 8-10 additional rods.

Hope you don't mind me interjecting, but I have a question...

I'd always understood that multiple rods should be at least rod length apart, but remember watching a John Ward video where he seemed to suggest they should be separated by their combined length - ie. 2x 1m rods should be at least 2m apart. Is this extra distance correct or necessary? Obviously greater distance wouldn't do any harm, but I'd never had a chance to confirm if it was a requirement.

 

[pc1966]

John Ward video where he seemed to suggest they should be separated by their combined length - ie. 2x 1m rods should be at least 2m apart. Is this extra distance correct or necessary? Obviously greater distance wouldn't do any harm, but I'd never had a chance to confirm if it was a requirement.

You get slightly better performance if they are separated by their combined length, but often not much. For example if you have 1m rods of 0.16m diameter, 150 ohm.m soil, and 6 arranged in a line using the standard formulae in the previously linked article we have:

• Single 1m rod = 40.6 ohm
• 6 rods @ 0.1m separation = 30.4 ohm
• 6 rods @ 0.5m = 11.49 ohms
• 6 rods @ 1m = 9.12 ohm
• 6 rods @ 2m = 7.94 ohm
• 6 rods @ 3m = 7.55 ohm
• 6 rods @ 20m = 6.88 ohm

The simple assumption of independent rods in parallel would have 6.77 ohm, so the "twice rod length" case is 17% worse than ideal and the "rod length" case is 35% worse than ideal (around 15% worse than twice-rod case).

If you have the space to separate them then it helps, but it quickly becomes of little additional benefit beyond 1-2 rod's length and with a significant number it might not fit your acceptable ground area.