Study specialized electrical engineering articles and technical papers in low/high voltage areas. Save 15% on PRO plan with code EEP21.

# Measuring Earth Resistance

## What affects grounding resistance?

First, the NEC code requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with soil. Soil composition, moisture content, and temperature all influence the soil resistivity, so it is recommended that the ground rods be placed as deep as possible into the earth to be most effective.

Nevertheless, four variables affect the ground resistance of a ground system:

### 1. Length/depth of the ground electrode

One very effective way of lowering ground resistance is to drive ground electrodes deeper. Soil is not consistent in its resistivity and can be highly unpredictable.

It is critical when installing the ground electrode that it is below the frost line. This is done so that the resistance to ground will not be greatly influenced by the freezing of the surrounding soil. Generally, by doubling the length of the ground electrode you can reduce the resistance level by an additional 40%.

There are occasions where it is physically impossible to drive ground rods deeper-areas that are composed of rock, granite, etc. In these instances, alternative methods including grounding cement are viable.

### 2. Diameter of the ground electrode

Increasing the diameter of the ground electrode has very little effect in lowering the resistance. For example, you could double the diameter of a ground electrode and your resistance would only decrease by 0%.

### 3. Number of ground electrodes

Another way to lower ground resistance is to use multiple ground electrodes. In this design, more than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods needs to be at least equal to the depth of the driven rod.

Without proper spacing of the ground electrodes, their spheres of influence will intersect and the resistance will not be lowered.

### 4. Ground system design

Simple grounding systems consist of a single ground electrode driven into the ground.

The use of a single ground electrode is the most common form of grounding and can be found outside your home or place of business. Complex grounding systems consist of multiple ground rods, connected, mesh or grid networks, ground plates, and ground loops.

These systems are typically installed at power generating substations, central offices, and cell tower sites.

Complex networks dramatically increase the amount of contact with the surrounding earth and lower ground resistances.

1. rogelio_beo
Jun 02, 2021

Dear, Sir,

Sir good morning my comment is regarding the residential power source supply Building.(PSSB) coming from Saudi electric company. WHY THE EARHTING AND NEUTRAL ARE IN ONE LOOP OR PARARELL CONNECTION. IF NOT AFFECTED THE TRANSFORMER IT SELF? AND ALSO THE LOAD SIDE OF THE BUILDING NOT AFFECTED DUE TO POWER FAILURE SOURCE SUPPLY?

Thank you very much
Best regards
foreman.

2. Dele Daramola
Nov 19, 2019

Sir, Your articles are amazing. I really appreciate your effort to bless and develop young Engineers.
Thanks in millions

3. arif
Nov 13, 2018

differnce between earthing and grounding

4. Ramesh Kumar
May 08, 2018

Dear Sir,

Plz advice if my reading showing me 0.28. Is this good reading or good resistance. Thank you

• Charles Lwanga
Feb 11, 2021

If you get this value in ohms then it is okay, 3 to 10 ohms at interval of 20 meters distance is just good.

Aug 21, 2017

Very informative. Thanks alot

6. Umesh Dutt
Mar 31, 2017

For subscription. Electrical engineer, working in defence manufacturing unit and involved in testing of control panel and other electronic equipment.

7. Wisdom Ayivor
Sep 09, 2016

I’m Professional Electrician at Ghana Energy commission. Sir your teaching is so good, l love it. I wish you all the best.

8. RAVIKUMAR
Jul 14, 2016

useful informations,thankyou sir.

9. mustafaelkarshini
Oct 28, 2015

10. t anand kumar
Aug 31, 2015

I am an electrical engineer. I want to learn about new methods of compensation for power factor improvements

• Girish R Nair
Jul 25, 2016

Capacitors are the best choices as there is no rotating parts.

11. jinesh
Jan 12, 2014

Dear Sir,
i am fresher electrical engineer and your work is really mind blowing…….ll read all your articals to improve my knowledge.Thanks a lot for providing all this @free base.
bdw can you have any articals regarding power system buses like Zbus and Ybus and all………? if than plz plz update same……
thanks a lot

### Learn to Design Power Systems

Learn to design LV/MV/HV power systems through professional video courses. Lifetime access. Enjoy learning!