Grounding Resistance Testing Methods
Wasim Asghar 
December 14, 2022Thousands of people die every year due to fire eruptions and electrocution caused by short circuits or lightning strikes. Most of these short circuits are caused by the usage of low-quality wires, substandard wire fittings, or compromised maintenance of electrical infrastructures. To eliminate the likelihood of such unfortunate events, it is crucial to find an alternative that minimizes the impact of current leakages and short circuits into the ground. The technique used is called grounding.
Ground resistance is the most important quantity used to calculate the current the ground can dissipate. To measure ground resistance, industry experts consult with Grounding Resistance Testing Methods issued and trusted by IEEE.
This blog post will discuss the three most practical and proven Grounding Resistance Testing Methods along with IEEE® standards related to testing ground resistance. Let’s cover each of these in detail.
3 Methods of Grounding Resistance Testing
We are going to cover the following three methods used for testing ground resistance.
Method 1 – Equally Spaced 4-Pin Method / Wenner Method
Wenner equally spaced 4-pin method is one the most preferred ground resistance testing method.
Procedure
- 4 electrodes of equal length ‘I’ are driven in a straight line at an equal distance ‘a’.
- The voltage between inner probes is measured.
- The current between outer probes is measured.
- Resistance and resistivity are calculated as shown below.
Mathematical Explanation / Formula
Method 2 – Unequally Spaced 4-Pin Method / Schlumberger Method
This is the modified approach of Wenner’s ground resistance testers to provide higher sensitivity for large spacing.
Procedure
- 4 electrodes of equal length ‘l’ are driven in a straight line, as shown below.
- The voltage between inner probes is measured.
- The current between outer probes is measured.
- Resistance and resistivity are calculated as shown below.
Mathematical Explanation / Formula
Method 3 – Driven Rod Method
This is based on the Fall-of-Potential approach and one of the most effective grounding resistance testing methods.
Procedure
- The test rod has a diameter of ‘d’, and it is driven into the ground to a length of ‘I’.
- Reference rods are driven to a shallow length in a straight line.
- Current is measured between Rod 1 and Rod 2.
- Voltage is measured between Rod 1 and Rod 3.
Mathematical Explanation / Formula
IEEE® Grounding Standards
As discussed earlier, Grounding or Earthing is an approach for electrical circuits integrated into the ground. It enables the dissipation of current leakage when electricity is discharged to the earth. This method is effective in eliminating the likelihood of any short circuit caused by wire or current leakage in the ground.
Since Grounding Resistance Testing is one of the core aspects of grounding, IEEE® documented a set of guidelines and procedures to measure ground resistance as well as ground potential for a grounding system. In power systems engineering, whether you are preparing for your PE Power exam or need references in your professional domain, It is recommended to consult the below three IEEE® Grounding Standards.
1. IEEE® Standard 80
Initially put forward in 1961, IEEE® standard 80 deals with the grounding of outdoor AC substations. According to the surveys, globally over 80% of utility engineers rely on a ground-grid design by ensuring IEEE® Standard 80. The latest amendments in the document further support users in further understanding ground grid design and its deployment across the globe.
The set of guidelines compiled in IEEE® standard 80 ensures secure grounding practices for primarily outdoor AC substations. These AC substations are to be used for power distribution and transmission. Whereas some instances of IEEE® 80 can also be adapted for indoor grounding.
2. IEEE® Standard 81
This IEEE® standard covers the procedures and standards used for the measurement of soil resistivity and grounding impedance of grounded systems.
Ground resistance testers are primarily used at lightning protection systems, AC substations, and industrial plants. The purpose is intended to safeguard expensive commercial or public equipment from potential hazards.
The US National Electric Code specifies that each substation must possess at least one grounding rod to protect the substations by:
- Ensuring that lightning strikes refrain from causing any potential damage to the system.
- Ensuring that an unwanted current from the neutral must not mistake a person standing close to the grid as the grounding rod. Otherwise, a person can be electrocuted.
These challenges can be addressed by setting up an efficient grounding system for the substation as per the guidelines instructed by IEEE® standard 81.
After the installation of a grounding system, it can be tested using the IEEE® Variation of depth method discussed above.
3. IEEE® Standard 142 (Green Book)
IEEE® Standard 142 signifies the guidelines related to the design and practical aspects of grounding.
An efficient design and deployment of an industrial-grade grounding system is undoubtedly a complex task. To ease this process, IEEE® Standard 142 has documented all the necessary knowledge that experts need to produce an effective grounding system. The references used in the document are remarkable and evident in their authority and technical proximity.
The IEEE® Standard 142 is applicable to all industrial-scale grounding infrastructures and helps you decide on the right type of grounding to use in a particular scenario.
Conclusion
Grounding Resistance Testing Methods discussed in this blog post are vital in determining the design and ensuring the proximity of the grounding system. The goal is to help experts in finding the perfect deployment location having the least possible resistance. The process depends upon calculating the resistivity factor of ground or soil, which may vary depending on various factors like the temperature and moisture on the deployment site.
Therefore, the inconsistent nature of soil resistivity in different geographics makes Ground Resistance Testing the most crucial approach in saving the electrical infrastructure as well as safeguarding human life.
Licensed Professional Engineer in Texas (PE), Florida (PE) and Ontario (P. Eng) with consulting experience in design, commissioning and plant engineering for clients in Energy, Mining and Infrastructure.