Earth Ground Resistance – why its important & how to measure.


If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people.

Importance of earthing or grounding
Importance of earthing or grounding

Why earthing/ grounding is necessary:

Poor earthing/ grounding not only contributes to unnecessary downtime, but a lack of good grounding is also dangerous and increases the risk of equipment failure. Without an effective grounding system, we could be exposed to the risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas.

What is electrical earth or ground:

The NEC, National Electrical Code, Article 100 defines a ground as: “a conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth.”

Types of electrical grounding:

  • Earth grounding
  • Equipment grounding.

Earth grounding is an intentional connection from a circuit conductor, usually the neutral, to a ground electrode placed in the earth. Equipment grounding ensures that operating equipment’s body is earthed.

These two grounding systems are required to be kept separate except for a connection between the two systems. This prevents differences in voltage potential from a possible flashover from lightning strikes.

Electrical Grounding standards

The NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less.The NEC code (1987, 250-83-3) requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with soil.
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Methods of Earth Ground resistance Testing

There are four types of earth ground testing methods available:

  1. Soil Resistivity (using stakes)
  2. Fall-of-Potential (using stakes)
  3. Selective (using 1 clamp and stakes)
  4. Stakeless (using 2 clamps only)

Fall-of-Potential measurement of earth resistance:

The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site.
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How does the Fall-of-Potential test work?

First, the earth electrode of interest must be disconnected from its connection to the site. Second, the tester is connected to the earth electrode. Then, for the 3-pole Fall-of-Potential test, two earth stakes are placed in the soil in a direct line—away from the earth electrode. Normally, spacing of 20 meters (65 feet) is sufficient. Then push the start button on megger meter.

measuring earth resistance with fall of potential method
Measuring earth resistance with fall of potential method

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