Grounding resistance measurement

Grounding or Earthing is the compulsory device for electrical installation.It’s the end point for electrical residue when leakage happen on electrical circuit or installation.

Good grounding can make a different of current balancing in phase during electrical leakage and Residual Current Circuit Breaker will activate if current leakage value over the sensitivity.( 10 mA,100 mA,30 mA,300 mA).

To get a good grounding,we should make sure the resistance is minimum.For the best resistance value is “0 ohm” or less than 5 ohm.That why grounding resistance measurement is important because we need to ensure our grounding is good.

How to make measurement for grounding resistance?

The potential difference between rods X and Y is measured by a voltmeter, and the current flow between rods X and Z is measured by an ammeter.

(Note: X, Y and Z may be referred to as X, P and C in a 3-point tester or C1, P2 and C2 in a 4-point tester.)

By Ohm’s Law E = RI or R = E/I,

we may obtain the ground electrode resistance

Example :

If E = 50 V and I = 1 A, then

R = E / I =50 / 1 = 50 ohm

It is not necessary to perform all the measurements when using a earth tester because it will measure directly by generating its own current and displaying the resistance of the ground electrode.

Position of the Auxiliary Electrodes when perform measurements

The goal in precisely measuring the resistance to ground is to place the auxiliary current electrode Z far enough from the ground electrode under test so that the auxiliary potential electrode Y will be outside of the effective resistance areas of both the ground electrode and the auxiliary current electrode.

The best way to find out if the auxiliary potential rod Y is outside the effective resistance areas is to move it between X and Z and to take a reading at each location.

If the auxiliary potential or  Y is in an effective resistance area, by displacing it the readings taken will vary noticeably in value. Under these conditions, no exact value for the resistance to ground may be determined.

On the other hand, if the auxiliary potential rod Y is located outside of the effective resistance areas, as Y is moved back and forth the reading variation is minimal.

The readings taken should be relatively close to each other, and are the best values for the resistance to ground of the ground X. The readings should be plotted to ensure that they lie in a “plateau” region as shown. The region is often referred to as the “62% are a . ”

I will explain in my next post about How to perform measurement for Ground resistance.It have several methods to perform :-


  1. M Ismael says

    Uusefull site for electrical engineers and electricians.

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