Earth Resistance Measurement

Presented by,

W. F. Wan Ahmad, PhD.Centre for Electromagnetic and Lightning Protection

Research(CELP)

Department of Electrical and Electronic EngineeringFaculty of Engineering

Universiti Putra Malaysia43400 UPM Serdang

Presentation outline

Introduction Methods of earth resistance measurement Earth resistance measuring instruments Earth resistance measurement procedure

Introduction

Earth resistance may be broadly defined as the resistance of soil to the passage of electric current.IEEE Std. 81-2012 has defined earth resistance, as the impedance excluding reactance between an earth electrode, grid or system and remote earth [IEEE Std. 81-2012].

Introduction (contd.)

Earth resistance measurement is conducted to determine the effectiveness of earth electrode systems such as grids, single or multiple earthing points and connections that are used in electrical systems to protect personnel and equipment [Megger].

 

Introduction (contd.)

The resistance of an earth electrode system has basically three components as illustrated in Figure 1;

 Resistance of the electrode itself and connections to it.Contact resistance between the electrode and the soil adjacent to it.Resistance of the surrounding soil.

 

Introduction (contd.)

Fig 1 Equivalent resistance of an earth electrode system

Introduction (contd.)

The resistance of the electrode materials such as copper, galvanized steel, and copper-coated steel are usually small so that their contribution to the total resistance is negligible.

The contact resistance between the electrode and soil is may also be considered negligible if the electrode materials are clean and unpainted when installed and the soil is firmly packed.

Introduction (contd.) Generally, the resistance of the surrounding soil will

be the largest of the three components.

An earth electrode system buried in the soil radiates current in all directions and eventually dissipates some distance away depending on the soil’s resistance to current flow as shown in Figure 2.

The earth resistance we’re concerned with here is the resistance to current flow from the electrode into the surrounding soil [Reeve, 2008].

Introduction (contd.)

Fig 2 Radiation of current through the soil

Methods of earth resistance measurementsIEEE Std. 81-2012 has outlined 3 methods;

Three point methodStaged fault testsFall of potential method (FOP)

FOP is the most accurate method for measurement of earth resistance and soil resistivity.

Also, it is the universally used method for earth resistance measurement.

Resistance measurements (contd.) The method involves placing a potential stake P in a

straight line between the earth electrode E under test and the current stake C making an angle θ = 00 between the voltage and current stakes as shown in Figure 3.

If the potential stake cannot be located in line with the earth electrode due to obstructions such as buildings, paved lots or underground metallic pipes, the potential stake may be located at angles ranging from 900 up to 2700 .

Resistance measurement (contd.)

Electrode under test Current stakeVoltage stake

Fig 3 Arrangement for earth resistance measurement

Earth resistance measuring instrumentsSeveral instruments for measuring earth resistance exist, but the most common ones are;Megger Earth Tester DET3TCFluke 1625-2 Geo Earth TesterKyoritsu 4106 Earth Resistance and Resistivity TesterAEMC Earth Resistance TesterMegger Earth Tester is the most commonly used in UPM due to its relative cost and ease of handling. Refer to Figures 4 and 5.

Fig 4 Megger Earth Tester DET3TC

Earth resistance measurement procedure

1. The first step in conducting earth resistance measurement is to calibrate the DET3TC Megger Earth Tester and ICLAMP to compensate for changes in environmental conditions during transportation from one location to another. See Figure 5.

2. The position of voltage stake (P) with respect to the earth electrode under test (EUT) is determined using the rule of 61.8% prior to commencement of the earth resistance measurement. Refer to illustration of Fig. 6.

Measurement procedure (contd.)

Fig 5 Calibration of DET3TC and ICLAMP

Fig 6 Typical arrangement for earth resistance measurement

Measurement procedure (contd.)3. Next the DET3TC, ICLAMP, voltage and current stakes are connected across the earth electrode under test (EUT) as shown in Figure 6. 4. The selector button on the DET3TC is then switched to 3-Pole option.5. When the test button on DET3TC is pressed, current I is circulated between EUT and current stake. 6. A potential difference V between EUT and the voltage stake is determined by the DET3TC.

Measurement procedure (contd.)

Fig 6 Connections at the rear of the DET3TC for measuring earth resistance

Measurement procedure (contd.)

7. The value of apparent resistance R of the EUT is then displayed on the DET3TC and recorded.8. The DET3TC is then switched off and transferred to the next EUT and procedure 3 to 7 is repeated until the EUT’s are exhausted or the job completed.

(contd.)

(contd.)