BRAC UNIVERSITY

DEPARTMENT OF ELECTRICAL AND ELECTRONIC

ENGINEERING COURSE CODE: EEE 431L/432

EXPT NO: 02
NAME OF THE EXPERIMENT: DETERMINATION OF EARTH RESISTANCE USING
EARTH TESTER

OBJECTIVE:
To determine earth resistance using Earth Tester.

THEORY:

➢ Earth Resistivity:

An earth resistance test (also called a ground resistance test) measures how effectively the
electrical grounding system in a facility dissipates fault currents into the earth. Proper grounding
protects people and equipment from electric shock, lightning, and surges.

➢ Importance of Earth Resistance Testing:

• Prevents electric shock hazards

• Ensures proper lightning protection
• Protects equipment from overvoltage and faults
• Required by many safety codes (e.g., NEC, IEC, IEEE)

➢ Common Earth Resistance Testing Methods

1. 3-Point (Fall-of-Potential) Test

Place-1:
Place-2:
Most accurate method, typically used with testers like the Lutron ET-3000 or PCE-ET 3000.

Setup:

• One rod is the earth electrode under test (E)

• One is a current probe (C), placed 30–50 m away
• One is a potential probe (P), halfway between E and C

How it works:

• Tester sends a known current from E to C

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• Measures voltage between E and P
• Calculates resistance using Ohm’s Law:

R=V/I

Pros: Highly accurate.

Cons: Needs open space to place probes (long distances)

2. 2-Point Test (Simplified)

Place-1:
Place-2:
Used when only one earth electrode is accessible.

Setup:

• Tester connects between the system under test and a known ground (e.g., building water
pipe)

Pros: Simple.

Cons: Accuracy depends on the quality of the reference ground

3. Clamp-On Test (No Probes Needed)

Uses a clamp meter to test ground loops without disconnecting the ground.

Pros: Fast

Cons: Works only if a complete ground loop is present

Ideal Earth Resistance Values

System Type Recommended Resistance

Residential ≤ 25 Ω (per NEC)
Lightning protection ≤ 10 Ω (ideally ≤ 5 Ω)
Sensitive electronics ≤ 1–5 Ω
Utility substations ≤ 1 Ω

Recommended Tools

• Earth tester (e.g., Lutron ET-3000, Megger DET3, Fluke 1623)

• 3 test leads (15 m, 30 m, 50 m)
• 2 ground stakes (rods)

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• Measuring tape

How Earth Resistivity is measured:

A four-terminal instrument is used to measure earth resistivity. Now however you use four small
sized electrodes driven down to the same depth and equal distances apart in a straight line
(Fig_1). Four separate lead wires connect the electrodes to the four terminals on the instrument
as shown. Hence, the name of this test: the four-terminal method.

Fig_1: Four-terminal method of measuring earth resistivity

Dr. Frank Wenner of the U.S. Bureau of Standards (now NIST) developed the theory behind this
test in 1915. He showed that, if the electrode depth (B) is kept small compared to the distance
between the electrodes (A), the following formula applies:

p=2πAR
Where p is the average soil resistivity to depth A in ohm-cm,
π is the constant 3.1416,
'A' is the distance between the electrodes in cm, and
R is the Megger earth tester reading in ohms.
In other words, if the distance A between the electrodes is 4ft, average earth resistivity to a depth
of 4ft can be obtained as follows:

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1. Convert the 4ft to centimeters to obtain A in the formula:
A = 4 x 12 x 2.54 cm = 122 cm

2. Multiply 2 πA to obtain a constant for a given test setup:

2πA = 2 x 3.1416 x 122 =766 cm

Now, for example, if your instrument reading is 60 Ω, the earth resistivity would be 60x766 or
45,960 Ω-cm.
Types of Soil Affecting Resistivity:

Whether a soil is largely clay or very sandy, for example, can change the earth resistivity a great
deal. It isn't easy to define exactly a given soil; “clay” can cover a wide variety of soils.
Therefore, we cannot say that any given soil has a resistivity of so many ohm-cm.

Fig_2: Deeper earth electrodes lower the resistance.

These graphs show the relation between character of soil and resistance of driven electrode at
increased depths.

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Features Of Earth Resistance (ET-3000):

This handheld meter supports three resistance ranges (19.99 Ω / 199.9 Ω / 1.999 kΩ) and earth
voltage up to 199.9 V AC. It features a large 18 mm LCD, data-hold, auto power-off, and built-in
overload protection. Powered by six AA batteries and includes spike/probe leads and carrying
case.

1.
 Earth Resistance and Earth Voltage measurement.
2. 3 ranges for earth resistance measurement, 19.99 Ω, 199.9 Ω, 1.999 KΩ.
3. 0 to 199.9 V for earth voltage measurement.
4. 18 mm, large size LCD display, easy to read-out.
5. Data hold function to freeze the display reading value.
6. Large Scale Integration (LSI) circuit provides high accuracy, reliability and durability.
7. Manual power on/off or auto power Off within two minutes after power On. 8. Built-in
over-input indication.
9. Durable & portable housing plastic case with the front protective cover.
10. Complete testing accessories included.

Fig: _3: Earth Resistance Tester (Model: ET-3000)

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Specifications:

• Resistance ranges: 19.99 Ω (0.01 Ω res), 199.9 Ω (0.1 Ω res), 1.999 kΩ (1 Ω res) •
Voltage: 0–199.9 V AC (0.1 V res)
• Accuracy: ± (2 % + 0.1 Ω) / ± (1 % + 4 digits)
• Response: ~4 s (resistance), ~1 s (voltage)
• Size ≈ 160 × 120 × 65 mm; weight: ~560 g
 Fig_4: Front Panel Description of Earth Resistance Tester

o 3-1 Display
o 3-2 Function Switch
o 3-3 Hold/Normal Switch
o 3-4 Power Button
o 3-5 Terminals (E/P/C)
o 3-6 Battery Cover/Compartment
o 3-7 Test leads (Black/Green/Red)

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o 3-8 Earth Spikes

Test Leads Connection:

Stick the Earth Spikes (3-8, Fig. 4), P and C, into the ground as shown below (Fig. 5). They
should be aligned at an interval of 5 to 10 meters from the earthed equipment under test. Connect
the Black lead wire to the terminal E of the instrument, the green wire to terminal P and the
Red wire to terminal C.

APPARATUS REQUIRED:
Serial No Name of Equipment Quantity
 1 Earth Tester (ET-3000) 1

2 Spikes 2

3 Connecting wires As per connection

CIRCUIT DIAGRAM/ EXPERIMENTAL SETUP:

Fig_5: Circuit Diagram

PROCEDURE:

1. Connect the earth tester as shown in Fig_5.

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2. Set the “Hold/Normal Switch” (3-3, Fig. 4) to the “Normal” position.
3. Set the “Function Switch” (3-2, Fig. 4) to the “2 K ohm” position.
4. Push the “Power Button” (3-4, Fig. 4) once a while. Then release the finger from the
“Power Button” will power on the instrument. The Earth Resistance value will be
indicated on the display.
5. If necessary, turn the “Function Switch” (3-2, Fig. 4) to the “200 ohm” or “20 ohm” and
make another measurement.
6. The instrument will be auto power off approx. two minutes after power on. During the
power
on, just
 push the “Power Button” will be manual power off.

OBSERVATION TABLE:

Resistivities of Different Places*

Place Resistivity (Ohm-cm)

Average Minimum Maximum

Place-1 13.215 13.1 13.33

Place-2 32.95 32.9 33

Place-3 0.565 0.13 1

____________________________
Signature of the Faculty

REPORT QUESTIONS:

1. You recorded three resistance values with the potential probe moved at 42%, 52%, and
75% of the distance from the test electrode. The values were 5.1 Ω, 5.3 Ω, and 5.6 Ω.
What does this indicate about the reliability of the test?

The resistance values (5.1 Ω, 5.3 Ω, 5.6 Ω at 42%, 52%, 75% probe positions) are close,
indicating a reliable test. Consistency suggests minimal influence from external factors or probe
placement errors.

2. Why is it important to use proper distances between the test electrode, potential probe, and
current probe?

Proper distances ensure accurate measurement by avoiding overlap of electrical fields between
electrodes, reducing interference and ensuring the potential probe measures true voltage drop.

3. If your test result was significantly higher than expected (e.g., > 30 Ω), what could be the
possible reasons? What corrective actions might be taken?

Possible reasons include poor soil conductivity, shallow electrodes, or dry soil. Corrective actions:
use deeper electrodes, add more grounding rods, or treat soil with conductive materials.

4. How would you interpret a decreasing resistance trend as you increase probe spacing?

A decreasing trend suggests the grounding system is effective, as deeper or farther soil layers may
have lower resistivity, improving current dissipation.

5. Suppose a building has a ground resistance of 50 Ω. Discuss the risks involved and how to
reduce the resistance to acceptable levels.

High resistance increases risks of electric shock, equipment damage, and poor lightning
protection. To reduce: install additional grounding rods, use deeper electrodes, or apply chemical
treatments to soil.

6. Why is achieving a resistance below 1 Ω critical in substations or sensitive facilities like

data centers?

Low resistance ensures rapid fault current dissipation, protecting sensitive equipment from surges
and ensuring safety in high-power or critical systems.