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# EARTH RESISTANCE

INTRODUCTION

Electrical equipment at all voltage levels must be earthed i.e. connected directly to an
electrode driven or buried in the ground. The resistance between the earth electrode is
critical to the safety of equipment and personnel. Usually it must be less than 100
ohms e.g.. 10 ohms for an overhead line pylon. This is not always possible because of
the high resistivity of some earthy materials.

THEORY

See 'EHV Transmission Line Reference Book' p.290 and 'Electrical Transmission &
Distribution Reference Book' Westinghouse Electric Corporation, Section I pp.570-
590.

## Where L= length of rod in ground, m, a= radius of rod, m

ρ= ground resistivity, ohm-m

A= sqrt (aS)

S= separation, m

APPARATUS

## YEW Earth Tester TYPE 3235 (L-91)

100' metal tape measure
Earth rod
Sledge hammer & crow bar

METHOD

1. Select an existing earth electrode or drive the earth rod into the ground to be
measured ensuring that there is at least 40-50 m of open ground adjacent to the
electrode.
2. Connect the earth tester as shown in FIG.5
3. Position Electrodes P and C at L= 5m, carry out a trial measurement. Adjust the
dial and momentarily depress the push-button. If the needle is off-scale, adjust the
dial again and depress the push-button momentarily. Repeat this until the pointer
remains on scale. The push-button can then be held down whilst the dial is
4. Take a series of readings by varying L. Plot a curve of R against L. Determine the
optimum value of L.
5. Drive a second earth rod 1m from the earth. Measure its resistance for the
optimum value of L, the length L, and the radius a
6. Connect the two earth rods together and measure there combined resistance at the
optimum value of L.

RESULTS:

## 1. Plot the results of step 4 on graph paper

2. Calculate ρ from the results of step 5.
3. Tabulate the resistance values.
4. Calculate the value of two rods in parallel.

DISCUSSION:

Compare the experimental curve with the expected theoretical curve. Is the
manufacturer's range for L reasonable? What was your optimum value of L? Compare
the measured & calculated values for the rods in parallel.

CONCLUSIONS:

## 1. How can the earth resistance be reduced?

2. Was the soil tested a good earthing material?
3. What is achieved by connecting rods in parallel?

## Read the following notes on ground resistance

GROUND RESISTANCE

## An understanding of the basic aspects of tower footing resistance characteristics and

measurements is very useful in evaluating the lightning performance. The resistance
of a rod electrode of length 2l and diameter 2a buried at an extreme depth in the earth
is given by

R= ρ ln (2l/a)
4 πl
where ρ is the ground resistivity.

If this electrode is buried with only half its vertical dimension in earth, the resistance
will be twice that given in the above expression. The resistance will be only
somewhat less than twice that given above if the electrode is buried at a depth small in
comparison to the length dimension.
The most useful form of the ground electrode is the driven ground. Ground rods are
usually supplied in-3 - 4.5m's in length and may be joined by couplings for longer
depths. Rod diameters are generally less than 2.4 cm. The resistance of a driven
ground rod in ohms is:
R=ρ [ln (2l/a)- 1]
2πl
where ρ = ground resistivity, ohm-m
l = length of rod, m
The diameter of the rod is of less significance since it affects only the logarithmic
term 2l/a, but the length is very important.
The resistance does not decrease directly with length, however, and a condition can
arise when further increase in length is accompanied by only a minor reduction in
footing resistance.

## Fig1.Resistane of vertical ground rods as a

function of length and diameter-soil
resistivity=100 m-ohms

250
Rod dia-2.54 cm
Rod dia =5.08 cm
200

Rod dia=7.62 cm
Resistance-ohms

150

## Rod dia =9.16 cm

100

50

Series1
0
0.2 1 10 100 Series2
Series3
length of rod-m
Series4

Fig. 1 shows the variation of resistance with rod length for various diameters. The
curves are drawn for a ground resistivity of 100 ohm-m.

Connecting driven ground rods in parallel can also lower ground resistance. If the
spacing between rods is large compared to the length of the individual rods, the
resistance will be reduced in proportion to the number of rods. If the rods are close
together, each rod will be in the intense electrical field of its neighbour. If the rods are
very close together, the over-all resistance becomes
R= ρ ln (2l/A)
2πl
where A represents the resistance of an equivalent rod. Fig. 2 shows how the
equivalent radius depends on rod geometry. If the rods are moderately close to each
other, the over-all resistance will be more than if the same number of rods was spaced
far apart. The increase in resistance depends on the number of rods involved
The fundamental method of measurement of ground resistance is shown in Fig.3.
Current is circulated between the ground under test and an auxiliary ground. This
should be preferably located at a distance that is large compared to the dimensions of
the ground under test since it is not desirable to have interaction of the ground current
distributions at the two electrodes. A voltage is then measured between the ground
under test and a reference ground located somewhere between the two current
carrying electrodes. This reference ground should also be so located that it is not in
the electric field of either the current carrying electrodes. Assuming that the current
density is negligible at the reference electrode, the resistance of the ground under test
is R = V/I. The measurement may be made by using a voltmeter and ammeter with
the current being supplied from the AC power lines. Alternatively, a bridge may be
used for measurement. Most often, the ground resistance is measured with self-
contained instruments such as the Megger ground resistance Tester.
The resistivity of earth may vary over extremely wide limits, depending on the
composition of the soil and the moisture content. Representative values are:

## Sandstone 108 ohm-meters

S S S

S S S S
A=SQRT(a S ) S
A cube
of groups of 2ground
root of(aS rods
) A= fourth ofof(sqrt(2)aS
S
I current

V voltmeter

## Adequate earthing systems for telecommunications, electronic and electrical

equipment, power lines and lightning arresters of steel towers and high buildings are
of extreme importance. Good earthing systems safeguard personnel and equipment
during fault conditions. They are also frequently necessary in minimising electrical
noise and for normal, reliable operation of electrical and electronic equipment. The
resistance of any earthing system should be periodically checked to ensure that it is
within limits.
The Type 3235 Earth Tester can be used to check almost any earthing system. It
incorporates a test for earth voltage. AC voltage up to 30V may be measured. High
earth voltages indicate either high earth currents (due to equipment faults) or a faulty
earthing system. High earth voltages indicate a condition that may be dangerous.
Its 3-electrode earth resistance measurement uses an AC potentiometer bridge. Two
earth electrodes are used besides the system earth under test.
Test electrode resistance is not critical, up to 10 k-ohms is tolerable; or a single "good
"earth test electrode is such as a metal water pipe may be used for 2-electrode
measurements.
The AC bridge uses a transistorised 500 Hz inverter and synchronous detector, so is
not affected by the normal DC or 50/60 Hz earth voltages of normal equipments.
Bridge balance is indicated by a rugged but sensitive taut-band suspension
galvanometer.

Measuring range:

## Earth resistance: 0 to 10 to 100 to 1000 ohms

Earth voltage: 0 to 30 v AC
Measuring frequency:

500 Hz

Measuring current:

Up to 20 mA 500 Hz)

Power source:

## Names and functions of components:

1. Measuring terminals

## E (system earth under test)

P and C (earth test electrodes)
E and P terminals are used to measure earth voltage.
E, P and C to measure system earth resistance
Dial scale: 0 to 1000 ohms

3. Indicator:

displays tester internal battery or system earth voltages, or indicates earth resistance
bridge balance
4. Push - button switch

Enables internal battery check and resistance measurement ranges. Must not be
operated during system earth voltage measurement.

## Ω- system earth resistance

V- system earth voltage
B-battery voltage check

6. Index

OPERATING INSTRUCTIONS

Electrode connections

a) E is the system earth under test. P earth test electrode is the centre point of the
resistance bridge, and is also used for earth voltage tests. C electrode supplies current
to the Resistance Bridge. Fig 5) l should be at least 5 to 10 meters,
E, P, C should lie in a straight line approximately, and earth resistance of the test
electrode should be low.

Practical considerations

There are often problems in siting suitable test earths, especially where system earth is
in a building surrounded by pavements and other buildings. Ground near tree or in
garden plots (where not above asphalt or concrete paving), water pipes (where
plumbing is all - metal and not internally insulated by carbonate deposits from hard
water) and manholes in building basements may provide access to suitable test earths.
EP should approximately equal PC

Principle of measurement

Fig. 6

## System earth resistance Rx

Measuring current I

## When the bridge is balanced (Galvanometer current is zero) voltage across Rx is

Ex = Ep = Eso........(1)

Ex = I Rx....(2)

## Since Eso = nI Rso

I Rx = nI Rso
Therefore,

Rx = n Rso.... (3)
According to equation 3, the log dial scale of potentiometer is calibrated a to show a
value n Rso corresponding to Rx.

## Measurement of earth resistance

(a) Suppose that the electrodes E and C are connected to source as in Fig.7 (a). P. D
between E and C is shown in Fig. 7 (b) or (c). If EC is great enough l1, an equivalent
area appears midway between E and C so that Ex (voltage drop by Rx) may easily be
distinguished from Ec (volt drop by Rc). Therefore, if another electrode P is buried
midway between E and C,

Ex = I. Rx.

## Therefore Rx = Ex/I.... (4)

(b) If EC is sufficient l2, the equipotential area does not appear at all as in Fig 7(c).
Because of this it is impossible to distinguish Rx from Rc. Generally a suitable
distance EC is 10 to 20 m.

Fig 7

(a) I Eo

E C

Rx l1 Rc

(b)

Ex = I Rx
Voltage drop

Ec = I Rc

Eo distance -m l1

(c) D
Ex
D
Ex
Ec
Eo l2
Ec

Eo l2

Fig. 6

1: n CT

Fig. 5

E P C

l l

l = 5 to 10 m

Fig. 4

1 6 2

3 5 4

3 5 4
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