Earthing

EARTHING

H. M. PAI

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EARTHING FOR EQUIPMENTS SUB - STATIONS RESIDENTIAL PREMISES RURAL AREAS

1985.

H. M. PAI















EARTHING

By : H. M. PAI B. Sc. Engg. ( Elec - Mech ) F. I. E. E C. Eng. ( India ) General Manager, Torrent Power Ltd., Jubilee House, Shahpur, Ahmedabad.















INDEX

1.

Terminology

1

2.

Earthing and I.E. Rules

2

3.

Earthing and grounding.

3

4.

Nature of earth electrode resistance.

4

5.

Measurement of earth resistivity.

5

6.

Earth Electrodes.

6

7.

Earth resistivity and gradient.

14

8.

Earthing and Bonding

15

9.

Sub-station earthing

17

10. General instruction for laying earthing grid.

22

11. Consumer installation earthing and protective multiple earthing.

23

12. Earthing in rural areas.

26

13. General question and answers.

27

APPENDIX Extracts from IS 3043 pertaining to :Fig. 1.

Effect of moisture on earth resistivity.

31

Fig. 2.

Effect of salt on earth resistivity

32

Fig. 3.

Fuse rating v/s earth circuit impedance

32

Fig. 4.

Pipe electrode erection details.

33

Fig. 5.

Earth electrode resistance details for various lengths.

34

Fig. 6.

Plate electrode erection details

35















PREFACE There are many ways of protecting both the installation, user and operation staff from the risk of electric shock of fire under fault condition. A shock risk arises when ever accidental contact is made between the live conductor and exposed metal work. This risk can be guarded against by efficient earthing. Fire risk in electrical installations also can arise due to earth fault current, particularly if the earthing and bonding arrangements arrangements are nor capable of carrying a sustained fault current without excessive heating. In outdoor substations step and touch voltage can cause risk to life of operation staff if voltage gradient is not kept within safe limits limits by a properly designed earthing grid. It could be seen that efficient earthing is must as far as electricity is concerned. The booklet on ‘Earthing’ covers all the essential points pertaining to the subject and it is hoped the book will be found useful and informative by all concerned. Thanks are due to the Management of the Ahmedabad Electricity Ltd., For the encouragement given. 1-1-1985

H. M. PAI















1.

TERMINOLOGY.

Bond :

To connect together electrically two or more conductors or metals parts.

Dead :

‘Dead ’ means at or about earth potential and disconnected from any live system.

Earth :

A connection to the general mass of earth by means of an earth electrode.

Earthed :

An object is said to be ‘earthed’ when it is electrically connected to an earth electrode.

Earth Electrode :

A metal plate, pipe or other conductor connected to the general mass of earth.

Earthing lead :

The conductor by which the connection to the earth electrode is made.

Potential gradient :

The potential difference per unit length measured in the direction in which it is maximum.

Resistance area of an earth electrode :

The area of earth within which practically the whole of the potential difference between the electrode and the general mass of earth occurs when it is carrying fault current.

Step potential :

The maximum value of the potential difference possible of being shunted by a human body between two accessible points on the ground separated by a distance of one pace which may assumed as one meter.

Touch potential :

The maximum value of potential difference between a point on the ground and a point on an object likely to carry fault current such that the points can be touched by a person.

















2. EARTHING AND I.E. RULES-SALIENT POINTS. Rule 33. In case of medium, High and Extra High Tension installations, a consumer has to provide his own earthing system over and above terminal provided by the supplier.

Rule 51 (D) All metal work associated with the installation other than that designed to serve as a conductor, be connected with earth.

Rule 61 (2) Before supply is made ‘on’ all earthing system shall be tested to ensure efficient earthing.

Rule 61 (5) All earthing systems shall be tested for resistance on dry during the dry season not less than once every two years.

Rule 61 (6) A record of every earth test made and the result there of shall be kept.

Rule 66 (a) Wherever conductors are enclosed in metal sheathing, the sheathing shall be earthed.

Rule 66 (b) The resistance of the earth connection should be low enough to blow fuse trip breaker in the installation concerned.

















3. EARTHING & GROUNDING. 3.1

Objective

Equipment earthing or grounding relates to the manner in which nonelectrical conductive material, which either encloses energized conductors or adjacent there to is be interconnected and grounded. The basic objectives being sought are the following :-

(a)

To ensure freedom from dangerous electric shock-voltage exposure to persons in the area.

(b) To provide current carrying, capability, both in magnitude and duration adequate to accept the earth fault current permitted by the protections system without creating a fire or explosive hazard to building or contents. (c)

To contribute to superior performance of the electrical system by isolating the defective equipment from the system.

It may not be out of place to clarify the difference between system grounding and equipment grounding. Equipment grounding is explained in the first para above. System grounding means a system of current carrying conductors in which at least one conductor or point (neutral in case of star winding of transformer or generator) is intentionally grounded, either directly or through some impedance.

3.2

Voltage Exposure.

Electrical accident statistics clearly indicate that many fires and personal injuries are caused as a result to current carrying parts coming in contact with metallic parts which are expected to remain non-energized. Proper earthing or

















interchangeably is obviously in error. So, where equipment cannot refer to earth potential while referring to voltage exposure magnitudes the term grounding is proper. In air craft, the main body of this the reference ground as for as voltage exposure magnitude is concerned and this has nothing to do with earth. So grounding is the proper term here. Similarly to an electrician working on the tenth floor of a modern steel structured building, the reference ground is the building frame and other metal equipments there. What might be the potential of earth is of negligible importance to this work on the tenth floor in short what is earthed can be called as grounded, but what is grounded may not have anything to do with earth.

4. NATURE OF EARTH RESISTANCE 4.1

Nature : The earthing resistance of an electrode is made up of :-

a. b. c.

Resistance of the metal electrode. Contact resistance between the electrode and the soil. Resistance of the soil, from the electrode surface outward to infinite earth.

The first two resistances are very small and can be neglected for all practical purposes. Around the electrode, the soil resistance is the sum of the series resistance of virtual shells of earth, located progressively outward from the rod. The shell nearest the rod has the smallest circumferential area or cross section, so it has the highest resistance. Successive shells outside this one have progressively larger areas, and thus lower resistances. As a radius outward from

















Table 4.1 Electrode resistance at a radius R cms from a 3 meter long- 16mm diameter rod-total resistance at R = 7.6 meter = 100 %.

Distance from Electrode surface. R=cm.

Approximate percentage of Resistance.

3

25

6

38

9

46

15

52

30

68

150

86

300

94

460

97

610

99

760

100

4.2

Multiple Electrodes.

Multiple electrodes in parallel are used to get a lower resistance than obtained by a single rod. Adding a second rod does not, however. Provide a resistance than obtained by a single, unless the two are several rod lengths apart. Normally the rods are placed one rod length apart i.e. if the first rod is of 3 meter. The next rod will be at least 3metres away from it. An experiment was carried with 24 rods, rods being placed one rod length apart, in a line, hollow triangle, circle, and square produced factors for each number as shown in table

















Table 4.2 Resistance of single rod. Multiplying factors for multiple rods. Number of rod

Total resistance in terms of ‘r’

2

0.58 r

3

0.43 r

4

0.34 r

8

0.21 r

12

0.15 r

16

0.12 r

20

0,1 r

24

0.09 r

So, if ‘ r ’ was 20 ohms, with 4 rods the total resistance will be 20 X 0.34 = 6.8 ohms and not 20 / 4 = 5 ohms Similarly with 20 rods it will be 2 ohms and not 20 / 20 =1 ohm.

5. MEASUREMENT OF EARTH RESISTIVITY 5.1

The resistivity of earth varies within wide limits between 1 to 10,000 ohm meter, ( 1 ohm meter =100 ohm Cm ) and is decisive importance in all problem involving earth as a return conductor. To measure earth resistivity, current is passed between two electrodes fixed ground, the voltage is measured between two intermediate Potential electrodes. Four terminal megger is used for this purpose and the general arrangements of connections are as shown in Fig. 5.1.



















In most practical work, the value of earth resistivity over a fairly large area is required. The electrodes therefore, should be spaced quite apart so that the



















5.3

Formulae The connections are done as shown in Fig 5.1.

Formula 1.

Where

r

=

192 S. R.

r

:

earth resistivity in ohm-cm.

S

:

Distance between electrodes in feet.

R

:

Megger reading in ohms.

r

=

2 TT S. R

r

:

earth resistivity in ohm-cm.

S

:

Distance between electrodes in meter.

R

:

Megger reading in ohms.

Formula 2.

Where



















5.4

Test Procedure :

At the selected test site, the four electrodes are driven into the earth along a straight line in the chosen direction at equal intervals ‘S’ as shown in Fig. 5.1. The depth ‘d’ of electrodes in the ground can be of the order of say 10 to 15 cms or more. It should be remembered that ‘S’ should be 20 ‘d’ or more, it ‘d’ say 15 cam, then ‘S’ should be 3 meters or more. The megger is placed on a steady and level base. The connection P1. P2, C1 and C2 are made as shown in Fig. 5.1. The readings on the megger are then taken while turning the crank at about 135 rev/min. Formula 1 or 2 can be used to get earth resistivity after getting reading ‘R’. But if ‘S’ is less than 20 ‘d’, than formula 3 will have to be used.

5.5

Earth resistivity :

Earth resistivity for different types of soils may have values as shown in table 5.1.

Table 5.1 Type of soil

Earth resistivity in ohm-cms.



















Table 5.2 Electrodes.

Resistance in ohms.

8’ × 5/8” dia Rod.

4.2

12’ × 5/8”

3.0

“

18’ Sq. CI plate.

6.0

24”

4.5

“

6’ × 1.1/4” dia pipe

4.75

6’ × 2”

4.43

‘

For soils of other resistivity, the figures in tables 5.2 are multiplied by the factor (Resisistivity/1000). Multiple electrodes are installed in almost all to get lower resistance. Precautions should be taken in such cases as follows :-

Multiple rods or pipes should be separated by a distance not less than their buried length. Plates should not be less than 10 apart.



















6.2

Made Electrodes :

Made electrodes may be subdivided into electrodes, steel reinforcing bars in below ground concrete, buried strips or cables, grids plates, and counterpoises. The type selected will depend upon the type of soil encountered and the available depth. Driven electrodes are generally more satisfactory and economical where bedrock is 3 meters or more below the surface, while grids, buried strips, or cable are preferred for lesser depths. Grids are frequently used for substation or generating stations to provide equipotential areas throughout the entire station where hazards to life and property would justify the higher cost. They also require the least amount of buried material for a given electrode resistance. Buried plates have not been used extensively in recent years because of the high cost as compared to rods or strips. Also when used in small numbers they are the least efficient type of rods of made electrode. The counterpoise is a from of buried cable electrode used to ground line towers and structures. When multiple electrodes are used spacing of less 3 meters do not provide the most economical use of materials. In selecting the number and size of grounding terminals their current discharge limitations must be recognized, if these are exceeded the earth around the electrode may be exploded by steam generation or may be dried out to the





















connection from a main reinforcing of each footing for attach to the building ground bus or structural steel. A convenient meant for a connection is to tack- weld a short connecting bar between one of the vertical reinforcing bars and one of the anchor bolts for connection above the footing surface. The steel frame of a building, attached to these anchor bolts, then becomes a highly effective grounding conductor, and is used as a grounding bus in many industrial buildings. Each such a footing electrode has resistance equal to or lower than that of a given rod of equal depth, The large number of such footing inherent to building will provide a net ground resistance considerably lower than that normally provided by other made electrode methods, generally below 1 ohm and frequently of the order of 0.25 ohm.



















Where the enclosing fence is within the perimeter of the buried grid the fence must be bonded to the grid to minimize the shock hazard from the touch potential of person touching the fence and, of course, standing on the ground which may be at a the perimeters of the grid the fence should be grounded to its least 1.22 meter beyond to its own electrode system. Which will include a cable buried in the earth approximately 0.91 meter outside the fence line, for the same reasons as discussed. Coarse cracked rock, usually granite, is normally spread all over the surface of the soil within such a substation grid area, not for housekeeping reasons, but to provide a high resistance surface treatment to reduce the hazard from step potential to persons within this areas buring a severe fault.

6.4

Plates :



















For ordinary soil condition, the 3 meter length of rod has become fairly well established as a minimum standard length to meet the code requirement of a minimum of 2.5 meter.

6.5.1 Methods Driving Rods. Sledging requires a minimum of driving equipment, but may require considerable time per food of rod. A modification of the sledging process, consisting of a chuck and sliding hammer, permits the work to be carried on at a level convenient to the worker with out a ladder or auxiliary platform. An additional advantage is that the blow is delivered to the rod at a point not far from the ground line, thus permitting rods to be driven to greater depths than would be possible by hand sledging. If rods are to be driven on a comparatively large scale, it is desirable to provide power driving equipment. Electric, pneumatic and



















6.6

Measuring Electrode resistance :

Four terminal earth testing megger, commonly know as earth tester is used for measuring the resistance of earth electrode. At the time of test, the test electrode and current electrode are used for the test. These auxiliary electrodes usually are of mild steel and have a diameter of 12.5 mm. these rods have to be driven up to 1 meter into the ground. The connection are shown Fig. 6.1.



















change to a market degree. An earth electrode or mat should therefore be driven or buried deep enough to be permanently in contact with moist should earth but where this is not possible or, as in the case of a large earth mat too costly, a welldistributed system of vertical rods, driven to a sufficient depth and bonded to the earth grid, will usually suffice.

Table 7.2 Type of soil.

Resistivity ohm - meter

Clay and loam

4 -150

Sandy clay

80 -200

Marsh peat

150 -200





















A voltage gradient in the earth occurs when current from an electrode flows through the earth resistance. It is theoretically infinite at the surface of the electrode. Practically, a danger exists to persons or large animals, particularly the latter due to their greater sensitivity and the greater distance apart of their legs, if the potential difference at the ground surface is sufficient to pass a dangerous



















transformers. Copper strip used for bonding conductors in distribution system are generally of following standard size.

(a)

4 cm by 5 mm (1.1/2 in by 3/16 in ) copper strip.



















point where connection to earth in made. The reason is to ensure that under fault condition there is no voltage difference between armoring and sheath which would cause arcing and subsequent pitting of the lead. At junction points between lines and cable, the cable sheath or sheath in the case three cable in trefoil must be bonded together and to the earth tower or pole. The base of the



















over the area is achieved. The mesh electrode is normally constructed of rectangular strip, copper of minimum size 2.5 cm by 3 mm or steel 5 cm by 5 mm of length not exceeding 100 m beyond which consideration of impedance at Hz indicates that no appreciable reduction is dissipation resistance would occur. Since the efficient design of earth electrodes requires then to have largest



















































































































































































































































Earthing

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