Earth 2

EARTHING ARRANGEMENT 1

EARTHING ARRANGEMENTS & LIGHTENING DISCHARGERS

· This Handbook covers Installations and Maintenance of Earthing Arrangements & Lightening Dischargers for Signalling and Telecommunication installations.

· Part ‘A’ of this Handbook covers Installation and Maintenance of earths for S&T equipments. This is also applicable to earthing provided in Railway Electrified areas, for earthing of cable screens and of the equipment in VF Repeater stations and Cable huts.

· Part ‘B’ of this Handbook covers Installation and Maintenance of Lightening Dischargers for S&T equipments as per RDSO Specification No.TC5-87 and IRS: S52-76.

CAMTECH/S&T July 1999


PART ‘A’: EARTHING ARRANGEMENTS

1.0 Introduction

It is an arrangement for electrical connection to the general mass of the earth.

1.1 Why Earth is required.?

The objective of the earth may be one or more of the following :

· To provide a return path. For example in block instruments, unbalanced HF serial circuits etc.

· To afford safety to personnel against shock by earthing the casing or other exposed path.

· To protect equipment against build up of unduly high voltages by earthing protective devices like surge, arrestors and lightening dischargers.

· To ensure safe and reliable operation of equipment by eleminating / limiting induced voltages as earthing of metallic sheathing and armouring of cables.

· To provide path for heavy fault currents to ensure effective and quick operation of protective devices, as in power supply induced systems.



An simple Earthing Aarrangement system is shown in figure No.1. It shows how earth is connected to the equipment. Earthing Arrangement is consists of following:

· Soil· Earth Electrode· Earthing Lead · Connecting wire to extend earth to equipment.

Figure 1


Battery Charger

Earth Electrode

Earthing

Lead

EarthPit

Equipment earthed

Termination Box

·

Connecting wire

Figure 1

xxx, 01/03/-1,


2.0 TERMINOLOGY

2.1 Earth

The conductive mass of the earth, whose electric potential at any point is conventionally taken is zero.

2.2 Earthing

It is an arrangement for electrical connection to the general mass of the earth.

2.3 Earth Electrode

A conductor or group of conductors in intimate contact with and providing an electrical connection to earth.

2.4 Earthing Lead

A protective conductor connecting the main earthing terminal to an earth electrode or to other means of earthing.

2.5 Earth Leakage Current

A current which flows to earth or to extraneous conductive parts in a circuit which is electrically sound.

2.6 Earth Resistance



The resistance of an earth electrode to earth is called Earth resistance. 3.0 USE OF EARTH IN S&T

3.1 Earthing of Signalling Equipment

Earthing of following signalling equipments is essential :

1. Signal Posts2. He lever frams and other metallic parts of the cabin

in contact with the lever frame.3. Metallic sheath and armouring of underground

Cable.4. Block instruments working on earth return circuits

through the respective Block filters.

3.2 Earthing of Telecom Equipment

It is necessary to earth all telecom. Equipments inclusive of transmitters, receivers and associated equipments, Sheath of Telecom cable for the following reason.

1. To prevent or to reduce the risk of cross talk.2. To complete earth return signalling circuits.3. To avoid risk of shock.4. To provide direct connection to the earth for

lightening protection.



4.0 TYPES OF EARTH

Two types of the Earth is used in S&T department of Indian Railways.

4.1 Protective Earth

Protective earth is required for protection of equipments and to avoid the risk of shock. In this the flowing current may be in the order of tens of amperes.

The basic objectives of Protective/ equipment earthing are:

1. To ensure freedom from dangerous electric shock voltages exposure to persons in the area.

When there is an un-intentional contact between an energized electric conductor and the metal frame or structure that encloses it, the frame or structure tends to become energized to the same voltage level as exists on the energized conductor. It will be dangerous to the persons who will touch this frame or structure as this current will pass to ground through his body.

To avoid this appearance of this dangerous, exposed shock hazard voltage, the equipment must be earthed. The equipment earthing conductor shall offer a low impedance path from the stricken frame



to the ground therefore the dangerous exposed shock hazard voltage shall disappear.

2. To provide current carrying capability, both in magnitude and duration, adequate to accept the ground fault current permitted by the over current protective system without creating a fire or explosive hazard to building or contents.

The earthing conductor must also function to conduct the full ground fault current without excessively raising the temperature of the earthing conductor or causing the expulsion of arcs and sparks that could initiate a fire or explosion.

3. To contribute to better performance of the electrical system.

4.2 Functional Earth

Functional Earth is required for following purposes:a. To complete the circuits of S&T systems

employing on earth return principals.b. To earth the power supply circuit and stabilize the

potential of the equipment with respect to earth.c. To earth screening conductors to reduce electrical

interference to the telecommunication circuits.In this case the flowing current is in the order of few milli amperes. If equipment requires both a protective earth and a functional earth connection, it is preferred that the two earths should be separated within the equipment so that power



system fault currents can not flow in the functional earthing conductors.

5.0 INSTALLATION

5.1 Location of Earth

The following are the preferred locations for efficient earth:

i) Wet marshy grounds or grounds containing vegetation or refuge, such as cinder, ashes and brine waste.

ii) Clay, loamy soil, arable land, clayey soil or loam mixed with small quantities of sand.

iii) Clay and loam mixed with various proportions of sand, gravel and stone.

iv) Damp and wet send pits.

A site should be so chosen that is not naturally well drained. A water-logged situation is not, however, essential, unless the soil is sand or gravel. In general no advantage results from an increase in moisture content above about 15 to 20 percent. Care should be taken to avoid a site kept moist by water flowing over it (for example, the bed of stream) as the beneficial salts may be entirely removed from the soil in such situations.

5.2 Soil Resistivity

Soil resistivity is depends upon the moisture content, chemical composition of the soil and concentration of salts dissolved in the contained



moisture. Size of grain, mode of distribution and closeness of packing also affect the resistivity as these factors control the manner in which the moisture is held in soil. Many of these factors vary locally and some seasonally, and as such soil resistivity varies not only from location to location but also from season to season. Besides, the areas where the soil is stratified, the effective resistivity also depends upon the underlying geological formation.

Temperature also affects the resistivity of the soil. However, it is of consequence only around and below the freezing point, which means that earth electrodes should be installed at depths where frost can not penetrate.

5.3 Treatment of Soil

Multiple rods in large number may sometimes fail to produce an adequately low resistance to earth. This condition arises in installations involving soils of high resistivity. To reduce the resistivity of soil, it is necessary to dissolve in the moisture normally contained in the soil some substance which is highly conductive in its water solution. The most commonly used substances are sodium chloride (common salt), calcium chloride, sodium carbonate, copper sulphate, salt & soft coke and salt & charcoal in suitable proportions.

In the case of salt & soft coke and salt & charcoal moisture the earth electrode should be surrounded in the earth pit by alternate layers of finally divided



coke, crushed coal or charcoal and common salt for at least 150mm all round. Though substantial reduction in earth resistance can be achieved by coke treated electrode, this method results in rapid corrosion of not only of electrode but also the associate bonding. Coke treatment shall be used when absolutely necessary and the coke treated electrodes shall not be situated within 6 metres of other metal structure.

With average and high moisture content, the above mentioned agents from a conducting electrolyte throughout the wide region surrounding the earth electrode. Approximately 900 of the resistance between a driven rod and earth lies within a radius of about two metres from the rod. This should be kept in mind when applying the agents for artificial treatment of the soil. The simplest application is by excavating a shallow basin around the top of the rod, one metre in diameter and about 30cm deep and applying the artificial agent in this basin. The basin should be subsequently filled several times with water which should be allowed each time to soak into the ground, thus carrying the artificial treatment in electrolyte from, to considerable depths and allowing the artificial agent to become diffused throughout the greater part of the effective cylinder of earth surrounding the driven rod.

5.4 Earth Electrodes

Although the earth electrodes material does not affect the initial earth resistance, care should be



taken to select a material which is resistant to corrosion in the type of soil in which it will be used. Under ordinary conditions of soil, use of galvanised iron or mild steel electrode is used. In cases where soil corrosion is likely to be excessive, it is preferable to use either copper or copper clad electrode. The electrodes shall be free from paint, enamel or grease.

Resistance of a electrode to earth depends to a larger degree upon its buried length and to a lesser extent upon its diameter. Therefore the electrode is chosen of such a diameter as can easily withstand the strain of driving.

5.5 Earthing Leads

Earthing lead shall be protected against mechanical damage and possibility of corrosion particularly at the point of connection of earth electrode.

The earthig lead should be of ACSR size 64 sq.mm.(19 strands of 2.11 mm dia) or copper wire of 29 sq.mm. cross sectional area (19 strands of 1.4 mm dia). In case the earth lead is buried underground, it should be protected from corrosion by an application of suitable anti-corrosive paint or bitumin or varnish.The length of the lead so treated should extend half a metre beyond the buried length. In general the earthing lead should be of adequate size to offer negligible resistance.



5.6 Preparation of Earth

a. Prepare a earth pit at the place as decribed in para 5.1. The size of the pit should be shown in figure 2.

b. Put earth electrode in the middle of the earth pit. The size of electrode should be as shown in figure 2 and as described in para 5.4.

c. Fill earth pit by alternative layers of finely divided coke, crushed coal or charcoal and common salt for at least 150 mm all round as described in para 5.3 and shown in figure 2.

d. Fill the remain earth pit by earth as shown in

figure(2).

e. The earth lead of the adequate size as described in para 5.5 shall be fastened to the bolt by means of nut & washer and then soldered as shown in figure2.

f. The walls of the pit surrounding the open pipe

shall be plastered and then filled with sand as shown in figure 2.

g. The surrounding of earth electrode should be kept moist by periodically pouring saline water through the pipe in order to keep resistance below specified value.



h. Where more than one earth is to be installed, they should be seperated by not less than 2000mm from each other.


Figure 2 : Drg. No. RDSO/TCA565

Note: All dimensions in mm

TC 566

Part No.DescriptionReqd.TC566Bottom TubeOneABrick MasonaryAs Reqd.BGI Pipe, Dia-20 BORE IS: 1239 As Reqd.CBolt,MS HEX,HD.M10x50 longOneDWasher(punched) MS, M 10 IS:2016TwoENut,MS,HEX, M10OneFGI Reducer Socket , Dia 40 to 20 OneGSand

F

G

B

A

1500

1000 TO2000

Alternate layersof Charcoal orCoke and Salt

EARTH

300

200 SQ 100 100 Ground Level

O



75

75

150

450

HolesDia 12

Parallel Pipe Threads

1500

75

25

48

40

BOOTOM TUBE

Note: All dimensions are in mm



Figure 3 : Drg.No.RDSO/TCA566Ground Level

2200

RDSO/ TC 21132

Alternative Layers of Charcoal or Coke and Salt

1300

300 x1200



900

500200

200

400

400

1400

H.F.Earthing Arrangement ( Earth Plate) RDSO/Drg. No. TC 21131

Figure 5

Drill Holes

Note: All dimensions are in mm.


Note :

1. Figure 2 shows Earth for Telecommunication, Block and Railway signalling to RDSO drawing No.RDSO/ TCA565.

2. Figure 3 shows an MS tube drawing No.RDSO/TCA 566 is used as a bottom tube in the earth for Telecom., Block and Railway signalling.

3. Figure 4 shows Radio frequency earths for wireless transmitting and receiving stations to RDSO drawing No.RDSO/TC 21131.

4. Figure 5 shows a galvanized steel plate to drawing No. RDSO/TC 21132 used in radio Frequency earth for wireless transmitting and receiving stations.



6.0 EARTH RESISTANCE

The total earth resistance is the sum of three separate resistances, viz,

1. Resistance of conductor jointing the earth electrode to the installation.

2. Contact resistance between the earth electrode and soil.

3. Soil resistance

Since the first two resistances are negligible so the earth resistance is determined by the nature of soil.

6.1 Measurement of Earth resistance


Earth to be testedA

B C

Ground

6m

6m

6m


Earth can be tested by means of a Wheatstone Bridge or a GPO Detector or a Megger Earth Tester. To test an earth, two iron bars with terminals fixed on them, are driven about 6 metre from the earth to be tested and 6 meter from each other as shown in figure 6:

The bars are used as temporary Earths and driven in the ground for 1 metre, 0.25 metre. Now pour salted water to ensure that the bars make a good connection with the earth.

In the above diagram A is the earth under test, B&C are temporary earths. Measure the resistances between A&B, A&C and B&C with the help of Wheatstone bridge or GPO detector or Megger earth tester.

Let, R1 is the resistance measured between A&B R2 is the resistance measured between A&C R3 is the resistance measured between B&C

Therefore the resistance of A can be findout by following formula:

Resistance of A = R1 + R2 - R3 earth. 2


Iron Bars

Figure 6


6.2 Limits of Earth Resistance

Maximum values of earth resistances specified for earthing of Signalling and Telecommunication equipments are as under:

Sr No.

Descriptions Earth Resistance

1 Telegraph and Block Instrument using earth return circuit

Should not be more than 10 W

2 Earths for surge arrestors/ lightening dischargers


3 Earthing of Signalling equipment


4 Earthing of signalling cable screen in AC electrified areas


5 Earthing of Telephone Exchange Should not be more than 5 W

6 Earthing of alluminium sheathed telecom cable in AC electrified area.


7 Earthing of equipment in VF repeater stations and cable huts.




7.0 MAINTENANCE

· Earths should be watered regularly.

· All earths and connections should be examined at the interval of not more than one month, to ensure that all connections are in tact and soldered joints are in proper condition.

· Resistance of every earth should be measured at intervals not exceeding one year. Earth resistance and date of last test should be entered in a register location-wise. Earth resistance and date of last testing should also be painted suitably on the wall of a nearby structure or post on a conveniently place sign board.

7.1 Token & Block Earth (SEM.944)

a. Token & Block earth and their connections must be examined at intervals of not more than one month.

b. Token & Block earths must be tested for resistance at intervals of not more than 12 month and where the resistance exceed 10 ohm, action should be taken to reduce the earth resistance.



PART B: LIGHTNING DISCHARGER

1.0 Introduction

The equipment which discharge the lightning is called Lightning Discharger (LD). It is a protection device.

The S&T equipments are to be protected from higher voltages which may arises either due to (i) Natural cause or (ii) Artificial cause.

1.1 Natural Causes

Amongst natural causes the main and the only cause is the lightning. The theory of lightning is somewhat complicated, but for practical purposes it may be considered as extremely high voltage discharge in an extremely short period. Typical figures are 1000KV in 1 microsecond.

1.2 Artificial Causes

· Direct contact between power line and S&T line.

· Parrallelism between power and telephone lines.

· Partial earthing.



2.0 Requirements

The main requirements of lightning dischargers are given below:

1. LDs should not operate for working currents.2. Speech or Signalling efficiency should not reduced

on its application.3. They should promptly operate at specified voltages

or currents.4. They should promptly isolate the apparatus and

prevent further action by lightning etc.5. The current rating should be always such that they

do not produce any heat in the componants of the main apparatus.

6. They should be as far as possible be self-restoring.7. They should not be costly.






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