EARTHING AND PROTECTION.ppt

7/28/2019 EARTHING AND PROTECTION.ppt

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EARTHING AND

PROTECTION

EARTHING


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REQUIREMENT OF EARTHING

Earthing arrangement of proper type is

required to protect the equipment and

working personals from high induced

voltage, due to lightning and high

electromagnetic induction.


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INSTALLATIONS TO BE

EARTHED The lever frame and other metallic frames of the cabin.

Metallic sheath and armoring of all under ground Main cables atevery1 Kilo meter.

Block circuits working on earth return through the respective BlockFilters.

The surge arrestors provided in Block filters as well as thoseprovided for Telecom equipments in switching stations

All Telecommunication equipments.

The signals provided on bracketed posts or gantries locatedabove contact wire level, falling with in 2 meters from electrifiedtrack.


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REASONS FOR EARTHING THE

TELECOMMUNICATION

EQUIPMENT

i) To prevent or to reduce the risk of cross

talk.ii)To complete earth return signaling circuits.

iii) To avoid risk of shock.

iv) To provide direct connection to the earthfor lightening protection.


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SOIL RESISTVITY

It is the resistivity offered by soil. It is indicatedby the symbol p (RO). it is a constant and

depends on the length l and area a of theconductor.

l

R = p --- where p = resistivitya


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RESISTIVITY

OR

THE SPECIFIC RESISTANCE The resistivity or the specific resistance is

defined as the resistance between opposite

faces of a conductor of unit length and unit cross

sectional area.

The value of resistivity is given Ohm- metre.

The resistance of the electrode depends on the

resistivity of the soil or Soil resistivity. To get low earth resistance( R ) the Resistvity of

the soil must be low.


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SOIL RESISTIVITY

The soil resistivity depends on

moisture content,

chemical composition of soil concentration of salts dissolved in the

contained moisture.

These factors vary locally and seasonally.


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VALUES OF EARTH RESISTACE

TO BE MAINTAINED

The resistance of the earth shall not

exceed maximum 10

In case of Telecommunication it is 5.

In case of Microwave it is less than1.


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TOTAL EARTHING RESISTANCE

IS THE SUM OF

The resistance of earth lead wires

contact resistance between the surfaceof the earth electrode and the soil

the resistance of the body of the soilsurrounding the earth electrode

Among the three, the resistance of theearth is primarily determined by thenature of the soil.


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EARTHING ARRANGEMENT

A single earthing

system consists of,

Soil.

Earth lead wire.

Earth electrode.

Connecting wire to

extend earth toequipment.


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SOIL

The soil is to be prepared to obtain optimumresistvity.

The site for earthing should be chosen in thefollowing order:

1. Wet marshy ground and grounds containingrefuse such as ashes, cinders.

2. Clay soil or loam mixed with small quantity ofsand.

3. Clay and loam mixed with varying properties ofsand, gravel and stone.

4. Damp and wet sand peak.

LIGHTENING CONDUCTOR


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LIGHTENING CONDUCTOR

OR

EARTH LEAD WIREIt is the metallic wire which connects the earthelectrode to the equipment / protector/ Installation.Size and metal of the conductor is given below,

Material Size G.I. Wire 8 mm Dia

G.I Strip 20mm x 3 mm

Copper wire 29 Sq.mm

( 19 strand wireof 1.4 mm dia)

ACSR wire 6 / 1 / 2.11 mm

PRECAUTIONS TO BE TAKEN IN


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PRECAUTIONS TO BE TAKEN IN

CASE OF LEAD IN WIRE

Protect them from mechanical damage

Apply anti-corrosive paint/ bitumen

compound on the portion of wire buried in

ground

The lead in wire of different earth must be

electrically insulated from each other, from

metallic structures etc.


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EARTH ELECTRODE

It may be a metal plate, pipe or other conductor or an array ofconductors electrically connected to the general mass of earth.

Type Size

Length in Mtrs Dia /Cross section

a) G.I. Pipe 2.5 to 3.5 above 38 mm (internal)

b) G.I. Angle 2.5 to 3.5 50 mm x 50 mm x 5mm

c) Copper Rod 2.5 to 3.5 16 mm


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PROCEDURE OF INSTALLATION

The hole can be made by manual

trenching or by using Earth auger.

The top of the electrode shall be 30 cm

above the ground.

After inserting the electrode, the hole shall

be filled with earth properly and water

should be spread to ensure good contact

between electrode and filling.


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The hole can be made bymanual trenching or byusing Earth auger.

The top of the electrode

shall be 30 cm above theground.

After inserting theelectrode, the hole shallbe filled with earth

properly and water shouldbe spread to ensure goodcontact betweenelectrode and filling.


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In the soils of high rersistivity, can be treated with saltand charcoal in appropriate proportion.

Earth pit of 600 mm dia and 2.5 mtrs depth shall beformed by excavation and the electrode shall be placed

at the center. The pit shall be filled alternately with layers of common

salt and charcoal each layer of about 2.5 cm thick up toa depth of about 200 cm from the ground level.

The pit shall be filled with several times with water, andthen covered with excavated earth and water shall besprayed to ensure good electrical contact.


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A brick wall of 400 mm height below the groundlevel shall be constructed in rectangular fashionas shown in the figure and walls shall beplastered and then filled with sand.

The surroundings of the earth electrode shouldbe kept moist by periodically pouring waterthrough the pipe in order to keep the resistancebelow specified value.

Coke treated electrodes shall not be situatedwithin 6 meters of other metal structure.


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LIMITS OF EARTH RESISTANCE

Type of earth Max. value

Telegraph and Block instrument return 10 Circuit earths

Earth for surge arrestors/ lightning

Dischargers for S&T equipment 10 Signalling equipment earths 10 Signalling cable screen earths in

A.C. electrified areas 10 Telephone exchange earth 5 Aluminium sheathed Telecom cable

screen earths in A.C. electrified area 1

Equipment earth in V.F. Repeater stations

and cable Huts 5 M.W. Station tower earth 1


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SINGLE EARTH SYSTEM

The Telecom installations shall use single earth

system in which the different earth connections

from equipments, towers, D.C. power supply,

metallic structures etc. shall be interconnected toeach other through low resistance earthing

conductors.

This method is recommended to keep all the

points to be earthed at approximately samepotential level.


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SEPARATION BETWEEN MAINS

EARTH AND TELECOM EARTH

The protective earth of Telecom system

shall not be connected to the earth of

mains power supply system.

A minimum distance of 10 Meters is

desirable..


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EARTH RESISTANCE

MEASURMENT Special instruments such as Megger earth testers

are recommended for making tests to avoid theeffect of back e.m.f. and stray currents.

While making the earth resistance measurements

two electrodes called as Auxiliary electrodes onenamed as current electrode(C) and the otherPotential electrode(P) are required to be installed inaddition to the original electrode ie. Earthelectrode(E). The two auxiliary electrodes to be

installed as given below, Earth resistance shall be measured once in a year by

suitable measuring instruments.


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EARTH RESISTANCE

MEASURMENT

The current electrode C must be

sufficiently for away from the Earth

electrode (E) under test about 30 to 40

Meters from E. The Potential electrode must be placed in

between E and C 15 to 20 Meters

distance taking exactly half of the distance

between E and C. All the three must be insame straight line.

METHOD OF EARTH


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METHOD OF EARTH

RESISTANCE MEASURMENT


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PROCEDURE OF TESTING

Follow the given procedure to get correct value of earth resistance.

1) Mark the two points named as P and C in a straight line with E,positioning at a distance of 20 Meters from E and 40 Meters fromE respectively as shown in the above figure. Drive the electrodesinto the ground up to 1 Meter. They are M.S. rods of 12.5mmdiameter.

2) Link C1 and P1 terminals of earth tester.

3) Connect E to C1 and P1 terminals, P to P2 terminal and C to C2terminal.

4) Now rotate the handle of the megger and adjust the resistance

knobs to bring the needle to null point. 5) The range switch to be kept on the required position, beforeadjusting the resistance knobs.

6) Multiply the reading of the resistance knobs with the rangeselected to get the value of earth resistance in Ohms.


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MAINTENANCE OF EARTHS

1) Check earth and its connections periodically at interval of not morethan one month, to ensure that all connections are in tact andsoldered joints are in proper condition.

2) Measure the earth resistance once in a year. Enter the value, date oflast test and location of earth should be entered in a register.

3) Earth resistance, date of last testing should also be printed suitablyon the wall of near by structure or post on a conveniently placedsign board.

4) Water to be added every day to the earth electrode in summer andonce in two days in other seasons.

5) If earth resistance is more than the nominal value either renew theold earth or provide a new earth.


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PROTECTION ARRANGEMENTS ACROSS A.C

MAINS SUPPLY TO TELECOM INSTALLATIONS

Low voltage lightningdischargers of normalrating 650V shall beprovided across the 230V mains power supply as

shown in the figure. In the case of high

tension supply (11 KV orabove) are terminatednear telecom installation

and suitable polemounted high voltagearrester shall beprovided.


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DIFFERENT TYPES OF

PROTECTIVE ARRANGEMENTSProtection arrangement shall be made for undergroundcable conductors, by providing gas discharge (GD) tubes& MOVR, at the following places.

1) Protection at subscriber premises.

2) Metallic sheath or armour of the cable shall beearthed and the sheath should connected to the body ofthe metal box.

3) At transmission/ switching end the metallic sheathshall be earthed and protective devices shall be providedfor each pair.

4) At transmission point between overhead lines andunderground cables are protected with GD tubes&MOVR if the distance exceeds 500 meters from thecable termination box.


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SURGE PROTECTION

Now a days all most all equipments used

in S&T department includes the electronic

devices which operates on low voltages.

To protect these devices from transient

over voltages produced due to lightning,

switching of inductive loads, ignition and

interruption of electronic arcs etc., suitablesurge protection arrangement to be done.


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SURGE PROTECTION

The low voltage equipments eg. UPS, Battery

charger, Inverter, control systems, etc are

provided with surge components like MOVRs,

avalanche diodes, Gas discharge tubes etc.inside the equipments.

The internally used Surge protection

Components prove to be inadequate towards

the surge protection solution. Hence it isrequired to take appropriate solution at different

levels.


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LIGHTNING PROTECTION

LEVELS

Class A protection:- Protection against thelightning on the structure housing theequipment.

This is provided with an external lightningconductor on top of the building connectedthrough a down conductor to ground (EARTH).This is known as class A protection.

By this arrangement 50% of lightning energyconnected to ground. Depending on the area,size of the structure to be protected, the type ofprotection varies.


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CLASS B PROTECTION

The first stage of protection provided before theequipment at mains distribution panel is called classB type.

These are Spark gap type operates on arc choppingprinciple and designed to handle lightning current

pulses of 10/350us. The sealed type should be used near places having

explosive gasses like gasoline.

Maximum operating voltage 1.1 times nominal supplyvoltage.

The voltage protection/clamping should be as lessthan 1.5KV.Its response time should be less than 200nano seconds.


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CLASS C PROTECTION

An Metal Oxide Varistors (MOV) is provided betweenPhase and Neutral and has a surge rating of 50KA8/20 us. This should be a single compact device ofproper rating and in no case a number of varistorsshould be provided in parallel. This device should have

following additional features: i) Indication to show health of the device which should

turn red when failed

ii) Thermal disconnection to the arrester when it startshaving heavy leakage current due to ageing after

handling repeated surges. iii) Potential free contact for remote monitoring.

Provision of one set of Class B and C devices ismust. Its normal arrangement is shown in the figure.


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CLASS D PROTECTION

It consists of a combination of MOVs and GDtube. This should have all the features asmentioned above for Class C device.

All external data/signaling (ac/dc) linesconnected to electronic equipment should beprotected by this arrangement.

One exception where Class D device should

not be provided is the cable conductorscarrying signal lighting fed as leakage in MOVsDue to ageing will have adverse effect onworking of Lamp Proving Relays (ECRs).


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PROTECTION ARRANGEMENTS


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SURGE PROTECTION DEVICES

(SPDS)

Electronic equipment can be protected from the

potentially destructive effects of high-voltage

transients.

Protective devices, known by avariety of names(including lightning barriers, surge arrestors ,

lightning protection units, etc.) are available.

The correct name (accepted internationally) is

surge protection devices or SPDs.


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FUNCTION OFSPD

Surge protection devices should ideally operate

instantaneously to divert a surge current to

ground with no residual common-mode

voltage presented at the equipment terminals. Once the surge current has subsided, the SPD

should automatically restore normal operation

and reset to a state ready to receive the next

surge.


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SPD DEVICES

They are,

1. Gas discharge tubes (GDTs),

2. Voltage-clamping diodes,

3. Metal-oxide varistors (MOVs) Feature are,

1. Rapid operation,

2. Accurate voltage control and

3. Automatic resetting once the overvoltage hasceased.


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LIGHTNING

Figure shows the

wind, temperature and

ice/water distribution

in a thundercloud.The violent up

draughts and

downdraughts in the

cloud centre generate

static charges.


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MAGNITUDE OF LIGHTNING

VOLTAGE

The voltage is estimated to be of the order

of 107 to 108 volts, i.e. 10 million to 100

million volts. The intense field which is

generated between the charge centerscauses ionisation of air molecules to take

place and a conducting channel is opened

which permits charge neutralisation tooccur i.e.,lighting stroke.


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SURGE CURRENT PULSE

In order to be able totest protectionnetworks and providecomparison

data, variousstandard waveformshave been proposedover the years.

These are all definedin terms of magnitudeand waveshape.


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SURGE CUTTENT PULSE

It can refer to current (normally associated withshort-circuit conditions) or voltage (associatedwith open-circuit faults). Waveforms are usuallydouble exponential rise and decay shapes,

specified by two time periods; rise-time to peakvalue and decay-time to 50% peak value.

IEC 60060-2 describes a particular method ofdetermining slopes, etc. and is more relevant formanufacturers of test apparatus.

Instrumentation protection circuits are almostalways specified in relation to 8/20s currentpulses of typically 3kA magnitude.


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2 ELEMENT GAS DISCHARGE

TUBES CONNECTION Gas discharge tubes (GDTs)

seek to overcome some of thedisadvantages of air or carbonspark gaps by hermeticsealing, thereby eliminatingenvironmental effects. Gasfilling enables spark dischargeconditions to be quiterigorously controlled since the

breakdown voltage of such adevice is related to gas

pressure and electrodeseparation for a particular setof materials.


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GAS DISCHARGE TUBES

Typically, low voltage protection devices

have electrode spacing of 1mm or so in an

argon/hydrogen mixture sealed within a

ceramic envelope at about 0.1 bar.

3 G


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3 ELEMENT GD TUBE

CONNECTION

EARTHING AND PROTECTION.ppt

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