Earthing System

EARTHING SYSTEM/PRACTICES IS3043-1987

EARTH • • •

A connection to the general mass of earth by means of an earth electrode. An object is said to be ‘earthed’ when it is electrically connected to an earth electrode. A conductor is said to be ‘solidly earthed’ when it is electrically connected to an earth electrode without intentional addition of resistance or impedance in the earth connection.

GENERAL DEFINATIONS 

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EARTH ELECTRODE: A metal plate, pipe or other conductor or an array of conductors electrically connected to the general mass of earth. EARTHING/GROUNDING RESISTOR: A resistor through which a system is earthed and which serves to limit the current flowing in the event of an earth fault. ARC SUPPRESSION COIL (PETERSEN COIL): An earthing reactor so designed that its reactance is such that the reactive current to earth under fault conditions balances the capacitance current to earth flowing from the line so that the earth current at the fault is limited to practically zero.

GENERAL DEFINATIONS 

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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 the distance of one pace which may be assumed to be 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.

BASIC OBJECTIVES  To

stabilise circuit potential w.r.t. ground & limit overall potential rise.  To protect men & material from injury/damage due to over voltage or touching.  To provide a low impedance path to fault currents to ensure correct operation of protective devices.  To keep max. voltage gradient of surface in substation within safe limits during ground fault.

RESISTANCE TO EARTH a) b) c)

Resistance of the (metal) Electrode. Contact resistance between electrode and the soil. Resistance of the soil from the electrode surface outward in the geometry setup for the flow of current from the electrode to the infinite earth. Soil resistivity is the single factor, which dominates in the arrival of earth resistance of an electrode.

SOIL RESISTIVITY Soil Resistivity depends upon following:  Moisture content of Soil,  Chemical composition & concentration of salts dissolved in the water,  Closeness of packing,  Grain size Nearly 90% of resistance between electrode & soil is within a radius of 2 meters from rod.

EFFECT OF MOISTURE CONTENT ON EARTH RESISTIVITY  Moisture

content is expressed in percent by weight of the dry soil.  Above 20% moisture, the resistivity is very little affected, while below 20% resistivity increases very steeply with decrease in moisture content.  General Range 10% (in dry season) to 35% (in wet seasons), with average of 16% - 18%.  If the water is relatively pure soil may have high resistivity and unless the soil contains sufficient natural elements to form conducting electrolyte.

ARTIFICIAL TREATMENT OF SOIL  NaCl

Common Salt  CaCl2 Calcium Cloride  Na2CO3  CuSO4  Salt

Sodium Carbonate

Copper Sulphate

& Soft coke  Salt & Charcoal  Fly Ash from thermal stations  Bentonite (for rocky terrain)

EARTH ELECTRODES  

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Under ordinary condition soil, use of copper, iron or mild steel electrodes is recommended. In case where soil conditions point to excessive corrosion of the electrode and its connections, it is recommended to use either copper electrode or copper clad electrode or zinc coated (galvanized) iron electrode. In direct current (DC) systems, due to electrolytic action which causes serious corrosion, it is recommended to use only copper electrodes. Electrodes shall be kept free from paint, enamel and grease. Earth electrode & earth conductor must be of same material as to avoid corrosion. Earth electrode should be capable of dissipating without failure, energy in the earth path at the point at which it is installed under any condition of operation on the system.

EARTH ELECTRODES  Rod

& Pipe Electrodes

Minimum Rod Dia. for Steel/GI = 16mm & for Copper = 12.5mm Minimum ID for Pipe Steel/GI = 38mm & for cast iron = 100mm Length of Rod/Pipe electrode must not be less than 2.5 m.

 Strip

or Conductor Electrodes

Strip electrode Minimum size 25mmX4mm Steel/GI & 25mmX1.6mm Copper. Conductor Minimum cross-section Steel/GI = 6 sq. mm & for Cu = 3 sq. mm Length of electrode must not be less than 1.5 m.

 Plate

Electrodes

Plate electrode Minimum size 60mmX60mmX6.3mm Steel/GI & 60mmX60mmX3.15mm Copper. Plate electrode shall be buried such that its top edge is at a depth not less than 1.5 m from surface on the ground.

EARTH RESISTANCE Station

 Large

Power Stations  Major Sub stations  Small Stations

Combined resistance of earths in the station – ohms (maximum) = 0.5 = 1.0 = 2.0

EARTH RESISTANCE MEASUREMENT A Current Source V

A Test Electrode

X=1m B Potential Electrode

C Current Electrode

DIFFERENT EARTHING METHODS Earthing Method

Points Earthed

Purpose of Earthing

Neutral Earthing

1. Transformer Neutral 2. Generator Neutral 3. Star point of Load 4. Neutral of Circuit 5. Start point of CT/PT secondary

1. Holding neutral at ground potential 2. Prevent arcing ground on OH lines 3. Discharge of Voltage surges 4. Path for out of balance current 5. Simpler earth fault protection

Equipment Earthing (Body Earthing)

Metallic non current carrying parts

1. Holding metallic parts at earth potential even on earth fault 2. Safety

Reference Earthing

Floating point in the circuit

Holding the point & the conductor at zero potential

Discharge Earthing

Earth terminal of 1. Earth switch & CT 2. Surge Arrester 3. Capacitor/filter bank

To discharge the surge voltages, Capacitor charge currents to earth.

THANKS FOR