2. Applications of earthing • Protect human against lightning and earth fault condition • Protect the premises against lightning and earth fault condition • Provide low resistance and safe path for lightning and fault current • All metallic enclosure and extraneous conductive parts are at equipotential • LV System Earth
3. Functions of Earthing Equipment Earth : Path for fault current, lower touch voltage, protection against electric shock Lighting Earth : Low resistance path to diverse the current under lightning attack. Telecom Earth : Signal Earth, reduce noise and interference, stabilize DC supply voltage and prevent electric shock Computer Earth : reduce interference, maintain supply voltages
4. Two classes of protection Class I protection – use of barrier/insulation and connection of protective conductor to equipment metallic enclosure in order to protect against electric shock Class II protection – beside of the basic insulation, addition layer of insulation apply to the enclosure. Therefore no extraneous conductive part. The additional layer is independent to the basic insulation so that under failure of basic insulation, it offers additional protection
5. Types of Earthing Supply System – Neutral Earth System Earth Electrical Safety Earth Lightning Earth Generator Earth Protection Earth (i.e. surge arrestor) Telecom / Computer Earth Shielding Earth Integrated Earthing System (Advocated) Electrostatic Earth (Clean Room / Hospital)
12. Factors affect to the earth impedance Soil Weather Electrode type Electrode size Near by utilities Electrode in parallel Distance between electrode
13. Soil Resistivity The resistivity of earth may vary over extremely wide limits, depending on the composition of the soil and the moisture content. Factors that affect resistivity Type of earth (eg, clay, loam, sandstone, granite) Stratification; layers of different types of soil (eg, loam backfill on a clay base) Moisture content Temperature Chemical composition and concentration of dissolved salt
16. Types Of Earth Electrodes • Solid Copper • Copper clad steel rod ( copper shrunk onto the core) • Copper Bonded steel core (coper is molecularly bonded to nickel plated steel rod)
17. Earth Resistance Of An Electrode soil exhibits a resistance to the flow an electrical current not an “ideal” conductor resistance (can never be zero) between the earth electrode and “true Earth”. The resistance between the earth electrode and “true Earth”
18. Rods Driven Vertically Into The Ground Rg = (𝞺/2𝞹L)[ln(8L/d)-1] where, ρ - Soil Resistivity in Ωm L - Buried Length of the electrode in m d - Diameter of the electrode in m
19. Combined Resistance Of n No Of Electrodes In which Where R=resistance of one rod Ω S = distance between adjacent rods m ρ = resistivity of soil Ω-m λ =is a factor selected from Table 2 or 3 of BS 7430 ‘n is the no of electrodes as given in Tables 2 and3
20. Resistance Of A Vertical Electrode With Infill Of Bentonite Or Concrete Where, 𝞺c – resistivity of the infill material d – diameter of electrode in m D – diameter of infill L – driven length of electrode
21. Where , L – length of the strip ‘h - the depth buried w - width of the strip P and Q are coefficient for strip or round conductor in Table 5 Approximate Resistance for a Strip or a round conductor
‧protective conductor and neutral conductor are independent• All exposed metallic part connected with protectiveconductor PE• System Characteristics:– Low earth fault loop impedance– High earth fault current• Advantages: earth fault protection device operates faster;allow multi point earth, better earthing continuity; minimizethe use of earth fault relay because of low earth fault loopimpedance• Disadvantages: high earth fault level; under earth faultcondition, low power factor (high inductance of long cable)
protective conductor and neutral conductor are combinedinto one• All exposed conductive parts connected to PEN• System Characteristics :– Low earth fault loop impedance– High earth fault current– More than one earth fault loops• Advantages : no earth wire required; allow of multi-pointearth, better earthing continuity; neutral never have floatvoltage; impedance of earth fault loop could be predicted• Disadvantages : If not multi-point earthed, and the neutralearth broken, the exposed metallic part may have floatvoltage; high earth fault level, intervenue the operation ofearth fault protective device, current operated type device isnot appropriated, voltage detected type could be employed
• Supply side has one point earthing only• Exposed metal connected to consumer earth system• System characteristics :– High earth fault loop impedance– Low earth fault current– Utility company need not to provide earth forconsumer• Advantages : Under earth fault condition, higherpower factor; save earth wires• Disadvantages : high demand of E/F relays,individual earth system need higher investment,higher touch voltage; induce Potential gradient
System not connected with the earth (highimpedance earth or isolated earth), all exposedconductive part connect to earth• Neutral is totally isolated• Infinity earth fault loop impedance• Deemed less chance of electric shock• Many disadvantages: phase current equal to livevoltage (because of floating Neutral); no zeroreference, may cause arcing when earth fault occurs;to small of earth fault level, current type earth faultdetector cannot work properly
