Methods of Earth Resistance Testing (Part-1)


Introduction:

  • The measurement of ground / Earth resistance for an earth electrode is very important for not only for human safety but also for preventing damages of equipment, industrial plants and to reduce system downtime.
  • It also provides protection against natural phenomenon such as lightning stock by providing path to the lightning current to the ground.
  • Ground resistance is the measurement of the resistance between conducting connection and earth Soil.
  • Earth Resistance should be Low as possible to provide low resistance path to leakage current to the earth.
  • Ground resistance depends on grounding electrode selection, soil resistivity, soil contact, and other factors

 Difference between Ground Resistance and Ground Resistivity

  •  Ground / Earth Resistance:
  • Ground Resistance is the resistance (Which oppose of current flow) of an installed earthing electrode system.
  • It is the resistance between a buried electrode and the surrounding soil.
  • It is measured in
  • Ground Resistance is measured with a four-point, three-point or clamp on tester.
  • Ground / Earth Resistivity:
  • Ground resistivity is a measurement of how much the soil resists the flow of electricity.
  • Ground resistivity is the electrical properties of the soil for conducting current.
  • It indicates how good the soil /Earth conducts electric currents. For the lower the resistivity, the lower the earth electrode resistance at that location.
  • Ground resistivity is theoretical resistance of a cylinder of earth Piece having a cross-section area of 1 Sq. meter.
  • Ground resistivity (ρ)is measured in Ohm centimeters.
  • Ground resistivity has nothing to deal with any installed electrical structure, but is a pure measurement of the electrical conductivity of the soil itself.
  • Ground resistivity is measured with a four-point tester.
  • Ground resistivity varies significantly according to the region, season and the type of soil because it depends on the level of humidity and the temperature (frost or drought increase it).

Purpose of Measurement of Earth Resistivity:

  • Earth resistivity measurements have a Main three purpose.
  • Earth resistivity data is used to use survey for Surface of Land to identifying locations, depth to bedrock and other geological phe­nomena.
  • Earth resistivity data is used for protective anticorrosion treatment of underground pipelines, because Earth resistivity is direct related on the degree of corro­sion of underground pipelines. Lower in resistivity increase in corrosion of Underground Pipes.
  • Earth resistivity directly affects the design of an Earthing system. When we design an Earthing system, it is advisable to locate the area of lowest soil resistivity to achieve the most economical grounding installation. If the lower the soil resistivity value, the lower the grounding electrode resistance.

Earth Resistivity depends on:

  • There are various that affect the ground resistance of a ground system

(1)  Diameter of Ground Rod:

  • Increasing the diameter of the ground electrode has very little effect in lowering the resistance.
  • Doubling diameter of ground rod reduces resistance only 10%.
  • Using larger diameter ground rods is mainly a strength issue. In rocky conditions, a larger diameter ground rod might be advantageous.

(2) Depth of Ground Rod:

  • As per NEC code minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with the soil.
  • Doubling depth of Rod theoretically reduces resistance 40%.
  • Earthing Spike (electrodes) deeper is a very effective way to lower Earthing resistance.
  • Actual reduction of resistance depends on soil resistivity encountered in multi-layered soils.
  • The resistance decreases rapidly as the length of the electrode increases and less rapidly as the diameter increases.

(3) Spacing of Ground Rod:

  • Earth resistance decrease when distance between adjustments earthing Rod is twice the length of the rod in Ground (in good soil).

t

Probe Spacing
Probe distance (m) Soil resistance, Re (Ω) Soil resistivity, ρρ (Ω m)
0.3 14.75 27.79
0.6 7.93 29.88
0.9 6.37 36.00
1.2 4.36 32.86
1.5 4.31 40.60

(4) No of Ground Rods:

  • Using multiple ground electrodes provides another way to lower ground resistance.
  • More than one electrode is driven into the ground and connected in parallel to lower the resistance.
  • The spacing of additional rods must be at least equal to the depth of the driven rod.
  • Two well-spaced rods driven into the earth provide parallel paths and act as two resistances in parallel. However the rule for two resistances in parallel does not apply exactly so the resultant resistance is not one-half the individual rod resistances.
  • The reduction in Earth resistance for equal resistance rods is
  • 40 % for 2 rods
  • 60 % for 3 rods
  • 66 % for 4 rods

(5) Material & Surface Condition of Ground Rod:

  • Grounding electrodes are usually made of a very conductive metal (stainless steel, copper or copper clad) with adequate cross sections so that the overall resistance is negligible.
  • The resistance between the electrode and the surrounding earth is eligible if the electrode is not free of paint, grease, or other coating, and not firmly packed with earth.
  • If the electrode is free from paint or grease, and the earth is packed firmly, contact resistance is negligible.
  • Rust on an iron electrode has little or no effect .But if an iron pipe has rusted through, the part below the break is not effective as a part of the earth electrode

(6) Moisture

  • Low-resistivity soils are highly influenced by the presence of moisture.
  • The amount of moisture and salt content of soil affects its resistivity.
  • Actually, pure water has an infinitely high resistivity. Naturally occurring salts in the earth, dissolved in water, lower the resistivity. Only a small amount of salt can reduce earth resistivity quite a bit.

(7) Temperature

  • Increase in temperature will decrease resistivity
  • Increase in temperature markedly decreases the resistivity of water.
  • When water in the soil freezes, the resistivity jumps appreciably; ice has a high resistivity. The resistivity continues to increase a temperatures go below freezing.

(8) Soil type

  • Some soils such as sandy soils have high resistivity that conventional ground.
  • Frozen and very dry soils are good insulators and have high resistivity.
  • In low resistivity soils, the corrosion rate is often greater than in high resistivity soils
  • The resistivity is much lower below the subsoil water level than above it. In frozen soil, as in a surface layer in winter, it is particularly high.

(9) Choosing Proper Instrument:

  • Use a dedicated ground tester for measuring earth resistance.
  • Do not use a generalized ohmmeter, multi meter or Megger for that.
Soil Resistivity (approximate ohm-meters)
Soil Description Minimum Median Maximum
Topsoil, loam 1 26 50
Inorganic clays of high plasticity 10 33 55
Fills – ashes, cinders, brine wastes 6 38 70
Gravelly clays, sandy clays, silty clays, lean clays 25 43 60
Slates, shale 10 55 100
Silty or clayey fine sands with slight plasticity 30 55 80
Clayey sands, poorly graded sand-clay mixtures 50 125 200
Fine sandy or silty clays, lean clays 80 190 300
Decomposed gneisses 50 275 500
Silty sands, poorly graded sand-silt mixtures 100 300 500
Clayey gravel, poorly graded gravel, sand-clay mixture 200 300 400
Well graded gravel, gravel-sand mixtures 600 800 1000
Granites, basalts, etc. 1000
Sandstone 20 1010 2000
Poorly graded gravel, gravel-sand mixtures 1000 1750 2500
Gravel, sand, stones, little clay or loam 590 2585 4580
Surface limestone 100 5050 10000

  

Soil Resistivity Ranges
1000 Ohm cm Wet organic soil
10000 Ohm cm Moist soil
100000  Ohm cm Dry soil
1000000 Ohm cm Bed rock
590 to 7000 Ohm cm Ashes, cinders, brine, waste
340 to 1630 Ohm cm Clay, Shale, Loam
59000 to 458000 Ohm cm Gravel , Sand , Stone with little Clay
300 to 500 Ohm meter Concrete
900 to 1100 Ohm meter Granite
20 to 2000 Ohm meter Sand Stone
100 – 15,000 Ohm cm Standard Design OK
15,000- 25,000 Ohm cm Standard Design Maybe
25,000 – 50,000 Ohm cm Special – Contact the carrier, owner or engineering

firm

50,000 + Ohm cm Very Special – Perhaps not practical

 

Ground Resistance Values
Industrial plant: 5 Ω
Chemical plant: 3 Ω
Computer System 3 Ω
Lighting Protection 1 Ω
Generating station: 1 Ω
Large HV sub-station, Generating Station (IEEE Std 142 clause 4.1.2) 1 Ω
Small Distribution sub-station (IEEE Std 142 clause 4.1.2) 5 Ω
Telecommunication facilities <5Ω
Water pipe ground should <3Ω

Method for Installation of Earthing Strip


(A) Purpose:

  • The method is to explain the procedure, which should be followed to install the Earthing Strip, Earthing Wire, and Earthing accessories as per the specification to achieve the standard requirements of the project.

(B) Equipment & Tools:

  • The equipment that will be used for Installation of Earthing Strip / Wire works are
  1. Ladder
  2. Spirit Level
  3. Drilling Machine
  4. Grinding Machine
  5. Cutting Machine
  6. Power tools
  7. Measure Tape
  8. Screwdriver
  9. Drill with bits
  10. File
  11. Galvanizing paint
  12. Bitumius Paint

(C) Test for Earthing Strip / Earthing Accessories:

  • Visual inspection:
  • Type of Earthing Strip and Accessories Material
  • Length , Width and thickness of Earthing Strip and Accessories
  • Galvanization thickness
  • Galvanization tests to be conduct.
  • Proper painting / Galvanization and identification numbers of the Earthing Strip and Accessories
  • The GS Flat to be supplied in 5.5 meters to 13 meters lengths.
  • The weight of GS Flat
  • MS flat shall conform to IS 2062 & its latest amendments for steel & Galvanization as per IS 4759 & its Latest amendments
  • Physical Damages Inspection:
  • Damage on Earthing Strip and Accessories
  • Damage on galvanizing
  • Testing of galvanizing:
  • Uniformity of coating Thickness Test
  • TRs not more than five year old shall be reviewed for acceptance.
Hot dip galvanization. (IS 2629)
Galvanizing Minimum thickness: Min. weight:
MS flats 5mm thick & over 75 microns (minimum) 610 gms. / sq. mtr.
MS flats under 5mm thickness 60 microns (minimum) 460 gms. / sq. mtr.
Pipes/ conduits with thickness over 5 mm 75 microns (minimum) 610 gms. / sq. mtr
Pipes/ conduits with thickness under 5mm 60 microns (minimum) 460 gms. / sq. mtr
GI Wire 20 Microns (Medium coated) 150 gms. / sq. mtr.

(D) Storage & Handling:

  • The Earthing Flat shall be supplied in standard lengths.
  • Materials should be stored according to a specification which is the maximum 1.5m height from the ground. Suitable support should be provided. The storage should be done in a designated area and proper covering should be provided.
  • Earthing Strip and Accessories (pre-galvanized, hot dipped galvanized) shall be stored in a dry place, fully enclosed / ventilated store.
  • When bringing down materials, they should be handled with care and lowered carefully to the ground. They should not be dropped.

(E) Preparation for Earthing Strip / Wire

  • Check and ensure that the correct size and type of Earthing Strip & accessories are ready for installation.
  • Ensure that the work area is ready and safe to start the installation of Earthing Strip.
  • Ensure that Earthing Strip and accessories received from site store for the installation are free of rusty parts and damages.

(F) Earthing Strip Installation:

  • Marking the Route:

  • Mark the route of Earthing Strip with marking threads.
  • The route of Earthing Strip to be coordinated with other services and shall be confirmed.
  • Minimum space from the building structure and other services to be maintained (200 mm from the nearest point) to facilitate easy handling and maintenance.
  • Satiating of Earthing Strip / Electrode:

  • Hot-dip galvanized strip steel is aligned on simple straightening machines or on a parallel by hammer.

AA

  • Installation on Wall / Ground:

  • GI strips used for earthing shall be minimum 6 mm thick and hot dip galvanized.
  • If round GI conductors are used it shall have double the calculated area of cross-section.
  • For installing earth leads on walls, special clamps are employed. They firmly accommodate the earth leads and are easily mounted. They are directly inserted in the wall or screwed to the wall. Fixing should be spaced not more than 1 m apart.
  • Joints and junctions of earth leads and earthing concentration leads are to warrant a durable, safe and electrically well conductive connection.

AA - Copy

  • Where a Copper conductor is to be joined to GI, the joints should be tinned to prevent electrolytic action.
  • If atmosphere is corrosive, GI conductors shall not be used for earthing.
  • Earthing strips may be placed together with underground cables in cable Trench, but the heat from the cable must not be able to dry out the soil.
  • Earth conductors in trenches having power or multi-core cables should be fixed to the walls near the top (for example, 100 mm from the top).
  • Copper earth strip supported from or in contact with galvanized steel should be tinned to prevent electrolytic action.
  • Sharp bends required in aluminum strip should be formed by the use of a bending machine.
  • Earthing Strip which install below ground should be covered adequate insulating Sleeve for avoid corrosion.
  • Earthing Electrode (Plate / Pipe):

  • Minimum distance between earthing electrode (Plate /Pipe) and adjacent civil structure shall be 1.5 meter.
  • Earthing grid should be run at a minimum depth of 50 cm below the ground.
  • Since earthing electrodes will be damaged by corrosion, they are not to be placed in aggressive soil, in the vicinity of rubbish or in running waters.
  • Transformer and generator neutral shall be double earthed. One independent earth electrode shall be provided for neutral earthing
  • Earth electrodes (Plate /Pipe) shall be embedded as far apart as possible from each other. Mutual separation between them shall usually be not less twice the length of the electrode and are to be arranged in such a way as to prevent them from affecting each other.
  • Earthing Bus-Bar:

  • As far as possible, all earth connections shall be visible for inspection.
  • All connections shall be carefully made, if they are poorly made or inadequate for the purpose for which they are intended, loss of life or serious personal injury may result.
  • No cut-out, link or switch other than a linked switch arranged to operate simultaneously on the earthed or earthed neutral conductor and the live conductors,
  • All earth electrodes shall be interconnected using the conductors of largest size in the earthing system.
  • All non-current carrying metal parts of equipment’s shall be double earthed using conductors of adequate size.
  • Earthing bus bars for screwing on wall / other constructions, distance of bores 35 mm.For connecting Flat strip with bore by flat head screws M10 (with anti-rotation feature), nuts and spring washer.

AA - Copy (2)

  • Connection of Earthing Strip / Wire in Earthing Bus-Bar or to the body of equipment etc, such that it should be easily disconnected for testing purpose.
  • By welding and drilling the zinc layer on the steel is damaged leading to stronger corrosion at the defective points.
  • Welded joints are to be thoroughly cleaned from scale by means of a welder’s hammer prior to applying the anti-corrosive tape.
  • Earthing Strip / Wire Jointing:

  • Bolted, welded and pressed joints are permitted, In this case welded joints are being preferred Joints must be protected from corrosion.
  • All Earthing Strip joined together with two bolt arrangement, cutting, bending, shaping jointing with nut bolts & lap welding joints at all junctions. All connection made by electric arc welding with low hydrogen content electrodes.
  • Joints shall be allowed to cool down gradually to atmospheric temperature before putting any load on it. All oxide films that may have formed during welding must be removed from the welded joints
  • Joints should be provided with coating alternative layers of red oxide and aluminium. Joints are to be covered with hot bitumen
  • The interfaces of all ‘mechanical’ joins. Should be protected with a suitable electrical joint compound, particularly any bimetallic joints. All bimetallic joints should then be encapsulated in a grease impregnated tape, mastic compound or bitumastic paint, etc., to exclude moisture, In general, aluminum should only be used above ground and the connections to earth electrodes made above ground with bimetallic joints.  

AA - Copy (3)

  • Joints using GI conductors should be welded as far as possible and kept separated from air by a thick coating of tar or similar non-hygroscopic materials. In case bolted joints cannot be avoided than there should be a minimum of 2 bolts for sizes up to 25 mm x 6 mm, 3 bolts for sizes up to 31 mm x 6 mm and zig-zag bolting for large sizes.
  • When making a bolted type joint the surface of the Aluminum strip should be cleaned thoroughly by wire brushing and greased or an approved jointing compound applied immediately to both mating surfaces. Bolts should then be tightened and all excess grease or compound wiped off and discarded.
  • All crossings of conductors in the main earth grid should be jointed. Compression type joints may be used for stranded conductors.
  • Non-conductor strip should be drilled for a bolt having a diameter greater than one-third of the width of the strip. If this diameter will be exceeded, than a wider flag should be jointed to the strip.
  • In case of bolted joints, at least a bolt M 10 has to be taken. For joining the earth lead to the auxiliary earthing electrode in case of applying the protective measure “voltage-operated earth-leakage protection” a bolt M 6 will suffice (Always hardened and tempered bolts with hexagonal head are to be used).
  • Connections to natural earthing electrodes are preferably to be made outside the soil. At points where this is impossible and at joining faces being not metallic-bright, toothed lock-washers are to be used. At joining faces being metallic-bright, joints between earthing electrodes may be made by applying spring lock washer resp. plain lock washers. At the joints of earthing electrodes protection against corrosion is of utmost importance. It must be durable and fully effective.

AA - Copy (3) - Copy

 

OVERLAPPING OF EARTHING STRIP

SR.NO SIZE OF EARTHING STRIP MIN.OVERLAPING
1 20×3 20MM
2 20×6 20MM
3 25×3 25MM
4 25×6 25MM
5 32×6 25MM
6 40×5 50MM
7 40×6 50MM
8 50×6 50MM
9 50×10 50MM
10 75×6 50MM
11 75×10 50MM
NO’S AND SIZE OF NUT BOLT FOR JOINTING EARTHING STRIP
SR.NO SIZE OF EARTHING STRIP MIN.NUT BOLT REQUIRED MIN.SIZE OF NUT BOLT
1 20×3 2 NO’S 8X25MM
2 20×6 2 NO’S 8X25MM
3 25×3 2 NO’S 8X25MM
4 25×6 2 NO’S 8X25MM
5 32×6 2 NO’S 8X25MM
6 40×5 4 NO’S 8X25MM
7 40×6 4 NO’S 8X25MM
8 50×6 4 NO’S 10X25MM
9 50×10 4 NO’S 10X25MM
10 75×6 4 NO’S 10X25MM
11 75×10 4 NO’S

10X25MM

  • Jointing conductors:

  • Aluminum to aluminum: When possible, 4 joints on strip conductor should be Bolted or arc welded using either the tungsten inert-gas arc ( TIC ) or metal inert gas arc ( MIG ) techniques. Oxy-acetylene gas welding or brazing may also be used.
  • Rectangular Strip can be joined or terminated by drilling and bolting.
  • When making a bolted type joint, the surface of the aluminum should be cleaned thoroughly by wire brushing and greased or an approved jointing compound applied immediately to both mating surfaces. Bolts should then be tightened and all excess grease or compound wiped off and discarded. To ensure adequate contact pressure and avoid overstressing, torque spanners should be used. The conductor manufacturer’s literature should be consulted for further details for the joints and procedures.
  • Aluminum to copper:

  • Joints between aluminum and copper should be of the bolted type and be installed in the vertical plane at a minimum distance of 150 mm above ground level.
  • The rating surface of the aluminum should he cleaned thoroughly by wire brushing and greased or an approved jointing compound applied and the copper tinned. Grease or an approved jointing compound should be applied to the melting surface of the aluminum.
  • After bolt tightening by torque spanner, excess grease or compound should be wiped off and discarded, and the joint protected from the increase of moisture by the application of suitable plastics compound or irradiated polyethylene sleeve with mastic lining. Alternatively, the joint may be protected by a bitumastic paint.
  • Aluminum conductor connections to equipment should, where possible, be in the vertical plane. Surface preparation of the aluminum and the making of the joint should be as previously described. The finished joint should be protected by a bitumastic paint.
  • Earthing strip shall not have any cut-outs or switches or links.
  • All material, fitting etc. used for earthing and earthing pit should be of IS specified make and standards.
  • Two separate and distinct connections shall be taken out from plate earthing.
  • Interconnection of earth and main branch of earth should be made in such a way that reliable and good electrical contact is established.
  • The path of earthing strip should be minimum as possible, be out of reach of any person. For earth resistance please refer to IS-3043:1966-10 page –32. h).
  • Anti-corrosive measures: Earth strip should be protected against mechanical damages and corrosion. Fittings should be resistant to the corrosive agencies or be otherwise suitably protected. Joints and bonds may be protected with bitumen or embedded in plastic compound according to the local conditions.
  • No conductor strip should be drilled for a bolt having a diameter greater than one-third of the width of the strip. If this diameter would be exceeded then a flat should be jointed to the strip
  • Aluminum or copper conductors should not be drilled for fixing to structures. Clips should be used that prevent contact between conductor and structure and which are of suitable material so that there is no electrolytic action between clip and conductor.
  • Fixings should be spaced not more than 1 m apart. Earth conductors in trenches containing power and/or multi-core cables should be fixed to the walls near the top (e.g. 100 mm from the top).
  • Copper earth strip supported from or in contact with galvanized steel should be tinned to prevent electrolytic action. If sharp bends are required in aluminum strip they should be formed by the use of a bending machine to avoid stress concentration. Aluminum is prone to corrosion when in contact with Portland cement and mortar mixes. Contact of aluminum conductors with such materials should, therefore, be avoided by the use of stand-off fixings.

(G) Standards:

  • IS:3043-1987 :Code of Practice for Earthing (first revision)
  • Indian Electricity Rules :1956 (latest edition)
  • National Electrical Code :1985 of Bureau of Indian Standards
  • IEEE Guide for safety in a. c. substation grounding. No. ANSI/IEEE Standard 80-1986.
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