Definition–To connect metallic parts of an electrical device/equipment/apparatus with the ground (which is a buried metal piece) through a conducting material like Copper, Aluminum, GI etc is called Earthing or Grounding.
There is no difference between earthing& grounding as both words are used in different parts of world for same purpose.
REQUIREMENT OF EARTHING/GROUNDING OF ELECTRICAL SYSTEM –
Earthing of electrical system is required for following purpose –
- It protects user from shock if metal body of equipment comes in contact with live wire,
- It protects property from damage/fire which may occur due to unearthed or inadequate earthing.
- It helps in detecting ground fault when occurs in the system,
- To keep system voltage constant with the help of neutral grounding,
- To protect High Rise Buildings from Lightening,
- To protect electrical system from Lightning etc.
SINGLE EARTHING & DOUBLE EARTHING–
Single earthing – It is done for 1-ph equipment/appliances,
Double earthing – It is done for 3-ph equipment/apparatus. The purpose of double earthing in 3-ph system is to ensure that if one earthing fails then 2ndearthing will ensure protection from shock.
TYPES OF EARTHING–
There are 3 types of earthing done for electrical system –
- Pipe Earthing,
- Plate Earthing (Cu or Al),
- Chemical Earthing.
As per IS:3043 the earthing material should have the quality to withstand against corrosion in the soil. Steel corrodes about 6 times as fast as copper when placed in the soil. The extent of corrosion depends upon the properties of soil. Generally the accepted correlation between the electrical resistivity of soil &corrosivity is given in the table below –
|Range of soil resistivity (Ohm-Meters)||Class of soil|
|Less than 25||Severely corrosive|
|25 – 50||Moderately corrosive|
|50 – 100||Mildly corrosive|
|Above hundred||Very Mildly corrosive|
Following methods can be adopted to safeguard conductor against excessive corrosion –
- Use of cathodic protection,
- Use current conducting, corrosion resistant coating on steel (for ex. Zinc coating),
- Use steel conductor with large cross-section having allowance for corrosion.
Based on the result of the field studies, following allowances in cross sectional area of the earthing conductor are recommended to take the effect of corrosion into account.
|Type of laying of earth conductor||Allowances to be considered in sizing|
|Conductor laid in soil having resistivity greater than 100 Ohm-meters||0 (no allowance)|
|Conductor laid in soil having resistivity from 25 to 100 Ohm-meters||15%|
|Conductor laid in soil having resistivity lower than 25 Ohm-meters or where treatment of soil around electrode carried out||30%|
So, for the purpose of determining allowance to be made for correction, minimum resistivity of soil is to be considered. The resistivity will be minimum in wet weather. However, the steel conductor in the soil of other types should be at least 6mm thick if it is steel flat and have a diameter of at least 16mm if it is in the form of steel road.
Recommendations for earthing from above details –
- CI (cast iron) pipe – Diameter – not less than 100mm, length – 2.5 – 3 mtr long, thickness- 13mm,
- MS (mild steel) water pipes – Diameter – 38-50mm, length – 2.5 – 3 mtr long, thickness- 13mm,
- GI (galvanized iron) – Diameter – 40mm, length – 3mtr long & will be placed at 1.25mtr below ground,
- MS (mild steel) rod – Diameter – 40mm, length – 3mtr long, long & will be placed at 1.25mtr below ground,
- PIPE EARTHING –
Pipe earthing is used for body earthing of the equipments. Earthing details of pipe is given below-
2.PLATE EARTHING –
Plate earthings are of two types–
- GI Plate Earthing –Normally GI plate earthing is done for body earthing of major equipment like Transformer, DG Sets, HT panel, LT panel etc.
- Cu plate earthing – Normally Cu plate earthing is done for neutral earthing for Transformer, DG Sets, UPS etc.
Details of earthing is given in the diagram given below –
SOME IMPORTANT PARTS OF EARTHING –
Sleeves – For identification of earthing (body or neutral), earthing strips/conductor are provided with heat shrinkable sleeves – Green sleeve for body earthing& black color for neutral earthing.
Earthing Conductor/wire/strips – It is used to connect the equipment with earthing pit. It can be a GI (Galvanized Iron) wire, insulated Cu (copper)wire, Al (aluminum) strips & Cu strips. The size of earthing conductor/wire/strips depends on the fault current of the equipment/system.
Earth Electrode –It is defined as a conductor or group of conductors in intimate contact with and providing an electrical connection to earth.
Earth electrodes are of two types – Pipe & plate. Pipe electrode is used in pipe earthing& plate electrode is used for plate earthing. Health of earth Electrodes ensures the healthiness of earthingsystem,therefore earth electrode should remain in healthy condition in the soil.
Earthing Pit – An earth pit is a 3 meter deep pit in which pipe is placed for pipe earthing or plate is placed for plate earthing. The dia of the pit for pipe should be at least 1ft & the opening of the pit for plate earthing should be 1.5ft X 3ft. The earth pit is filled with 150 mm charcoal & 150mm salt layer alternately. Charcoal & salt is filled in the pit -1) from bottom to upto bottom of the earthing chamber in case of pipe earthing& 2) 150mm layer surrounding the GI/Cu plate in case of plate earthing.
Earthing Chamber – It’s the upper most part of the earth pit wherein a funnel & fish plate are visible. Earthing conductor/wire is connected with pipe/plate through a fish plate.
Funnel is used for watering the pit in order to keep the soil wet & wet soil keeps the resistivity of the ground at lower level.
Charcoal & Salt –
Flow of current depends on the value of resistance of the path, so for better earthing system the value of earth resistance of pit should be as low as possible.The value of earth resistance depends on the resistance value (or resistivity) of the soil. This can be achieved by mixture of equal amount of charcoal & salt. How this is achieved is given below –
- The function of charcoal & salt is to keep the soil wet,
- When salt melts/drips, it creates moisture which is observed & maintained by charcoal,
- Being a good conductor, mixture of charcoal& salt increase the conducting area around the plate &,
- This decreases the resistivity of the soil.
METHODS OF JOINTING OF EARTHING STRIPS –
Suitable methods of jointing are a taper pin driven into a reamed hole and riveted over or a copper stud screwed into a tapped hole and riveted. Such joints should be protected by a heavy coat of bitumen.
MEASUREMENT OF EARTH RESISTANCE –
Resistance of earth could be measured with the help of Earth Testerwith three point method which is done as shown in the diagram given below –
Use following method to measure resistance of earth electrode –
1) Material required for earth resistance testing – a)an earth tester, b) 2 nos spikes (metal/MS rods) of one meter length, 3) some flexible wire of size 1.5 sq meter.
2) An earth tester has a voltage source.
3) Disconnect earth strips/wires which is connected with earth electrode,
4) Connect earth electrode with common point C1 of earth tester through a 1.5 sq wire,
5) Location of earth tester should be close to the earth pit.
6) Connect P2 point of earth tester with a spike which should be driven in the earth 15 meters away from the earth pit,
7) Connect C1 point of earth tester with a spike which should be driven in the earth 30 meters away from the earth pit,
8) All these three items electrode & both spikes should be in the same line,
9) Rotate handle of earth tester, the needle will show the value of earth resistance.
RESISTIVITY OF EARTH–
The resistance to earth of a pipe or plate electrode reduces rapidly within the first few feet from ground (mostly 2 to 3 meter) but after that soil resistivity is mostly uniform.
After about 4 meter depth, there is no appreciable change in resistance to earth of the electrode. Except a number of rods in parallel are to be preferred to a single long rod.
The resistance to earth of a given electrode depends upon the electrical resistivity of the soil in which it is installed. The type of soil largely determines its resistivity. Earth conductivity is, however, essentially electrolytic in nature and is affected, by the moisture content of the soil and by the chemical composition and concentration of salts dissolved in the contained water. Grain size and distribution, and looseness of packing are also contributory factors since they control the manner in which the moisture is held in the soil. Many of these factors vary locally and some seasonally.
How to decrease the resistivity of soil –
If resistance value of earth increases, following methods should immediately be applied to reduce the soil resistivity surrounding the earth electrode.
1) Multiple rods can be used connected in parallel,
2) To reduce soil resistivity, it is necessary to dissolve in the moisture. The most commonly used substances are NaCl (Sodium Chloride), CaCl2 (Calcium Chloride), Na2CO3 (Sodium Carbonate), CuSO4 (Copper Sulphate), salt, soft coke and Salt & Charcoal in suitable proportions. These substances are highly conductive in its water solutions.
(Note: It is recommended to do annual or biannual measurement of earth resistivity to find out if any treatment is needed or not.)
Moisture content is one of the controlling factors in earth resistivity. It has been experienced that moisture upto 20% increases resistivity very abruptly. As the percentage of moisture decreases further, the resistivity increases further. But,that above about 20 percent moisture, the resistivity is very little affected. A difference of few percent moisture will therefore, make a very marked difference in the effectiveness of earth connection if the moisture content falls below 20 percent. The normal moisture content of soils ranges from 10 percent in dry seasons to 35 percent in wet seasons, and an approximate average may be perhaps 16 to 18 percent.
HOW TO REDUCE EARTH RESISTANCE –
- Joints should be connected/tightened properly either by riveted or welded joints. There should not be any paint/oxidation between the joints,
- To avoid joints from rusting, joints should have coating of bitumen,
- Pour sufficient water in the pit,
- Electrodes should be connected in parallel,
- Keep earth electrode at sufficient depth,
- Use sufficient good quality of charcoal & salt as these two helps to increase the effective area of electrode.
LOCATION OF EARTH PIT–
The location of earth pit also plays an important role in term of resistivity.Like sandy area (beds of river or mountain streams), rock area, soil area etc they all offer different soil resistivity.
A site should be chosen in the order of preference given –
- Wet marshy ground,
- Clay, loamy soil, arable land, clayey soil, clayey soil or loam mixed with small quantities of sand;
- Clay and loam mixed with varying proportions of sand, gravel and stones;
- Damp and wet sand, peat.
1)A soil area should be avoided if it is backfilled with malba/debris in the earthing area.
2)Dry sand, gravel chalk, limestone, granite and any very stony ground should be avoided, and also all locations where virgin rock is very close to the surface.
MAXIMUM ALLOWABLE EARTH RESISTANCE –
- Large Power Stations = 0.5 Ohm
- Major Sub-stations = 1.0 Ohm
- Small Sub-station = 2.0 Ohm
- Max allowable earth resistance in other cases = 5.0 Ohm
- Any point of time the resistance between earth pit & earthed body should be less than – 1 Ohm
EARTHING IN AIRCRAFTS, AUTOMOBILES AND OFF SHORE INSTALLATIONS-
It is important to know that how earthing is done in Aircrafts, ships or offshore installations because –
- Earthing or grounding of an electrical installation is necessary that a person does not get an electrical shock when he/she comes in contact with it. It is because earthing provides a low path for current to flow as compared to human body.
- Earthing helps us to identify to locate the fault easily.
Aircrafts – As earthing can’t be done separately so Aluminum frame/body is the earthing point in the Aircraft.Aircraft can build up massive charges flying through thunderstorm or even otherwise. All aircraft have Wire like attachments to their wings & flaps called Static Wicks that help dissipate charge while flying. Also bonding wires are used when the craft is on ground especially during fueling. Grounding is the reason why a bonding wire is first lowered from a helicopter before lifting people aboard.
Ships – Both Ships& Submarines are large steel (usually) hulled object in direct contact with the earth (Seawater is a very good conductor). The distribution system on ships is usually an Insulated Neutral system which prioritizes continuous machinery operation over safety of humans.
Cars–The neutral of the battery is earthed with either chassis or metal body of the car. So, current flows form the positive terminal of the battery, passes through the component (like bulb etc) & back to the battery through the body of the car.
The neutral terminal of the battery should be connected with thick cable.
3.CHEMICAL EARTHING –
Chemical earthing is also called Maintenance Free Earthing. Chemical Earthing consists of 1)Earthing Electrode/Rod, 2) Backfill Chemical Compound& 3) Earthpit cover.
- Earthing Electrode – Earthing electrodes are available in GI or Copper material. The function of electrode is to direct the fault current into the ground. Cu electrode is most preferred choice for almost all types of earthing. Its available in different diameters & lengths like40mm(D) X2000mm (L),40mm(D) X3000mm (L), 50mm(D) X2000mm (L), 50mm(D) X3000mm (L), 76mm(D) X2000mm (L), 76mm(D) X3000mm (L) etc.
- Backfill Compound – The function of backfill compound is to reduce the soil resistance & creates a low resistance zone around the electrode. It is available in powder form & it is mixed with water before pouring it into the pit. This compound is compacted on adding water & has capability of absorbing moisture up to 13 times its dry value.
Advantage of back filled compound over salt & charcoal –
- Environment friendly, it has no negative impact on environment,
- No need of water filling,
- It has no negative effect on electrodes,
- Pit Covers – Earth pit covers are used to protect earthing terminations from damage. These covers may be brick/concrete/ readymade heavy duty Polypropolyne/FRP. These Pit covers should be of heavy duty in nature.
Detail of chemical earthing is given in the below diagram –
Comparison between Conventional & Chemical earthing –
|Conventional Earthing||Chemical Earthing|
|It’s a costly Solution||It’s a cost effective Solution|
|Low product Life of plate /pipe earthing||Product life more than 25 Years|
|Require more space||Require less space|
|Need Regular maintenance like watering pit||Free from Maintenance|