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05-10-2010, 10:34 AM
Post: #1
Elecrical Earthing System

.ppt  Earthing.ppt (Size: 91.5 KB / Downloads: 251)


Introduction

What is Earthing


-The process of earthing is to connect all the objects which could become charged to the general mass of the earth, to provide a path for electrical current and to hold the parts as close as possible to earth potential.
-For tying the electrical supply system to earth, supply transformer neutral conductor (Y point of 3 phase Supply) is connected to earth using an earthing electrode or the metal sheath and armouring of a buried conductor.

Why Earthing required


-The purpose of grounding system is to provide a low impedance electrical contact between the neutral of an electrical system and earth
-1. To provide safety during normal or fault condition.
-2. To stabilize the voltage during transient conditions and therefore to minimize the probability of a flashover during transients.
-3. To dissipate lightening strokes
-Due to impedance of the system, the potential of the grounding system may become different than the potential at various points on earth during abnormal operating condition or fault condition.
-Due to this potential difference a touch and step voltages are subjected by the persons which generate an electrical current through his body. This flow of current is a source of danger.
-Electrical body current below 116/(sqrt(t) mili amps, where t is shock duration time
-Vtouch, allowable = 0.116(1.5 + 1000)/ (sqrt(t)
-Vstep, allowable = 0.116(6.0 + 1000)/ (sqrt(t)
03-03-2011, 02:57 PM
Post: #2
RE: Elecrical Earthing System

.ppt  EARTHING.ppt (Size: 3.04 MB / Downloads: 234)
EARTHING
What is earthing?

• The earth is made up of materials that is electrically conductive.
• It is very important to earth the line and electrical equipment.
• It will be electrically unsafe without earthing.
• The pole/ body of equipment connected solidly to earth is called earthing .
Main objectives of the earthing
• Provide an alternative path for the fault current to flow so that it will not endanger the user .
• It case of short circuit or leakage, current will pass with minimum resistance to earth so that maximum current will flow through effected circuit so that fuse will blow or circuit breaker to trip.
• This will isolate the faulty line or equipment from live circuit
Requirement of Good earthing
• The chief requirement of good earthing is LOW soil resistivity.
• Soil Resistivity (specific resistance of the soil) is usually measured in Ohm metres.
• Soil resistivity varies greatly from one location to another.
• For example, soil around the banks of a river have a resistivity in the order of 1.5 Ohm metres. In the other extreme, dry sand in elevated areas can have values as high as 10,000 Ohm metres.
• The Earth resistance mainly depends on
1) Type of earth soil.
2) Temperature of earth.
3) Humidity in earth.
4) Minerals in earth.
5) Length of electrode in the earth.
6) Electrode shape and size.
7) Distance between two electrodes.
8) Number of electrodes.
• Measurement of Earth Resistance
• Three point method
• Four point Method
• C1 & P1 are shorted to each other and connected to the earthelectrode (pipe) under test.
• P2 & C2 are connected to the two separate spikes driven in earth.
• If we rotate generator handle with specific speed we get directly earth resistance on scale.
• This instrument injects a current pulse into the ground conductor and calculates the value of the ground conducto resistance from the current pulse amplitude.
• Some instruments can store the result of a number of readings which simplifies field record keeping.
• Maximum earth resistance allowed
• Major power station 0.5 W.
• Major Sub-stations 1.0 W
• Minor Sub-station 2 W
• Neutral Bushing. 2W
• Service connection 4 W
• L.T.Lightening Arrestor 4 W L.T.Pole 5W
• H.T.Pole 10 W
• Tower 20-30 W
BONDING
• Process in which all NON-CURRENT CARRYING METALLIC parts of the electrical wiring system are permanently joined together to form a CONTINUOUS grounding path
• Reduces the differences in potential between each electrical system
• May be caused by power surges, lightning strikes or other types of ground fault type occurences.
• Parts of the Electrical System
• Circuits
• Service Entrance Panel
• Metal Water Pipe
• Metal Gas pipe
Methods of Earthing
1) PLATE EARTHING
• In major power stations and major sub-stations 12 mm thick, 1200 m long, 1200 mm wide Cast Iron plates are used.
• For minor sub-stations 18 mm broad, 50 x 50 cm. G.I. plates are used.
• These plates are dug vertically in the pit. Coal, sand and salt are filled in the pit each of 150 mm layer.
• The plate should be dug deep so that soil will be wet from all sides.
Pipe earthing:
• The pipe type earthing is generally provided outside the base of the tower.
• earthing is given in A hole of the required diameter and depth is augured in the earth for the earthing pipe.
• The earthing pipe is then put inside the hole.
• A mixture of coke and salt is filled in the hole in which the earthing pipe is provided.
• The earthing strip which was fitted to the stub of the tower leg is then connected to the earthing pipe.
• The Railway authorities specify that the size of the pipe used for earthing should be of 38 mm diameter. Therefore, for towers on both sides of the Railway crossing, 2 pipes connected together are to be used for earthing.
• In case of difficult locations, the pipe may be laid horizontally or slanting and within the tower base or foundation pit.
METHODS OF REDUCING EARTH RESISTANCE
• By adding mixture of salt and water to the earth pit.
• By adding salt, charcoal and sand mixture to the pit.
• By using a bigger grounding plate
• By burying the ground plate as deep as possible
• By having parallel ground plates with a distance of 10m between grounds
• By using salt, charcoal etc., to reduce resisitvity
TYPES OF GROUNDING
Electrical power distribution systems can be either
• ungrounded,
• solidly grounded, or
• resistance grounded.
Ungrounded system.
• there is no intentional connection between the neutral or any phase and ground.
• the system is capacitively coupled to ground.
Advantage:
• Offers a low value of current flow for line-to-line ground fault (5A or less).
• Presents no flash hazard to personnel for accidental line-to-ground fault.
• Assures continued operation of processes on the first occurrence of a line-to-ground fault.
Disadvantages:
• Difficult to locate line-to-ground fault.
• Doesn’t control transient overvoltages.
• Cost of system maintenance is higher due to labor involved in locating ground faults.
Solidly grounded system
• the neutral (or occasionally one phase) is connected to ground without an intentional intervening impedance.
• It is commonly used in low voltage distribution systems.
• usually recommended for overhead distribution systems supplying transformers protected by primary fuses.
• not the preferred scheme for most industrial and commercial systems, again because of the severe damage potential of high magnitude ground faultcurrents.
Low Resistance Grounded System:
• The low resistance grounded system is one that has the neutral connected to ground through a small resistance that limits the fault current.
• The size of the grounding resistor is selected to detect and clear the faulted circuit..
• The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.
• Limits transient overvoltages during ground faults.
• not recommended for low voltage systems due to the limited ground fault curren.
• Ground fault current typically in the 100 – 600 Amp range.
High-resistance grounding
• High-resistance grounding (HRG) systems are commonly used in plants and mills where continued operation of processes is paramount in the event of a fault.
• It is normally accomplished by connecting the high side of a single-phase distribution transformer between the system neutral and ground, and connecting a resistor across the low-voltage secondary to provide the desired lower value of high side ground current.
• With an HRG system, service is maintained even during a ground fault condition.
• If a fault does occur, alarm indications and lights help the user quickly locate and correct the problem or allow for an orderly shutdown of the process.
• An HRG system limits ground fault current to between 1A and 10A.
Standard Industrial System Grounding Methods
Factors to be considered while designing the grounding system

– The area available for installation of the grounding system. This could lead to the requirement and utilization of chemical rods, or wells.
– Water table and seasonal changes to it.
– Soil condition ,resistivity,elevation above sea level and hard rocky soil are concerns that would need to be addressed.
– Available fault currents (i.e., three (3) phase, line-to-ground, and line-to-line-to ground, etc.).
– NEC and ANSI/IEEE requirements. Also include here the requirements of the process equipment to be installed.
– Consideration to the number of lightning strikes and thunder storm days per year.
– Utilization of area were ground system is to be installed, (i.e., do not install under paved parking lot).
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