24-12-2012, 06:38 PM
EARTHING SYSTEM
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INTRODUCTION:
-Earthing is an integral part of any electricalinstallation. Earlier, very little was known about the requirement of earthing. Even now, when a wealth of information is available about the necessities and methodology of earthing systems.
SOME FUNDAMENTAL TERMS:
-EARTH:
From electrical engineering point of view of Earth is define as the conductive mass of the earth, whose electrical potentialat any point is conventionally taken as zero. Being electrically neutral and at zero potential, the earth provides a common reference for voltage measurementEARTHING: An electrical connection to the general mass of earth, whose dimensions are very large in comparison to the electric system being considered. The term ’Ground’ and ‘Grounding’ are synonymous with ‘earth’ and ‘earthing’.
EQUIPOTENTIAL BONDING:
Electrical connection putting various exposed conductive part and extraneous conductive parts at a substantially equal potential.
EXPOSED CONDUCTIVE PART:
A conductive part of an equipment which can be touched and which is not a live part but which may become live under fault conditions.
EXTRANEOUS CONDUCTIVE PART:
A conductive part liable to transmit a potential including earth potential and not forming part of the electrical installation.
NEED FOR EARTHING:-
Earthing is of utmost importance for safety of plant, equipment, property and human as well as animal life. In the absence of a well designed effective earthing system, earth fault conditions may lead to tremendous loss of property and lives. The main objectives of earthing are as follows ……….1)To ensure safety of life and property from hazards of electric shock and electric fires.2)To ensure that system voltages on healthy lines remain within reasonable limits under fault conditions thereby preventing insulation breakdowns.
EARTHING CATEGORIES:-
There are two types of earthing.
A)SYSTEM EARTHING ,
B)EQUIPMENT EARTHING.
SYSTEM EARTHING :
This is primarily concerned with the protection of electrical equipment by stabilizing voltages with respect to ground. This is basically achieved by earthing the neutral of the supply system. Ungrounded neutral supply systems are not in use mainly for the following disadvantages :
1) Under a single line to earth fault the voltage to earth of the two healthy phases rises from phase value to line value i.e. Root 3 times.
2) The capacitive current in the two healthy phases increases to root 3 times the normal value. 3)The capacitive current in the faulty phase is 3 times its normal value.4)The capacitive fault current gives rise to arcing ground condition. Under such condition the phase voltage may rise to 5 to 6 times its normal value causing insulation breakdown.5)It is difficult to detect and isolate a single phase to earth fault.
Resistance Grounding :
Neutral earthing is done through a resistor to limit the earth fault current. Neutral earthing resistors are normally designed to carry their rated current for a short period ,usually 30 seconds. Resistance grounding is normally employed on systems operating at voltages between 2.2 KV and 33 KV, when the total power source capacity exceeds 5000 KVA, as the current characteristics of such systems usually give rise to excessive currents under ground fault conditions.
TWO IMPORTENT INDIAN ELECTRICITY RULES FOR LOW AND MEDIUM VOLTAGE :
Rule 61(1):
states that neutral conductor of a 3phase,4 wire system shall be earthed by not less than two separate and distinct corrections with a minimum of two different earth electrodes or such large number as may be necessary to bring the earth resistance to a satisfactory value.The earth electrodes so provided,may be interconnected to reduce earth resistance. Rule 61(2): states that the frame of every generator,motor and the metallic parts (not intended as conductors) of all transformers and any other apparatus used for regulating or controlling energy and all medium voltage energy consuming apparatus shall be earthed by two separate and distinct connections with earth.