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OBJECTIVE OF EARTHING
Prime Objective of earthing is to provide a Zero potential surface in and around and under the area where the electrical equipment is installed or erected.
To achieve this objective the non-current carrying parts of the electrical equipment is connected to the general mass of the earth which prevents the appearance of dangerous voltage on the enclosures and helps to provide safety to working staff and public.
CLASSIFICATION OF EARTHING
Earthing can be classified into the following categories based on the purpose for which the part of the equipment connected to the general mass of earth.
Earthing associated with current carrying parts of the equipment is called system earthing. The system security, reliability, performance, voltage stabilization, all relied only on the system Earthing.
Earthing Neutral of Transformer,
Surge arrester Earthing
IE 32: Identification of earthed & earth neutral conductors & position of switches & cut-outs therein
1. An indication of a permanent nature shall be provided by the owner of the earthed neutral conductor, to enable such conductor to be distinguished from any live conductor.
2. No cut out, link or s/w other than a linked switch arranged to operate simultaneously on the earthed neutral conductor with the following exceptations:
A) a link for testing purpose, or
B) a switch for use in controlling a generator or transformer
IE RULE 33: Earthed terminal on consumer’s premises:
The supplier shall provide and maintain for the consumer’s use a suitable earthed terminal in an accessible position at or near the point of commencement of supply
IE RULE 61: Connection with earth:
Normally >125V & of at medium voltage, neutral conductor of a 3-phase, 4 wire system & the middle conductor of 2-phase, 3 wire system shall be earthed by not less than two separate & distinct connections 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.
IE RULE 67: Connection with earth:
All non- current carrying metal parts associate with HV/EHV installation shall be effectively earthed to a earth mat which will;
a) limit the touch & step voltage to a tolerable value
b) limit ground potential rise
c) maintain the earth resistance value to operate the protective device
Types of Earth Megger
Two types :
a). Four terminal Earth Megger
Megger which is exclusively used for Soil Resistivity Measurement. This can be converted as 3 terminal by connecting P1 & C1 together.
b). Three terminal Earth Megger.
Three Terminal Meter
i)For 3 Terminal Meter
Four Terminal Meter
(ii) For 4 Terminal Meter
Current Terminal – C or H
Current Terminal C1, C2
Potential Terminal – P or U
Potential Terminal P1, P2
Earth Terminal – E
STANDARD EARTH MEGGER VALUE
a. Switchyard 1 Ohm
b. for Minor Sub Station 2 Ohm
c. Power House 0.5 Ohm
d. Distribution Transformer 5 Ohm
Application of Bendonite Powder
Powdered charcoal and salt put surrounding the earth pipe. Calcium chloride, magnesium sulphate may be administered. If salt is put corrosion of pipes will be earlier.
Use of Bentonite Powder in Soil Treatment:
1. Bentonite is a clay with outstanding electrical properties. It swells to several times its original volume when suspended in water; it binds the water of crystallization and the water absorbed during the mixing process is retained over a long period. Bentonite suspension in water when used to surround the earth electrode virtually increases the electrode surface area.
2. Use of bentonite around the earth electrode results in reduction of ground resistance by about 25-30 ohms.
3. Bentonite has a tremendous capacity to absorb water and retain it over along period. The best results are obtained by mixing bentonite with a very large quantity of water, say in the ratio of 1 : 4.
1. A cover of bentonite power for 300 mm dia. Around the electrode would be adequate in most of the cases to obtain a significant reduction in earth resistance.
2. Even during the summer months, bentonite suspension retains the moisture where as the natural soil dries up.
3. Bentonite or similar material may be used to advantage in rocky terrain.
Treatment of Soil using Fly Ash
Use of Fly Ash in Soil Treatment:
Recent CPRI studies reveals that Fly Ash from Thermal Stations has equivalent chemical composition and hence can be used for the electrical installations in areas of high ground resistivity. Fly Ash can also be used as a chemical treatment material to reduce soil resistivity when compared to bentonite and salt charcoal combination.
DANGERS OF IMPERFECT EARTHING
(a) If the Transformer neutral is not earthed properly, in the event of an earth fault in the system a condition known as “arcing ground” will occur resulting in high voltages.
(b) If lightning arrester earth is not proper, in the event of lightning discharge, the lightning arrester will become
in-effective and this may cause injury elsewhere in the equipment.
© In case AB switch handle is not earthed properly, during
the operation of the AB switch, the touch voltage limits
may get exceeded and cause injury to the operator.
(d) In effective earthing in a distribution system may, not only result in life hazards but also may affect metering.
If proper standard and effective earthing adopted, it will minimize accidents and maximize the security and reliability and supply.
Regular condition monitoring of healthiness of earth and maintenance of earth system ensures safety to the operator and customer
Sizes of Earth conductors as per IS 3043/IEEE 80
Main earthing ring 75*10mm GI flat
HT/LT switch gear 50*6mm GI flat
HT equipment 25*3mm GI flat
(CB,ISO,LA,NGR,CT,PT) 25*3mm GI flat
Details of 400KV Earthmat
Equivalent length of earthmat area 600m
Equivalent width of earthmat area 160m
No of conductor along lengthwise 10
No of conductor along widthwise 8
No of electrode 40
Length of electrode 3m
Conductor cross-section 804.27sqmm
Conductor dia 32mm
Tolerable touch voltage 674.08v
Attainable touch voltage 228.242v
Tolerable step voltage 2204.18v
Attainable step voltage 85.1736v
Grid resistance 0.02703ohm