EARTHING

Earth: In electrical engineering ‘earth’ means the conductive mass of the earth whose electric potential at any point is conventionally taken as zero.

Earthing: Earthing means connecting the non current carrying metallic parts of electrical apparatus or the neutral point of the supply system to the general mass of earth by means of an earth electrode in such a manner that immediate discharge of electrical energy can take place at all time without effecting any danger.

System earthing & equipment earthing: System earthing means the intentional connection of the neutral point i.e. the star point of generator, transformer, rotating machine and grounding transformer to the general mass of earth whereas equipment earthing means the intentional connection of the non current carrying metal parts of the motor body, generator body, switchgear structure, transformer core and tank, sheaths of cable, body of portable apparatus etc to the general mass of earth.

Advantage of neutral point earthing: (1) Under earth fault condition the phase to earth voltages do not rise to high value and (2) under earth fault condition it ensures efficient and fast operation of protective gear.

Objects of equipment earthing:

(i) To eliminate the danger of serious injury from electric shock to persons who may come in contact with the metal frames of electrical equipment which may have become live due to the insulation failure.

(ii) To flow the earth fault current easily through earthing causing operation of over current protective system or earth fault relay without creating a fire or explosive hazards to building and contents and

(iii) To contribute to better performance of the electrical system.

Good earthing: It is that earthing which gives very low resistance to the flow of heavay leakage current of a circuit to earth.

Methods of neutral earthing:

(i) Solid earthing: When a direct metallic connection is made between the system neutral and earth then it is called as solid neutral earthing.

(ii) Resistance earthing: When a current limiting device resistance, metallic or liquid, is introduced in between neutral and earth connection to limit the earth fault current then it is called as resistance neutral earthing.

 (iii) Reactance earthing: When an impedance, predominantly inductive, is inserted in between the neutral point of generator or transformer and earth connection then it is called as reactance earthing. The advantage are as follows: (i) For the same maximum earth fault current the reactor device can be made smaller than the corresponding resistance and (ii) Dissipation of electrical energy in the reactor device is very much less.

 (iv) Arc suppression coil earthing: Arc suppression coil earthing is a logical development of reactance earthing and is based on a value of reactance in the system neutral such that the reactance current due to the coil exactly neutralizes the network capacitance current at the fault. The resultant current being nil is quite inadequate to maintain any arc at the earth fault point. Hence the inductive coil is named as arc suppression coil which is usually provided with tapings so that its value can be adjusted to suit the capacitances of the system. It is also called in the name of its inventor as Peterson coil earthing.

Electrodes used for earthing: (i) Pipe electrode, (ii) Plate electrode, (iii) Rod electrode and (iv) Strip electrode.

Pipe electrode: It shall not be smaller than 38mm internal diameter if made of galvanized iron or steel and 100mm internal diameter if made of cast iron. The length of the pipe electrode shall not be less than 2.5metres.

Plate electrode: Plate electrodes when made of galvanized iron or steel. The size shall not be less than60cm x 60cm x 6.30mm and the plate electrodes when made of copper the size shall not be less than 60cm x 60cm x 3.15mm.

Rod electrode: Rod electrodes of steel or galvanized iron shall be at least 16mm in diameter and those of copper shall be at least 12.5mm in diameter. The length of rod electrode shall not ge less than 2.5metres.

Strip electrode: It shall not be smaller than 25mm x 1.60mm if of copper and 25mm x 4mm if of galvanized iron or steel. The length shall not be less than 15metres.

The distance of the earth electrode from the building should not be less than 1.5metres (5ft)

Earthing electrode for low & medium voltage installation as per I.E. rule:

(i)For low voltage :  at least one earth electrode

(ii) For medium voltage: at least tow earth electrode

(iii) For motor installation of any voltage at least two earth electrodes are required.

Double earthing is used (i) to give minimum resistance to the flow of leakage current during any fault happened on the equipment and (ii) to give more reliability i.e. if one earth becomes out of order second earth will function.

The size of the earth conductor depends upon the full load current of the installation for which it is used. As the sizes of the phase conductors for motors of different capacities are different for different values of load current therefore the sizes of earth conductors for motors of different capacities will be different.

Minimum cross section of protective conductors: (i) When the cross sectional are of phase conductors of the installation does not exceed 16mm², the minimum cross sectional are of the corresponding protective conductor will be same as that of phase conductor but in any case it will not be less than 2.5mm², if mechanical protection is not provided.

(ii) When the cross sectional area of phase conductors of the installation exceeds 16mm2 but not 35mm², the minimum cross sectional are of the corresponding protective conductor will be 16mm².

(iii) When the cross sectional area of phase conductors of the installation exceeds 35mm², the minimum cross sectional are of the corresponding protective conductor can be calculated by the formula S = I.√t/k,

where S= Cross sectional are in mm²

             I= a.c. r.m.s. value of fault current which can flow through the protective device in ampere

             T= operating time of the disconnecting device in seconds and

             K= factor dependent on the material of the protective conductor, the insulation and other

                   part, the initial and final temperatures, the value of which may be taken from LS-3043.

Earth resistance: Earth resistance of an electrode means the combined resistance of (i) resistance of the metal electrode, (ii) contact resistance between the electrode and the soil and (iii) electrical resistance of the soil surrounding the metal electrode set up for the flow of current outward from the electrode to infinite earth. For practical purposes first two factors can be neglected because they are very small fractions of an ohm.

In an electrical installation, the earth resistance should be:

                          32

-------------------------------------------  or 5 ohms whichever is less

Fusing current or tripping current

If the earth resistance exceeds its permissible value then in the event of earth fault, the fault current may not reach a sufficient value to operate the protective equipments such as fuses or relays causing high potential gradient at the surface of the ground which may constitute a danger to life. So to avoid the dangerous situation the earth resistance should not be higher.

Max. permissible value of earth resistance:

(i) Large power station           0.5 ohm

(ii) Major power station         1.0 ohm

(iii) Small substation               2.0 ohm

(iv) In all other cases              0.8 ohm

Earth resistance depends on the following factors:- (i) Condition of soil, (ii) Temperature of soil, (iii) Moisture content of soil, (iv) Size and spacing of earth electrodes and size of earth wire, (v) Depth at which the electrode is embedded and (vi) Material of electrodes and earth wire.

Effect of moisture content on earth resistivity: Moisture content is one of the controlling factors in earth resistivity which is expressed in percent by weight of the dry soil. Dry earth weighs about 1440 kg per cubic meter and thus 20% moisture content is equivalent to 288 kgs of water per cubic metre of dry soil. Above about 20% moisture the soil resistivity is very little effected while below 20% the resistivity increases very abruptly with a few percent decrease in moisture content.

Moisture alone is not the predominant factor in the low resistivity of soils for example, earth electrodes driven directly in the beds of rivers or mountain streams may present very high resistance to earth and again if the water is relatively pure he soil resistivity will be high unless the soil contains sufficient natural elements to form a conducting electrolyte. High moisture content in soils increases the solubility of existing natural elements to improve the soil conductivity.

Effect of temperature on earth resistance:  The temperature co-efficient of soil resistivity is negative but a negligible for temperature above freezing point. Below 00C as the wate4r in the soil begins to freeze, the temperature co-efficient increases tremendously and with the decrease in temperature the resistivity of soil rises enormously. It is therefore, recommended that in areas where the temperature is expected to be quite low the earth electrodes should be installed well below the frost line otherwise the earth resistance will become higher due to high resistivity of upper frozen soil.

Higher value of earth resistance can be improved by (i) using low resistance copper strip for every protective conductor, (ii) using more number of electrodes in parallel and larger mesh and (iii) using high conductivity treated soil around the earth electrode.

Arrangement for improving earth resistance in a dry place: In a dry place, where the soil resistivity is comparatively higher, multiple electrodes may sometime fail to produce an adequately low resistance to earth. The alternative is to reduce the resistivity of the soil immediately surrounding the earth electrode. To reduce the soil resistivity com high conductive substance are mixed with the natural soil in suitable proportion. The most commonly used substances are sodium chloride (NaCl), calcium chloride (CaCl₂), sodium carbonate (Na₂CO₃), copper sulphate (CuSO₄), soft coke and charcoal. In this arrangement the pipe earth electrode should be placed at a depth of minimum 3.75 metre and digging around the earth electrode to a depth of 2.5 meters from the bottom the pit should be filled with treated soil of about 0.3 to 0.5 meter around the pipe and crammed tightly. The pipe electrode should be perforated and provided with the arrangements of pouring the water at the top of the electrode. The pouring of water through pipe helps to dissolve the artificial agents and thereby reduces the soil resistivity. As a result the earth resistance is improved.

Ø  Type of earthing.

SN	PLATE EARTHING	PIPE EARTHING
1	Size of plate: 60mm x 60mm x 3mm	Pipe size: dia 25mm, 2.5 M length,
2	Length of pit – 3 mtrs	Length of pit – 3 mtrs
3	Plate will vertical	No of holes with 7cm gap in the pipe
4	Layer of salt & charcoal	Layer of salt & charcoal
5	Not less than 8 SWG copper wire	Not less than 8 SWG copper wire

Ø  Why charcoal is used?

It is a soft conductor so its conductivity increases

Ø  Why salt is used?

It maintenances wetness

Ø  Why grey color is used for electrical panel & motors?

Cooling efficiency in grey color is more compared to other colors.

Ø  What are the insulation materials?

Mica, Wood, Bakelite, Asbestos, Rubber, PVC, glass.