chemical earthing

Chemical Earthing:

An effective earthing system is a basic requirement of power distribution/supply system for operation and safety reasons. Lightning, insulation failure, over current etc are common faults that they keep the electrical system in danger and can lead to shock and finally sometime be the cause of casualty. A well designed earthing system of any electrical supply is required for –

a)      Equipment earthing for personnel safety. 

chemical earthing

b)      System neutral earthing 

c)      Protection against Static and Lightning discharges.  –

d)     Separate clean earth system for UPS, Instrumentation, DCS, ECS

Earthing system has been covered in various standards -

a)      India: IS 3043 :1987 - Code of practice for Earthing

b)      Great Britain   : BS 7430 - Code of practice for Earthing

c)      USA: UL467  : Grounding and Bonding Equipment

d)     Germany         :DIN VDE 0100 – Earthing arrangements, protective conductors, equipotential bonding conductors

e)      France             : NFC 15-100 – Low voltage electrical installations

The purpose of grounding is to ensure that all equipments other than live parts shall be at earth potential. To ensure that operator or attendant or any person shall be at earth potential at all times to avoid shock in case of short circuit or fault current. Hence, low earth resistance is obtained by an efficient earthing. Conductors of suitable section are used in order to carry expected current. The grounding has many advantages as such it eliminates the surge voltage and also discharge the over voltage to the ground. The grounding provides the safety to the equipment and improves the power reliability.

Grounding:

The current carrying part is connected to the ground. Grounding may not have zero potential. It is mostly used to protect machine/equipment. It is applied to provide the return path to the current. It has type like solid, resistance, and reactance grounding. It facilitates in balancing the unbalance load and as a safe guard to reduce insulation voltage.

Example: Neutral grounding of transformer and generator.

Earthing:  

The non-current carrying parts are connected to ground and it is also called body earthing. Earthing has generally zero potential. It is used for human safety. It is applied to discharge the current to earth. It has type like pipe, plate rod and strip earthing. It facilitates for avoiding electrical shock.

Example: enclosure or body earthing of any electrical equipment /machine.

Characteristics of good Earthing system

·         Low resistance/ impedance

·         Excellent electrical conductivity

·         Should require  minimum maintenance

·         High corrosion resistance

·         Conductors of the sufficient mechanical strength with standing high fault currents with no evidence of the fusing or deterioration.

·         Lower earth resistance ensures that energy is dissipated into the ground in less time.

·         The choice of the material for grounding conductors to be taken in care because grounding system will be buried in the earth for many years.

·         High hot dipped galvanized electrodes should be used to for this purpose.

·         Mechanically robust and reliable.

·         Dry sand, lime stone, granite and any stony ground should be avoided

·         Earthing electrode should not be installed on high bank or made-up soil.

Conventional Earthing

Conventional earthing uses a copper plate or GI plate buried at the bottom of pit. The bore is filled up with layers of salt and charcoal. Salt dissolve in the water and dries up gradually. It needs maintenance and pouring of water at regular intervals. Earth value will vary due climate conditions e.g. summer, winter and rain. It has variable ohmic value and minimum in the rainy season.

Note: Readers are requested to visit Earthing Chapter in protection system.

Chemical earthing

Chemical earthing uses bentonite or graphite based compound along with earthing electrode either a pipe or a rod made of copper or galvanized iron (GI) that helps in improving soil condition and reducing the soil resistivity.

The pipe containing the flat strip is packed with chemical compound and called backfilled. The surface area for dissipation of heat generated is increased. Installation is easier as electrode is dug into ground up to appropriate length of 2 meter or 3 meter. Where the resistivity of soil is high, it is more effective as such the backfilled compound provides low resistance value. no need to pour water at regular interval except in sandy soil so it is also called maintenance free earthing. It has longer life compare to conventional earthing. It has Constant ohmic value.

chemical earthing compound

Generally salt and charcoal are used to improve the earth pit resistance which is as per IEEE 80 14.5 (a). It has certain drawback as salt and charcoal leach to the soil so we have to maintain the earth pits in regular intervals.

Chemical compound enhances the soil conductivity like Bentonite and other chemicals. As per IEEE 80 14.5 (d), there are certain compounds which has resistivity less than 0.2 ohm meter  which has a life span of more than 10 to 12 years.

Component of chemical earthing

·         Pipe or Rod

·         Backfilled compound

·         Earth pit cover  

·         Fasteners

·         Connecting link

·         Earth strips

Here we will discuss backfilled compound and cover, rest is same as in conventional earthing.

Earth Pit Cover

The conventional pit cover is made of RCC or cast iron. It is cumbersome and costlier. In comparison to conventional pit cover, the chemical earth pit cover is of light weight and cheaper.

It is made of heavy duty FRP (Fiber Reinforced Plastic).FRP material also provides mechanical strength & less water absorption. Green colour body and green top cap matches the earthing.

Typical size: 260 mm x 220mm x160mm or 350 x mm275mm160 mm.

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Backfill Earthing Compound

It improves conductivity of the earth electrode and ground contact area. Compound enhances the earthing effectiveness, especially in areas of poor conductivity (rocky ground, areas of moisture variation, sandy soils etc.).

Compound typical name: marconite or bentonite

Features:

·         Typically it is non-corrosive in nature having low water solubility but highly hygroscopic.

·         It is non explosive, does not cause burns, irritation to eye, skin etc.

·         It has high conductivity, improves earth’s absorbing power and humidity retention capability.

·         It is so called maintenance free

·         Typically it is thermally stable between -10 deg.C to + 60 deg.C ambient temperatures.

·         It is suitable for copper or GI type of electrode and different kinds of soils of different resistivity.

·         granule size 0.1 mm to 3 mm

·         It is little alkaline in nature with pH value >7 but <9.

·         User demands -Test certificate from NABL approved lab.

Quantity of backfill compound

For Example:-

·         To backfill an earth pit containing 1 no. copper earth rods in 3.m deep, 0.25m diameter bore 1 holes will require the following volume of Bentonite clay compound:

·         Volume of bore hole

·         =  π r² x h

·         =  π x (0.25/2)² x 3.0

·         = 0.1473m³

·         Assumption: 1 x 25kg Bentonite = 0.0566m³ (Wet)

·         Number of Bags = 0..1473 / 0.0566= 2.6 Bentonite Bags

·         In Kg = 25 x 2.6 = 65 KG

Value of the resistance is moderate when the rod length is between 2.5-3 meter. So it is meaningless to shorten the rod length or increase the rod length. Normally the pipe or rod has approximate same value although rod has longer mechanical strength and life.

Distance between earth electrodes.

In practice, the distance is equal to the driven depth. Little is to be gained by separation beyond twice the driven depth. A substantial gain is effective even of 2 meter of separation.Pl refer to IS-3043-1987.

In the high resistivity soil area, bentonite clay can be used to decrease the grounding resistance effectively. It consists of hydrous aluminium silicate and acts as an excellent backfill if sufficient amount of water is added to it. Credit- 16th national power systems conference, 15th-17th December, 2010

Typical test value

Case-I                                                

·         Length and dia of the earthing electrode       : 3m & 75 mm

·         Type of compound                                          :marconite

·         Length and dia of the rod                               :3m & 16mm

·         Material of rod                                                :copper

·         Rated short time withstand current                :20 KA for 1 sec

·         Earth resistance                                               : 1.56 Ώ

·         Resistivity of concrete                                    : 30-90 ohm-M

Case-II                                               

·         Length and dia of the earthing electrode       : 3m & 100 mm

·         Type of compound                                          :marconite

·         Length and dia of the rod                               :3m & 40mm

·         Material of rod                                                :copper

·         Rated short time withstand current                :40 KA for 1 sec

·         Earth resistance                                               : 1.03 Ώ

·         Resistivity of concrete                                    : 30-90 ohm-M

Typical pipe specification

1) Type of Earthing                 : Sealed maintenance free chemical filled earthing electrode.  

2) Diameter of the pipe           : 48 mm

 3) Thickness of the pipe         : 3.9 mm

4) Length of the pipe              : 3000 mm

5) Galvanized Thickness         : 80-100 micron  

6) Type of galvanizing            : Hot dipped  

7) Size of the conductor         : 25x8 G.I. strip  

8) Terminal diameter               : 12mm

9) Chemical used in the pipe   :  CCM (Crystalline conductive material), Approx. Weight-5kg

Price and technical specification

Sr,	Description	Unit of measurement	Price each(Rs,)
1	Supply and installation of Maintenance free Chemical Earthing using Electrode of size 40 mm dia, 03 meter long MS rod , complete with excavation, civil works, FRP  . The voltage between Neutral & Earth not to exceed 1 volt & IR value less than 1 ohm. The earth resistance shall be as per IS 3043 including  Labor Charges for completing  the process Installation,  commissioning & Testing of earthing pits –	Each	28650
2	Backfill Earthing Compound, type tested from NABL accredited lab, pH value >7 but <9. Resistivity < 0.2 ohm meter, granule size 0.1 mm to 3 mm.	KG	15
3	GI strip 50mmx6mm Flat , Hot dip,  86 to 100% Zink in single band	Meter	270
4	GI strip 25mmx6mm Flat, Hot dip,  86 to 100% Zink in single band	Meter	130
	GI strip 32mmx6mm Flat, Hot dip,  86 to 100% Zink in single band	Meter	167

Note: 50 kg backfill compound will be required per electrode. 10 m GI strip will be required per electrode.