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.

petroleum rule

Petroleum Rule

Petroleum industry is so called hazardous industry; numerous regulations have been framed by various statutory/regulatory authorities in the country to safeguard the interest of its stakeholder and environment. Over the years; the regulatory agencies have played a significant contribution in the promotion of industrial safety in the petroleum industry in India. Few of them are-

Petroleum rule

a)      Oil Industry Safety Directorate (OISD)

b)      Chief Controller of Factories (CIF)

c)      Chief Inspector of Boilers (CIB)

d)     Chief Controller Of Explosives (CCE)

e)      Chief Electrical Inspector (CEI)

f)       Director General of Civil Aviation ( DGCA)

g)      Tariff Advisory Committee (TAC)

h)      Bhaba Atomic Energy Commission (BARC)

i)        Director General of Mines and Safety (DGMS)

j)        Regional Transport Authority (RTA)

k)      Director General Of Dock Safety (DGDS)

l)        International Maritime Organization (IMO)

Other few petroleum rules adopted by the regulatory authority of India are –

a)      Petroleum Act, 1934

b)      Calcium Carbide Rules, 1987

c)      Petroleum Act, 1934

d)     Petroleum Concession Rules, 1949

e)      Petroleum Rules, 2002

f)       Petroleum and Natural Gas Rules, 1959

Petroleum Rules of 1976 deal with the safety guidelines / regulations for:

a)      Import

b)      Transport

c)      Storage

d)     Refining

e)      Blending Testing of Petroleum/ products

Gist of rule pertaining to electrical maintenance/design/inspection are-

Part-A: Electrical Installation

a)      There shall be no fire or artificial light or smoking on board near ship or other vessel or within 30 meters of the place where the petroleum is being loaded or unloaded.

b)      The pressure of the electric circuit shall not exceed 24 volts

c)      Electric wiring shall be heavily insulated and be adequate for maximum loads to be carried;

d)     To be provided with suitable over-current protection in the form of fuses or automatic circuit breakers and installed so as to be protected from physical damage and contact with possible product spill either by location or by being encased in metal conduct or other oil resistant protective covering; and

e)      All junction boxes sealed.

f)       The generator, battery, switches, fuses and circuit brackets shall be carried in the cab of the vehicle or in the engine compartment and the battery shall be in an easily accessible position with a heavy-duty switch close by to cut-off the battery.

g)      Generators, motors and switches thereof which are not installed within the engine compartment shall be of the approved flame-proof type.

h)      All petroleum pipelines entering any tank vehicle loading or unloading area shall be electrically continuous and be efficiently earthed.

i)        An earth bus with a flexible cable having robust clamping device shall be provided adjacent to the loading point.

j)        (3) Sound and electrically continuous hoses or metal pipes shall only be used for loading or unloading of a tank vehicle where stand pipes or metallic loading arms are provided; swivel joints shall be electrically continuous.

k)      The tank, filling pipe and the chassis of the tank vehicle shall, during loading of a tank vehicle, be efficiently bonded and connected with the earth bus by means of a flexible metal wire or tape.

l)         The bonding and earthing connections shall not be disconnected until loading of the tank vehicle has been completed and the filling and dip pipes thereof have been securely closed.

m)    No mechanically propelled vehicle for the petroleum shall be loaded or unloaded until its engine has been stopped and battery is isolated from the electrical circuit.

n)      The engine shall not be restarted and the battery shall not be connected to the electrical circuit until tanks and valves have been securely closed.

o)      The pipeline shall be protected against corrosion by suitable coating, strapping and where necessary by cathodic protection.

p)      No electric wiring shall be installed and no electric apparatus shall be used in any refinery, installation, storage shed and service station or in any other place where petroleum is refined, blended, stored, loaded or unloaded.

q)      Petroleum having flash point below 650C or any inflammable gas or vapour in a concentration capable of ignitions is likely to be present; is said hazardous area.

r)       A hazardous area shall be deemed to be-

(i) A zone "0" area, if inflammable gases or vapours are expected to be continuously present in the area; or

(ii) A zone "1" area, if inflammable gases or vapours are likely to be present in the area under normal operating conditions; or

(iii) A zone "2" area, if inflammable gases or vapours are likely to be present in the area only under abnormal operating conditions or failure or rupture of equipment.

s)       If any question arises as to whether hazardous area is a zone "0" area or a zone "1" or a zone "2" area, the decisions thereon of the Chief Controller shall be final.

t)       No electric apparatus shall be allowed in a zone "0" area.

u)      A flame proof or intrinsically safe apparatus of a type or types approved in writing by the Chief Controller.

v)      An industrial-type apparatus housed in an enclosure or a room which has been made safe by pressurizing or purging with a plenum of atmosphere free from significant concentration of any inflammable gas or vapour and so arranged and inter-locked that in case of failure of the pressurizing or purging agent, the electricity supply is automatically cut off or a warning is automatically given to a person in attendance who shall take suitable measures to prevent a hazard.

w)    All electric apparatus installed or used in a zone "2" area shall be a non-sparking apparatus of a type approved by the Chief Controller.

x)      All conductors of an intrinsically safe circuit in connection with an intrinsically safe apparatus installed in any hazardous area shall be so laid as to prevent invasion of such circuit by current arising from contact or electrostatic or electromagnetic induction from any other circuit. Conductors of intrinsically safe circuits shall be effectively protected against mechanical damage.

y)      All electric wiring in a hazardous area, other than the conductors of an intrinsically safe circuit, shall be effectively sealed at all joints.

z)      Approved armoured cable with correctly designed terminations, complete with armour clamps, the armouring being carried and electrical clamps to provide mechanical support to the cable and electrical continuity.

aa)   single or multicored insulated cables accommodated in solid drawn heavy gauge screwed galvanized conduits used in conjunction with approved flame-proof fittings, the conduit being sealed at both ends and installed in such a manner as to permit internal condensation to drain to a point or points from which it may be removed; where a conduit runs from a zone "1" area to zone "2" area or an area which is not hazardous area, an adequate seal shall be provided outside the boundary of zone "1" area.

bb)   Single or multicored mineral insulated cable of approved type in conjunction with approved flame-proof type glands at all joints and terminations.

cc)   Bare conductors contained in an approved flame-proof or forming part of an intrinsically safe circuit.

dd) Insulated cables without metallic sheathing or armouring but accommodated in a conduit shall not be buried in the open ground in any hazardous area.

ee)   The electric supply circuits of each electric pump in a hazardous area shall be-

(i) Separately protected by a fuse or circuit breaker set to operate when the current in the circuit so exceeds the rated current for such a period of time as to involve danger and

(ii) Provided with an individual isolating switch at the main supply point for each electric pump including its integral lighting system, if any.

Part-B: Earthing and Boding

a)      All electrical systems, equipments, all structures, plants and other non-current-carrying metallic parts of major electric apparatus or any major metallic object in any place where petroleum is refined, blended, stored, loaded or unloaded shall be efficiently earthed.

b)      The resistance value of earthing system shall not be more than 4 ohms in the case of electrical systems and equipment or a valve.

c)      The resistance value shall be 10 ohms in the case of all non-current-carrying metallic parts of major electric apparatus.

d)     All joints in pipeline, valves, plants, storage tanks and associated facilities and equipments for petroleum shall be made electrically continuous by bonding or otherwise; the resistance value between each joint shall not exceed 1 ohm.

e)      A piping which is not in electrical contact with the associated tank or vessel shall be efficiently connected to such tank or vessel by a flexible conductor and earthed.

Part-C: Cathodic Protection  

a)      Cathodic protection system if employed; shall be designed and installed such that it does not  affect adversely metallic bodies in the zone of protection and  to eliminate the danger of sparking in a hazardous area.

b)      The metallic structures, pipelines, valves, plants and associated equipments under cathodic protection shall not be broken/disconnected for repair or maintenance unless a heavy gauge conducting cable is clamped to each side of the intended break for establishing between them an electrical bond and the cable shall remain clamped until repair or maintenance work is completed and connection restored.

Part-D: Protection against Stray Current

a)      Where high values of stray currents are likely to exist, both the rails of spur lines shall be insulated from a railway siding which is used for the loading or unloading of tank wagons.

b)      On electrified railway systems, live-contact rails and overhead electric lines shall be terminated outside the area where tank wagons are loaded or unloaded, such rails or overhead lines shall not be allowed within a refinery or an installation.

c)      No joint in a pipeline or associated equipments shall be broken

Part-E: Portable Electric Apparatus

a)      Any portable electric lamp or apparatus shall not be used unless approved by the Chief Controller’

b)      No mains operated portable lamp shall be operated at a voltage exceeding 25 volts above the earth Provided that the Chief Controller may, at his discretion, allow higher operating voltage not exceeding 55 volts above earth.

c)      All portable lamps or apparatus other than a self-contained lamp or apparatus shall be connected to the mains as the Chief Controller may specify.

Part-F: Maintenance of Apparatus and Wiring

a)      All electric apparatus and wiring shall be maintained in hazardous area all the time so as to retain the characteristic for which their approval has been granted.

Part-G: Repair and Test 

a)      No flame-proof or intrinsically safe apparatus shall be opened and no work likely to impair the safety characteristics of such apparatus or electric wiring connected thereto shall be carried out until all voltage has been cut off from the said apparatus or wiring.

b)      The voltage shall not be restored until the work has been completed and the safety characteristics provided in connection with the apparatus and wiring have been fully restored as original.

c)      Notwithstanding anything contained in this rule, use of soldering apparatus or other means involving flame, fire or heat or use of industrial type apparatus in a zone "1" area shall be permitted for the purposes of effecting repairs and testing and alterations, provided that the area in which such apparatus or wiring has been installed, has first been made safe and certified by a competent person after testing with an approved gas-testing apparatus to be safe and free from inflammable vapours, gases or liquids and is maintained in such conditions, so long as the work is in progress.

Part-H: Certificate of electric installation

a)      A certificate from competent person shall be required before engineering any electric circuit and any electric apparatus in hazardous area for the first time and after each repair, maintenance or alteration work carried out.

b)      The certificate shall be preserved by the occupier of the premises and shall be produced to the Inspector on demand:

c)      The certificate issued for each repair and maintenance work need not be preserved for a period exceeding six months.

Part-I: precaution against corrosion

a)      All electric apparatus and equipment carrying electric wiring in a hazardous area shall be regularly treated with a suitable protective paint.

b)      The certification label indicating the flame-proof or intrinsically safe nature of the apparatus or equipment shall not be painted.

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