DESIGN OF EARTHING SYSTEM

A suitably designed earthing has been provided as a safety measure for the electrical system of ROP.

 Soil  Resistivity

 The measured value of 100 ohm-m has been taken for the calculation of the step and touch potentials.

Fault current levels

System Voltage	Short circuit level
415 V	50kA

Fault Current Duration

 The fault duration of  0.5 sec is considered for sizing of earthing conductor.

Jointing Method

 Joints of equipment earthing conductor with main earthing conductor & main earthing conductor to electrodes should be designed such that it resists deterioration and fusing of brazed / bolted joints under the most adverse condition of fault current magnitude and fault duration.

The maximum allowable temperatures for copper and steel are as follows : (The information of steel are given for comparison)

Copper	For brazed joints	-   450 deg C
	For bolted joints	-   250 deg C
Steel	For welded joints	-   620 deg C
	For bolted joints	-   310 deg C

Corrosion allowance

The degree of corrosion for copper in soil is negligible, whereas corrosion of steel in most soils increases with the decrease in resistivity of soil. Since the soil resistivity data of proposed site indicates values above 100 ohm meter at 0.6 to 1m depth the intensity of corrosion expected is very mild and no corrosion allowance is normally required. However, to ensure the reliability of underground steel earthing conductors corrosion allowance of 15% is to be considered, in case steel is used as an earthing conductor. The steel above the ground level is galvanised to protect from the atmospheric corrosion.

Depth of burial of conductor

The earthing grids may be buried at a depth varying from 0.6 to 1m. By increasing the depth of burial from 0.6 to 1, the reduction obtained in length of buried conductors is not considerable, as compared with the cost of excavations. In locations where extra cost of excavation is not required, the depth of burial is taken as 1m. The conductor will be laid along the width in the ground, in case flat conductor are adopted.

Applicable codes/classification

I)  IS 3043 - 1989             Code of practice for earthing.

II) IEEE  Std.  80 - 1986  Guide for safety in alternating current substation grounding.

Material Selection

The two important factors considered while selecting the material for the earthing conductors are:

i)   resistance to corrosion in the surrounding  medium .

ii)  conductivity (so that the voltage drop in the conductor  is negligible in comparison to that in the soil).

      The Parameters of Copper & Steel are indicated for comparison in Table XV.

Copper is an ideal earthing conductor material as it meets all the requirements. Installation and handling of copper conductors are easy because of their small size and cross section. Hence copper has been chosen as the material for the main earthing conductor in the plant. For the earthing of cable trays, fences and lighting posts, steel has been used.  Since steel is not resistant to corrosion to the same extent as copper, a corrosion allowance of 15% has been considered to ensure the reliability of underground steel earthing conductors.  The steel above the ground level is galvanised to protect it from atmospheric corrosion.

Earthing provided in the main process building as well as in the sub-station is made of annealed, soft drawn type bare copper conductor. Stranded Copper conductors have been used for formation of earthing grid and also for connections from equipment to earthing grid.

Standard procedures / formulae as recommended in IEEE-80 and IS 3043 have been used for the calculation of various parameters used in the design of the earthling system.  These include cross section area of earthling conductor, permissible and actual values of step  &  touch  potentials, 

resistance of earthing grid, distribution of zero phase sequence current between earth wire and ground mat etc.   

Layout of earthing

Total length of buried conductor required to control the step and touch potentials is about 10 km. Spacing of parallel conductors for formation of the grid is about 10m.  The earthing grids are buried in the soil at a depth varying from 0.6 to 1.0m based on soil resistivity value of above 100 ohmmeter at the plant site at these depths.

i) Outdoor earthing

Grounding loop of copper conductor of 190 square mm size has been laid at a distance of about 1.5m around sub-stations, main process building, service building and other allied structures at a depth of about 1m. The grounding loop in each area is connected to the adjacent grounding loop of another area by a minimum of two independent copper conductors of 190 square mm size. Copper plate electrodes 6 mm thick & 600 square mm area are located along the run of the earthing loop.

Transformer neutral, lightning arresters, down conductors of lightning protection system & communication circuits are all directly connected to the plate electrodes.  These electrodes in turn are connected to the plant's earthing system.  Earth leads from transformers' neutral are connected to two separate earth electrodes.  Facility has been provided to disconnect the earth electrodes from rest of the system to enable measurement of the resistance of the earth electrodes.  Properly treated earth pits have also been provided at suitable locations near transformer neutral grounds for the discharge of lightning current.

ii) Indoor earthing

2x95 square mm copper conductor lay in each floor of the building form the main earthing loop inside the buildings.  This is connected to the outdoor earthing loop at least at two points. Horizontal run of the conductor in each floor is provided by placing the conductor between layers of reinforced steel with a provision of 100mm covers.  In the vertical run along the wall, column, ceiling etc., the conductors are cleaned along the surface at regular intervals.

iii) Equipment earthing

Tails from the main earthing system and/ or the earthing rods provided near the equipment are connected to the earthing terminals of the equipment.

Materials and the size used for equipment earthing in the plant is indicated in Table XVI.

iv) Earthing of motors

The frames of all electric motors are connected to the grounding system by two distinct paths. Sizes of the grounding leads have been decided based on the power ratings of the motors.

v) Earthing of electrical panels

The internal earthing bus of each electrical panel s connected at both ends to the plant's ground system by means of an earthing conductor.

vi) Instrumentation earthing

All the instrument chasis and housings have been connected to the plant's earthing network with earthing conductors of suitable sizes.  The signal ground (Zero signal reference point) has been insulated from the chassis ground in all the control equipment.  Within any one measuring system the signal ground is connected to the plant's ground at only one point.   The signal shield is connected to the zero signal reference potential at only one point namely where the signal source is earthen.  (Applicable for low-level signals such as thermocouples).  The overall shield is connected to ground at only one end of a cable run.

vii) Earthing of cable armour

The metallic sheaths of single conductor power cables carrying AC is grounded at one end only. Rest of the sheath is kept insulated from ground in order to eliminate circulating current inside the sheath.  The metallic sheath of a three-phase cable is connected to plant's ground bus at both the ends.

viii) Earthing of other structuresAll steel structures accessible to personnel such as metal enclosures, cable trays, steel stairways etc., are connected to the plant's earthing system.  Security fences and lighting posts are earthen independently through conductors of galvanised steel.

7.7.9        LIGHTNING PROTECTION SYSTEM :

The Lightning protection system shall be designed as per guidelines of IS–2309 : 1989 – Code of practice for protection of buildings and allied structures against lightning and Indian Electricity Rules. All the structures in PREFRE complex, including the stack, shall be covered in the lightning protection scheme adopted for ROP Project. The lightning current is discharged to earth without passing through the structures or causing damage by fire, flash-over etc. The lightning protection –

system shall consist of horizontal roof conductors with air termination and down conductors, made of copper, for the main plant and allied buildings. The tall structures like stack and boiler chimney

shall be provided with vertical rods connected together by horizontal roof conductors and down conductors, all made of copper.  Vertical rods are provided to give additional protection.  All the down conductors are connected to the earth electrodes.  The earthing electrodes of the lightning protection system are interconnected with the main earthing grid of the plant.

Design of the lightning system

The lightning protection system for ROP has been designed generally in accordance with the code of practice IS 2309-1989.

The main buildings of the plant namely the Low Block, the High Block and the Stack have been provided with independent lightning protection systems.

Lightning protection system for the stack is designed with vertical air termination and horizontal

conductor on roof periphery.

7.8  ELECTRICAL FIRE PROTECTION

Various design provisions made in Electrical system of ROP Project for prevention and propagation of fire in critical areas are summarized herein under:-

7.8.1 CONTROL ROOM

Fire retardant low smoke (FRLS) cables are used for control wiring of all the systems in control room.Power wiring and control wiring are segregated through separate cable trays and/or conduits. Cables used for power wiring is departed suitably.  Load factors used in design of such cables are restricted to 50% only to avoid overloading of cables under any circumstances.

Cable fire sealing will also be provided for cables entering in the control room in due course of time.

7.8.2 CELL AREAS

No electrical installations are made in any cell except dissolver cells. Electrical cables used in dissolver cells are fire retardant low smokes, radiation resistant CSPE sheathed type. Dissolver cell loads (crane, lighting etc.) shall be fed by class-IV power supply. Cable fire sealing will be provided for all the cables entering dissolver cell for containing spread of cable fire.

Electric motors used in dissolver cells are totally enclosed type and will not catch fire generated outside. Dissolver cell lighting is connected through a special shielding plugs, which is not likely to catch and/or transmit fire to any other object, whatsoever.

Load factors used in design of such cables are restricted to 50% only to avoid overloading of cables under any circumstances.

7.8.3 DAY TANKS

Concrete fire barriers with 3 hours fire rating shall be provided between day tanks for DG sets. The fire barrier shall be provided between day tanks upto a height of 1 m above the top of day tank.

7.8.4 PLANT AREAS        

Cable fire sealing and fire resistant coating will be provided for al the cables installed in important plant areas viz. FHA, laboratories, tank space, MUA, access galleries, sampling corridors etc. for prevention and propagation of electric cable fires.

Power and control cables are segregated and separated in different cable trays to prevent propagation of fire from power to control cables. Control cables are installed at bottom most tier of cable trenches in perforated covered cable trays for the same reasons.