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Guideline for Earthing of Buildings & Industrial plants

This guide covers the earthing of Domestic, Commercial buildings and Industrial Plants. This guide is prepared after referring to BS 7430, IEEE 142, EN 50522 and IS 3043. Readers should note that this guide is supplementary to these standards. Readers have to refer the standards before designing a system.

Before starting the design the designer should have Earth resistivity of the site, Fault current calculation results, Details about the regulation, Select the material of earthing conductor, Earthing conductor Sizing, Layout of the industry/Site/Plant, Type of source earthing to be used, Resistance of earthing system to be achieved

The Objective of earthing design is to carry out Sizing of earthing conductors, Earthing system resistance calculation and determine number of vertical driven earth rods required, Preparation of earthing layout, Estimation of total quantity of earthing materials and Prepare Bill of material BOM or Bill of Quantity BOQ. Earth Fault current is calculated using standard IEC 909 either manually or using a computer program

Earthing Conductor material can be copper, aluminium or steel . The choice of material depends on the owner’s specification, type of site etc. Earthing conductor sizing S = I tk     (BS 7430 - CL 9.7), I is fault current in A rms, and t fault duration in sec, S is in mm2

Different types of LV earthing system (BS 7430) are TN-S, TN-C, TN-CS, TT, IT. Selection of the type depends on local regulations. Source side earthing type can be T or I, T : effectively earthed ,  I : un earthed. Load side earthing can be T or N. T : load side has own earthing terminal or earth electrode, N: Load side earthing system connected to source side earthing

Earth Loop Impedance is the impedance seen by the fault current from the fault location upto the neutral point of the source where the current returns back. This impedance does not influence the fault current if solidly grounded system is used. It affects the potential rise of non conducting metal parts during a fault.

Resistance of earthing system, the earthing system consisting of several earth electrodes and interconnecting horizontal conductors gives a earthing resistance which is much less than the resistance of single electrode. Different standards specify different values. Some standards don’t specify any value. However it is a practise to achieve the resistance of the earthing system to be 1Ω.

Different types of electrodes are Plate, Pipe/Rod, Strip, Mesh. A given earthing system may consist of all of these or few of them. Normally strip electrode (Horizontal) and Rod electrode (vertical) will be used more often. Plate electrode is used to earth the neutral of LV transformers due to large area of the plate.

R plate  = 4A, = resistivity of soil Ωm, A area of plate m2 (BS 7430 CL 9.5.2, IS 3043 CL 9.2.1). R rod = 1n2πLln 8Ld -1+LSln 1.78n2.718   (BS 7430 Cl 9.5.4), n number of rods, s spacing between rods.

Typical Earthing Layout of LV MV Substations

Detailed guidelines for Earthing of Buildings and Industrial plants consist of 35 sections with 34 pages. This detailed guideline gives many practical examples, calculations, illustration which will help an application engineer to actually design a plant. This full guideline can be accessed in below link.

Contents of the detailed guideline are as given below

1.0 Scope
2.0 Exclusions
3.0 Reference Standard
4.0 What is the function of earthing
5.0 What is the difference between Earthing of Substation and Building
6.0 Basic requirement before starting the earthing design
8.0 Objective of earthing design
9.0 Fault current calculation results
10.0 Material of earthing conductor
11.0 Earthing conductor sizing
12.0 Current density limitation at electrode
13.0 Minimum dimensions of conductors
15.0 Source earthing
15.6 Transformer neutral earthing
15.7 Earthing of UPS neutral
16.0 Types of earthing based on resistance
16.1 Solidly grounded system
16.2 Unearthed or ungrounded system
16.3 Reactance earthed system and resonant earthed system
16.4 Resistance earthed system
16.5 Types of Resistance earthed system
17.0 Different types of LV earthing system (BS 7430)17.1 Earth Loop Impedance
18.0 Potential gradient around the earth electrode
19.0 Earth Resistivity measurement
20.0 Resistance of earthing system
21.0 Measurement of electrode resistance
22.0 What all needs to be earthed
22.1 Source Neutral earthing
22.2 Cable Armour earthing
22.3 Cable Tray earthing
22.4 Electrical Panel and Distribution boards earthing
22.5 Junction Box earthing
22.6 Motor and Push button station earthing
22.7 Lighting poles and fixtures earthing
22.8 Tanks, Vessels, Piping earthing
22.9 Package equipments earthing
22.10 Lightning Protection system earthing
22.11 Electronic equipments earthing system
22.12 Earthing of utility pipes
22.13 Earthing of steel reinforced bars of structures and buildings
23.0 Earthing Schematic
24.0 Types of Electrodes and their resistances
24.1 Resistance of Plate electrode
24.2 Resistance of Rod or Pipe electrode
24.3 Resistance of Rod electrodes in parallel
24.4 Variation of resistance of electrode due to length and diameter of the rod (Table 5 and Table 6)
24.5 Resistance of straight Strip
24.6 Resistance of Mesh
24.7 Resistance of electrodes encased in Low resistivity materials
24.8 Earthing of steel reinforced concrete foundations
25.0 Treated earth electrodes
26.0 Auxiliary earth grid
27.0 Typical Earthing Layout of LV MV Substation
28.0 Layout Requirement
28.1 Spacing between electrodes
28.2 Distance between electrode and building wall
28.3 Depth of horizontal conductor or connecting conductors
29.0 Stray Currents
30.0 Common mode noise
31.0 Typical Calculation of Earth electrode resistance of substation
31.1 Resistance of Rod electrodes in parallel (BS 7430)
31.2 Resistance of straight Strip (BS 7430)
31.3 Resistance of Mesh (BS 7430)
32.0 Type of joints
33.0 Recommended dimensions of earthing Conductor (Table 7)
34.0 Typical Earthing design of Oil and Gas installation
34.1 Calculation and steps for typical Oil and gas installation earthing design
35 Short Circuit Current calculation for fault at Motor terminal and Earthing conductor sizing.
36 Fault Current paths
37.0 Reference

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