CHAPTER - Four Grounding and Lightning arrestors 4.1. Introduction Electrical connection of neutral point of a supply system or the non current carrying part of electrical equipments to the general mass of earth in such a manner that all times on immediate discharge of electrical energy Why is the practice of earthing electrical system? To provide a sufficiently low impedance To retain system voltages with in reasonable limits under fault condition ( Lightning\, switching surges, or contact with higher voltages system) The potential of an installation is measured with respect to the general mass of earth or commonly called Earth 1
Earthing Conductors: A protecting conductor connecting the main earthing terminal of an installation to an earth electrode or other means of earthing. Bonding: Is the practice of intentionally electrically connecting all exposed metallic parts not designed to carry electricity in a room or building to protect from electric shock. Equipotential Bonding Electrical Connection maintaining various exposed conductive parts and extraneous conductive parts as substantially the same bonding 2
Bonding Conductor: A protective conductor providing equipotential bonding There are different earthing arrangements: IT system TT system TN system TN-C-S The first letter defines the situation of the neutral point in relation to earth T: Solidily earthed to neutral I : unearthed or high impendence earth to neutral The Second letter defines the connection method of electrical installation exposed conductive path T: The exposed conductive parts are interconnected and solidly earthed, regardless of whether the neutral part is earthed or not N : The exposed conductive parts are directly connected to neutral conductor 3
The third and Fourth letter indicates the arrangement of earthed supply conductor system C: Combined N : Separate IT system earthing The neutral is earthed or unearthed via high impendence An impendence b/n 1 kohm and 2 kohm are frequently used TT Earthing system The neutral is directly earthed, The exposed conductive parts of the loads are interconnected, either together or in groups or individually earthed 4
TN Earthing system The neutral point is directly earthed The exposed conductive parts of the loads are connected to the neutral conductor There are two possible There are two possible system depending on whether the neutral conductor (N) and protective conductor (PE) are on the same or not TN C Earthing system The Neutral and protective conductors form a single conductor called PEN It is advised to connect PEN to ground TN s Earthing system The Neutral and protective conductors are separate It is advised to regularly to connect the protective conductor to earth 5
TN C S Earthing system With this arrangement the main distributor neutral is also used to return earth fault current arising in consumer installation safety to the source. To achieve this the distribution will provide consumer earthing terminal which is linked to the coming neutral conductor Grounding Design consideration System Earthing The system earthing, that is to say the connection between the transformer neutral points and earth, is of high importance to the behavior of a power system during an unsymmetrical fault Equipment Earthing Earthing all metal work of electrical equipments earthing other than which are normally live or current carrying. Soil resistivity The resistance to earth of an electrode of given dimension dependent on the electrical resistivity of the soil in which it is installed 6
Element of Electrode Electrode Driven to earth wider earth contact area usually 1 to 2 m length and 16 mm diameter will give relatively low resistance Plate Electrode They offer a better grounding system than electrodes are generally used in permanent communication and computer sites Iron or steel type: Not less than 6.3mm thickness ( B/s Iron rusts) Copper type: Not less than 3.15mm thickness Plate electrodes shall be the size of at least 60cm X 60cm Plate electrodes shall be buried at a depth of not less than 1.5m 7
Soil resistivity testing The resistance of an electrode highly influenced by a resistivity of the soil Soil resistivity is the measure of a resistance b/n opposite sides of a cube of soil with a side dimension of 1 meter R l A Where : ρ resistivity in Ω m l length in m A Area in mm2 Soil resistivity values widely depending up on the type of terrain Eg: silty on a river bank 1.5 Ω m Dry sand or granite 10,000 Ω m Factors affecting resistivity Type of earth ( Clay, loam, sand stone, granite) Stratification : layer of different soil Moisture content ( M ρ ) Temperature Chemical composition and concentration of dissolved salt 8
Type of soil or water Typical resistivity ( Ω m) Sea water 2 0.1 to 10 Clay 40 8 to 70 Ground well and spring water 50 10 to 150 Usual limit (Ω m) Clay and sand mixture 100 40 to 300 Sand 2000 200 to 3000 Solid granite 25,000 10,000 to 50, 000 Ice 100,000 10,000 to 100,000 Soil resistivity testing Methods Wenner Method It requires the longest cable lay out, largest electrode spreads and for a large spacing one person per electrode is necessary to complete the measurement V d d d I V 2 d I 2 dr Where d spacing b/n in m 18 9
Schumbefer array The outer electrode are moved four or Five times of each move of the inner electrodes Lowe voltage reading are obtained during this method V d a a d I 2 d R 2a 2 d R x where, x 2a 19 Driven rod Method The driven rod method called three pin or Fall 0f potential method b It is normally suitable for use in circumstances such as transmission line structure earths or areas of difficult terrain 0.62d d V I 2 br 8b ln( ) D Where: b The driven electrode length R=V/I D diameter of driven rod 20 10
Protection against electric shock Direct contact Refers to a person coming in contact with a conductor which is live in normal circumstance Indirect contact Refers to a person coming in contact with an exposed conductive parts of which in not normally alive bus has become alive accidentally Protection against electric shock Direct contact By means of barrier enclosure ( Key, By mean of obstacles ( By replacing out of arm s reach By using Residual current device ( RCD) fast sensitive tripping device 11
Protection against electric shock Indirect contact 1 st Level protection: Earthing all exposed conductive parts electrical equipments [ Equipotential bonding ] 2 nd Level protection: Automatic disconnection of the supply Protection against electric shock Indirect contact 1 st Level protection: Earthing all exposed conductive parts electrical equipments [ Equipotential bonding ] 2 nd Level protection: Automatic disconnection of the supply 12
Protection against lightning Lightning can be linked to distrubtive electrical discharge due to the dielectric breakdown of the air between the clouds or between clouds and the ground. Breakdown, which is visible in the form of lightning flash, it accompanied by a sound wave, thunder Types of lightning within cloud cloud to cloud cloud - to - ground Protection against lightning 13
Protection against lightning Thank you! 14