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Safety measures   electrical maintenace
 

Safety measures electrical maintenace

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    Safety measures   electrical maintenace Safety measures electrical maintenace Document Transcript

    • Vijay Balu Raskar – Electrical Maintenance – Polyphase Circuit Junior Science College ( Std – XI ) - Syllabus Theory – “ Safety Measures” Why Earthing is required???  Earthing means connecting the electrical equipment to the general mass of the earth by a wire which having negligible resistance. The potential of the earth is zero for all purposes.  Earthing or grounding is done for safety of equipment and human beings (including all animals and plants). Human Safety: If a person touches an appliance which has an earth fault in it he will not get an electric shock as his body (standing on the earth) and the equipment's body are at the same potential provided the equipment is earthed properly. Thus proper earthing protects a person from getting electric shock. Equipment Safety: The outer housing of an electrical equipment is earthed by directly connecting it to a earth grid or earth electrode, thereby providing a low resistance path to ground. In case of a fault involving earth the live part of the equipment gets connected with the low resistance earth path. This produces high earth fault current and the protective devices in the circuit disconnects the circuit from the power source thereby reducing further damage to the equipment. Other Safety:Neutral of electrical equipment are also earthed for equipment safety. Like, neutral of generators in power plants are earthed through Neutral Grounding Resistor to limit the earth fault current. Three phase transformer's neutral are earthed to provide neutral point to supply single phase loads like lighting and small appliances. ______________________________________________________________________________ Types of Earthing:There are two types of earthing (a) Plate earthing and (b) Pipe Earthing The important methods of earthing are the plate earthing and the pipe earthing. The earth resistance for copper wire is 1 ohm and that of G I wire less than 3 ohms. The earth resistance should be kept as low as possible so that the neutral of any electrical system, which is earthed, is maintained almost at the earth potential. The typical value of the earth resistance at powerhouse is 0. 5 ohm and that at substation is 1 ohm. (a) Plate Earthing:     Material Used :- copper plate or GI plate Dimension of plate: Copper plate of 60cm x 60cm x 3.18 mm thick or  GI plate of the size 60cm x 60cm x 6.35 mm thick  Copper plate is rarely used. GI plate is kept vertically at a depth of 3 m. Plate should be embedded in Coke or charcoal layers of 15cm around plate to reduce earth resistance. The material of earth wire and earth electrode should be same. Suggestions or difficulties are most welcome at vijayraskar2003@yahoo.co.in
    • Vijay Balu Raskar – Electrical Maintenance – Polyphase Circuit Junior Science College ( Std – XI ) - Syllabus  Plate is placed upward to connect maximum levers of earth.  If G.I. plate used then earth wire, nut, bolts & washers should be GI only.  If copper plate used then earth wire, nut, bolts & washers should be copper only.  In summer season, the soil becomes dry & this will increase the earth resistance.  To reduced earth resistance, add water in funnel which is placed at the top side. Disadvantages of plate earthing: Its very difficult to find fault, disconnection of earth wire and plate which is under the ground. Hence, repairing cost is high. (b) Plate Earthing:     Material Used :- GI pipe only Dimension of plate:GI pipe of the size 38.1mm in diameter & 2m long. GI pipe length is 2.75 if the soil is rocky and dry. Pipe is placed vertically upward and 4.75m in depth. Suggestions or difficulties are most welcome at vijayraskar2003@yahoo.co.in
    • Vijay Balu Raskar – Electrical Maintenance – Polyphase Circuit Junior Science College ( Std – XI ) - Syllabus  Pipe should be between alternate level of salt and Coke or charcoal 15cm around the pipe which reduces the earth resistance.  The material of earth wire and earth electrode should be same.  Earth wire should be GI wire only.  The earth wire 19mm diameter pipe is taken out through a 12.7mm diameter pipe and 60cm below the ground.  To reduced earth resistance and earthing efficiency, add water in funnel which is placed at the top side which increase the moisture level. Indian Electricity Rules for Earthing: The earth wire and electrode should be same. Suggestions or difficulties are most welcome at vijayraskar2003@yahoo.co.in
    • Vijay Balu Raskar – Electrical Maintenance – Polyphase Circuit Junior Science College ( Std – XI ) - Syllabus  The earthing plate should be at least 1.5m away from the building whose installation is to be earthed.  The size of the earthing wire should not be less than half of the live conductive wire.  The earth wire should be taken through GI pipe of 12.5mm diameter to give protection to earthing wire against damaging.  Use mixture of salt and coal around electrode to provide effective earthing.  All the earthing wires loop and connected to main board and then connected to earthing electrode.  The earthing electrode should be placed vertical inside the earth. So, it comes in contact with all different layer of earth.  A coating of bitumen compound should be given over the socket joint to protect it against corrosion.  Avoid jointing a branch earth wire to the main earthing wire. If interconnection is done than solder the joint. Apply bitumen coating at each joint.  When connecting earthing wire to electrode then use clean socket terminals and file the terminals which creates better contacts. Effects of electric shock on human body: Heart issues like damaging the heart into small pieces.  Stops breathing due to blockage in the nervous system.  Burning of the body.  Partial mental issues.  Physical damages.  Muscles, ligaments and tendons may tear as a result of the sudden contraction caused by an electric shock. Tissue can also be burned if the shock is lasting and the current is high.  Blindness:- Electric shock can also affect the eyes, causing cataracts to develop over time. Other disorders can appear in the weeks or months following the accident, depending on which organs the current passed through. Suggestions or difficulties are most welcome at vijayraskar2003@yahoo.co.in