Earthing UNIT. Learning Objectives. Introduction. To understand purpose of Earthing. To learn system of earthing.

Transcription

1 274 Electrical Technician UNIT 10 Earthing Learning Objectives To understand purpose of Earthing. To learn system of earthing. Introduction The very purpose of earthing is to safe against dangers of shock and fire etc. It is essential to have good and effective earthing or grounding The word earthing and grounding have the same meaning but the equipment is different from neutral point earthing. This chapter deals with the equipments earthing only. A poor safety ground or one that is wired incorrectly is more dangerous than no ground at all. The poor ground is dangerous because it does not offer full protection while the user is lulled into a false sense of security. The incorrectly wired ground is a hazard because one of the wires and the safety ground are transposed; making the shell of the tool hot the instant the plug is connected. Thus the unwary user is trapped, unless by poor chance the safety ground is connected to the grounded side of the line on a single-phase ground system, on no ground are present on the ungrounded system, this instance the user again goes blithely along using the tool until he encounters a receptacle which has bits wires transposed or a ground appears on the system.

2 Paper - II Electrical Engineering Materials and Wiring 275 Because there is absolutely Fullproof method of insuring that all tools are safely grounded (and because of the tendency of the average technician to ignore the use of the grounding wire) the old method of using a separate external wire has been discontinued. Instead a 3-wire, standard, colorcoded cord with a polarized plug and a ground pin is required. In this manner safety ground is made a part of the connecting cord and plug. Since the polarized plug can be connected only to a mating receptacle, the user has no choice but to use the safety ground. All new tools properly connected, Use the green wire as the safety ground. This wire is attached to the metal case of the tool at one end and to the polarized grounding pin in the connector at the other hand. It normally carries no current, but is used only when tool insulation fails, in which case it short circuits the electricity ground around the user to ground and protects him from shock. The green lead must never be mixed with the black or white leads which are the true current carrying conductors. Check the resistance of the grounding system with a low reading ohmmeter to conform that the grounding is adequate (less than 1 ohm is acceptable). If the resistance indicate greater than 1 ohm, use a separate ground strap. Some old installations are not equipped with receptacles that will accept the grounding plug. In this events use one of the following methods: 1. Use an adapter fitting 2. Use the old type plug and bring the green ground wire out separately. 3. Connected an independent safety ground line. When using the adapter, be sure to connect the ground lead extension to a good ground. (Do not use the centre screw which holds the cover plate on the respectable). Where the separate safety ground leads are externally connected to a ground, be certain to first connect the ground and then plug in the tool, first remove the line plug and then disconnect the safety ground. The safety ground is always connected first and removed last Purpose of Earthing Fig 10.1 illustrates an electric iron connected to supply through 3-core cable. The line and neutral and connected to heating element where as the earth wire (green shown dotted) is connected to the metal frame of the iron. The earth terminal in turn connected to the neutral earthing at sub-station through mass of earth. Due to insulation damage or otherwise, the live wire touches the metal frame of iron, it effectively gets connected to the earth.

3 276 Electrical Technician If the person holds the iron current will not pass through his body because the resistance of the earth will be less than the human body resistance. Hence the person will be protected from the electric shock. Meanwhile the current will pass through the low resistance path of the earth wire causing the fuse wire in the live wire to blow and thus protect the circuit. Fig Electric Iron Connected to Supply with 3-Core Cable If the earth wire is not provided (i.e. 2-core cable is used) then the live wire accidentally makes contacts with the iron frame, the latter will be at the same potential as the live wire (i.e. 230-v). If the person holds the wire unknowingly the current flows through the body of the person as shown in the fig 10.2 which is very dangerous to life. Sub Station Metal Body of Consumer D.B Electric Iron Neutral Earthing Fig Fault Current Path When no earth is provided Hence from the above it is clear that the non-current carrying metallic parts in the neighborhood of electrical circuits must be earthened, which ensures safety to human life. The non-current carrying parts include the following: (i) Motor body, switch gear metal enclosure, transformer tank, conduits of wiring etc. (ii) Support structures, tower poles etc.

4 Paper - II Electrical Engineering Materials and Wiring 277 (iii) Sheaths of cables (iv) Body of equipments such as iron, oven heater, kettle etc. Electricity is dangerous. Death can occur even with 230-V shock.. The equipment earthing is essential safety measure Types of Earthing The various methods of earthing are as follows: 1. Rod earthing 2. Strip or wire earthing 3. Pipe earthing 4. Plate earthing 1. Rod earthing This method of earthing is simple, cheap and does not need the excavation. It is suitable for the areas which are having loose soil conditions or sandy. In this system of earthing a 16mm dia. GI solid rod ( or 125m dia copper rod 25mm GI pipe) of length not less than 2.5m driven vertically into the ground either manually or by hammer. Sometimes it may be necessary to drive more than one rod to reduce the earth resistance to desired value. 2. Strip or wire earthing In this method of earthing a metal strip or wire is used as earthing electrode. This type of earthing is preferred at places where soil is rocky with a earth bed over it, because the excavation is difficult at such places. In this system 24x4mm GI strip or 25x1.6m copper strip (or 6mm 2 GI wire or 3mm square copper wire) is buried in horizontal trenches of minimum depth 0.5m. The length of the strength or wire depends upon the requirement of earth resistance. It shall however be not less than15m. Sometimes it may be necessary to use more than one-strip/ or wire laid in parallel or radial trenches. Fig Strip Or Wire Earthing

5 278 Electrical Technician 3. Pipe earthing In this method of earthing,a 38mm internal diameter, perfornated galvanized pipe of length 2.5mm is placed vertically (up right) in a permanently wet soil. Where the rock is encountered at the length of less than 2.5m the electrode may be buried inclined to the vertical. Fig Pipe Earthing The inclination should not be more than 30 degrees from the vertical. The pipe is surrounded by a pieces of coke or charcoal and salt in alternate layers of about 15cm around the pipe is used to decrease the resistance as shown in fig Another pipe of 19mm dia and length 1.25m is connected through the buried pipe through reducing socket.

6 Paper - II Electrical Engineering Materials and Wiring 279 At the top of 19mm pipe a funnel is fited and is fastened in a cement concrete work. For effective earthing water should be poured, 2 to 4 buckets now and then through funnel particularly in summer. The earthwire (either GI wire or pipe) is carried in a GI pipe of 12.7mm diameter at a depth of 60cm from the ground level. If it is necessary to reduce the depth of burial of a electrode under unavoidable circumstances, this can be done without increasing the earth resistance. It is achieved by using number of electrodes connecting them together. The distance between two elecxtrodes in such case should not less than twice the length of the electrode. Acast iron cover is hinged in a small masonary work on the top of the earthing to facilitate its identification and periodical checking etc. 4. Plate earthing Fig Plate Earthing

7 280 Electrical Technician In this type of earthing a copper plate of dimension 60cm x 60cm x 3.15mm or a GI plate of 60cm x 60cm x 6.3mm is used as an earth electrode. Plate electrode should be buried with its face vertical such that the top edge is at a depth of not less than 1.5m below the surface of the ground. The electrode is surrounded by alternate layers of broken pieces of coke or salt. The earth wire is securely bolted to the earth plate with the help of bolt, run and weather made of copper for copper electrode and GI for GI electrode. A cast iron cover is provided at the top of the earthing with a hing to facilitate its identification and for its period checking. When resistance of one plate earthing is higher than the required value more than one plates should be earthed and connected together Selection of Earthing Rod earthing is used very rarely but can be seen at agricultural pumps to glow the lamps. Strip or wire earthing is used only at rocky soil areas. To provide the earthing for refrigerators, air cooler, heater, geyzer, electric iron (domestic appliances) etc., in house wiring installations and to provide the earthing for pole mounted substations as well as distribution poles, pipe earthing is sufficient. Plate earthing is used for large installations such as sub-stations, transmission towers etc. The maximum permissible values of each resistance for satisfactory operation are given below: Large power stations=0.5 ohms Major sub-stations=1.0 ohms Small sub-station=2.0 ohms In all other cases=8.0 ohms Factors effecting the earth resistance 1. Soil condition (black soil, red soil, rocky soil, etc) 2. Moisture content in the soil. 3. Temperature of the soil. 4. Size of the earth electrode 5. Depth of the electrode embedded below the ground level.

8 Paper - II Electrical Engineering Materials and Wiring Material of the electrode 7. Quality of coal or charcoal pieces or powder. 8. Quantity of coal and salt Material Estimation S.No. Specification of Material Qty. Remarks 1. G..I. Plate 60 cm x 60cm x 6.30 mm 2. G.I. wire (8 SWG) 4 m mm GI pipe 2 m 4. 19mm GI Pipe 1.5 m For watering 5. G..I. nuts, bolts, check nut and washers 6 sets Four at point A and two for funnel 6. G.I. bend For connecting earth wire 7. G.I. lugs 3 No. 8. Cast iron frame with hinges 30 x 30 cm 9. Cast iron cover 30 x 30 cm 10. Funnel with wire mesh LS = Lump Sum 11. Charcoal or coke 20 kg 12. Salt 20 kg 13. Cement concrete L.S. 14 Caution plate painted Example 10.1: Draw a neat sketch of suitable earthing to be provided for a 12kw industrial load and prepare the quantity estimate of material required. Indicate the details giving dimensions. Solution: Refer art for the sketch of pipe earthing with its dimensions.

9 282 Electrical Technician Schedule of Material S.No. Specification of Material Qty. Remarks mm dia G.I. pipe perrforated 19 mm G.I. pipe 12.7 mm GI pipe G.I. wire 8 SWG G.I. (38x19mm) reducing socket G.I. lugs Nuts, bolts, locknut and washer for 19mm G.I. bends 12.7 mm 30x30 cm cast ironframe with hinges 30 x 30 cm cast iron cover Funnel with wire mesh Charcoal or Coal (pieces) Cement concrete Caution plate painted 2.5 m 1 m 2 m 6 m 2 No. 2 sets 2 No. 20 kg L.S. LS = Lump Sum 10.4 Measure of Earth Resistance Earth resistance can be measured by Earth Tester or Megger. For satisfactory operation, the approximate permissible values of earth resistance for various applications are given below: Large power station... = 0.5 ohms Major sub stations... = 1.0 ohms Small sub stations In all other cases... = 2.0 ohms... = 5.0 to 8.0 ohms

10 Paper - II Electrical Engineering Materials and Wiring 283 Short Answer Type Questions 1. What do you understand by the term earthing? 2. What is the necessity of earthing? 3. Explain the rod earthing? 4. Explain the strip or wire earthing? 5. What are the factors effecting earth resistance? Or State the factors on which the resistance of earth depends. 6. List the points to be earthened? 7. Draw the neat sketch of a dimensional pipe earthing and estimate the quantity of material required for the same. 8. Draw the neat sketch of the dimensional plate earthing and estimate the quantity of material required for the same. Activities 1. Visist a pole mounted/plinth mounted transformer and watch earthing system. 2. Visit a shopping Mall and watch the earthing method. 3. Visit a near by flour mill and watch earthing system.