Earthing fault

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  • 1. 2011 Electrical Earthling Construction, Faults & Protection Given the importance of electric power as one of the basic elements of economic and social development has provided a lot of countries support the electricity sector, by producing power electricity projects resulting in a significant expansion in using devices & equipments run on electricity. [ÉáátÅ T{Åxw mx|Ç Modern Academy for Engineering & Technology 1/1/2011
  • 2. ELECTRICAL EARTHLING Introduction Given the importance of electric power as one of the basic elements of economic and social development has provided a lot of countries support the electricity sector, by producing power electricity projects resulting in a significant expansion in using devices & equipments run on electricity. In other way exist the electrical dangerous like fires, electric shocks and other so we must use … (Earth Leakage Circuit Breakers) for electrical leakage protection. (Will be mentioned in details…) Likely to feel the normal person that often do not impact the ground on the electrical systems or devices through its normal operating, Which give the false impression that it is possible to separate the ground without notice any effects as a result it appears (seemingly only) that the b ground good contact with the ground poor is not a do not know the importance of the effectiveness of the ground unless it conducted periodic surveys from time to time. The grounding is required to provide for the safety of the electrical system and the staff a t the facility and this is known in general among some of people, but is not clear the most of them how to achieve that. B enefits: Firstly: personal protection of electric shocks caused by Isolation failure. Secondly: protection from Electrical Discharging. Thirdly: Protect devices from sudden changes in Voltage source. Fourthly: Reduce the likelihood of damage as a result of lightning or fault currents, Lightning Strikes, Static Discharges, EMI and RFI signals and Interference. Fifthly: Functional earthling in electrical power system. T he agencies and organizations all have recommendations and / or standards for grounding, to ensure that personnel safety is being protected. The organizations that provide guide lines/rules for grounding are: The International Electrotechnical Commission (IEC), European Committee for Electrotechnical Standardization (CENELEC), Underwriters Laboratories (UL), National Fire Protection Association (NFPA), American National Standards Institute (ANSI), Mine Safety Health Administration (MSHA), Occupational Safety Health Administration (OSHA), Telecommunications Industry Standard (TIA) and others. I nstrument And Devices Must be Earthed: To make Good ground Network, it is necessary to ground the following elements: - All metal objects vertically and more than 240 cm in length or extended horizontally and more than 150 cm in length and are exposed to contact. - All electrical appliances. - All Albriz exits ‫ رج ا ا‬and lighting units. 2 Page Hossam Ahmed Zein
  • 3. ELECTRICAL EARTHLING General Concepts Step Voltage : The difference in surface Potential experienced by a person bridging a distance of 1 meter with his feet without contacting any grounded object. Touch Voltage ; Potential difference between rise and surface potential at the point where the person standing while in the same time having his hand contact with grounded structure. Mesh Voltage : The max. touch voltage within a mesh of groun grid. Transferred Voltage : A special case of touch voltage where a voltage is transferred into or out a substation from or to a remote point external to the substation site. 3 Page Hossam Ahmed Zein
  • 4. ELECTRICAL EARTHLING Standards Ideal Ground Resistance should be Zero ohm But practically, it must be lower than 25 ohm for power systems & 5 ohm For Telecommunication System achieved by increasing no. of ground electrodes. It is at least 2,4 m in length and in contact with the soil. There are 3 variables that affect the resistance of a ground electrode: a) The ground material Itself. b) The length/depth of the ground electrode. • Very effective way of lowering resistance is to drive ground electrodes deeper. Because the earth is in layers resistivity changes and varies considerably on the layer and the depth within that layer. c) Diameter of the ground electrode. • Increasing the diameter of the ground electrode has very little effect in lowering the resistance. For example you could double the diameter of a ground electrode and your resistance would only decrease by as much as 10 %. d) Chemical Treatment OF the soil: • Making a hole Beside the grounding electrode with max. distance 10 cm and filled of salts Mg3SO4 ‫آ ت ال‬ Or Cu2SO4 ‫ آ ا س‬Or ‫ي‬ till 30 cm from surface level . • By making circular tenth around the electrode with (45 cm-Diameter) & (30 cm-Depth) and filled with pre-mentioned chemicals ,with no direct contact to the electrode other else corrosion Occur. The chemicals recommended to be Cu2SO4 with around (18-40 Kg) as it’s cheap , Good electrical conductivity and the long run effect (2 years) • Flood the grounding hole with water for good absorption of salts (chemicals) periodically that what rain water do. copper conductor diameter to the Ground Largest copper conductor Diameter (2mm) Electrode (2mm) 1 1 1.5 1.5 2.5 2.5 4 4 6 6 10 10 16 16 16 25 16 35 25 50 35 70 50 95 70 120 70 150 95 185 120 240 150 300 185 400 4 Page The allowed Current passing through the Grounding conductor Hossam Ahmed Zein
  • 5. ELECTRICAL EARTHLING Instantaneous Current Allowed Current for long Conductor (A/sec.) time (A) Diameter (2 mm) Al Cu AL Cu - 2500 - 150 16 2700 4000 160 200 25 3700 5500 200 280 35 5300 8000 250 480 50 7400 11500 320 590 70 10500 11600 430 780 95 21000 32500 760 1380 185 Relation between Leakage Current Intensity & Its passing through time in human body Biological effect on Human passing through time Current (mA) body ‫سو‬ ‫ا ر‬ 0.5 – 0 ‫رو‬ ‫س‬ ‫أا‬ ‫ك‬ ‫رإ أ‬ ‫ا‬ ‫نا‬ 5 – 0.5 ‫نا‬ ‫را‬ ً ‫ء‬ ‫را‬ ‫ل‬ ‫ا‬ ‫ةد‬ 30 – 5 ‫ا مو‬ ‫ار ع‬ ‫و‬ ‫ا م‬ – ‫ا‬ ‫م‬ ‫ما‬ ‫ا‬ 50 – 30 ‫إ ء‬ ‫ر‬ ‫ا‬ ‫ةا‬ ‫أ‬ ‫ت‬ ‫ة‬ – 50 ‫ا‬ ‫ر ر‬ ‫ء‬ ‫إ‬ ‫ةا‬ ‫ل‬ ‫أ‬ ‫ا‬ ‫ر ر‬ ‫ء‬ ‫إ‬ ‫ةا‬ ‫أ‬ ‫ت‬ ‫ة‬ ‫أآ‬ ‫ت أو‬ –‫ء‬ ‫إ‬ ‫ةا‬ ‫ل‬ ‫أ‬ 5 Page Hossam Ahmed Zein
  • 6. ELECTRICAL EARTHLING 6 Page Hossam Ahmed Zein
  • 7. ELECTRICAL EARTHLING E) Number of ground electrodes • In this design, more than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods needs to be at least equal to the depth of the driven rod. F) Ground system design • Mentioned After …. 7 Page Hossam Ahmed Zein
  • 8. ELECTRICAL EARTHLING Ground electrode: You can use one of the following means of grounding pole, namely: 1 - Extensions of metal pipes for water. 2 - Reinforcing rods ‫ أ خ ا‬of the building. 3 - A metal conductor is extended around the building and not less than 75 cm from the surface of the earth. 4-Specialised Industrial Electrodes: i. Made Electrode: It is a metal rod or pipe not less than 240 cm in length buried vertically in contact with the soil unless the ground is rocky and can be placed diagonally 45 degrees to the vertical level, or buried in a trench ‫ ق‬at a depth of 75 cm from the surface of the earth at least. ii. Plate Electrode: It is a sheet metal may be copper (1.5 mm-thick) OR Iron (6.35mm-min. thickness) , with min. expose area 186 m2 a. Free Of grease or Oils because they undermine the viability of the properties of the grounding of the conductivity. The Electrode consists of three basic components : 1. Ground conductor 2. The connection/bonding of the conductor to the ground electrode 3. The ground electrode itself. The resistance of a ground electrode has 3 basic components: A) The resistance of the ground electrode itself and the connections to the electrode. • it's connection is generally very low, ground rods are generally made of highly conductive/low resistance material such as copper of copper clad. B) The contact resistance of the surrounding earth to the electrode. • The Bureau of Standards has shown this resistance to be almost negligible providing that the ground electrode is free from paint, grease etc. and that the ground electrode is in firm contact with the earth B) The resistance of the surrounding body of earth around the ground electrode. The resistance of a ground 8 Page • The ground electrode is surrounded by earth which is made up of concentric shells all having the same thickness. Those shells closest to the ground electrode have the smallest amount of area Hossam Ahmed Zein
  • 9. ELECTRICAL EARTHLING resulting in the greatest degree of resistance. Each subsequent shell incorporates a greater area resulting in lower resistance. This finally reaches a point where the additional shells offer little resistance to the ground surrounding the ground electrode. Types Of grounding According to the Protective Device Earth grounding is to ensure that operating equipment within a structure is properly grounded. is an intentional connection from a circuit conductor usually the neutral to a ground electrode placed in the earth. Equipment grounding Complex networks increase the These two grounding systems are required to prevent amount of contact with the differences in potential from a possible flashover from a surrounding earth and lower lightning strike. ground resistances. According to the Design Simplicity • Simple grounding systems consist of single 1. Complex grounding systems consist of ground electrode driven into the ground. The multiple ground rods, connected, mesh or use of a single ground electrode is the most grid networks, ground plates, and ground common form of grounding. loops. • Where: outside your home or place of o Where: at power generating substations, business. central offices, and cell tower sites. According to its Design techniques a. Single ground electrode. b. Multiple ground electrodes connected. c. Mesh network. d. Ground plate. 9 Page Hossam Ahmed Zein
  • 10. ELECTRICAL EARTHLING According to its fuctionality :- 1. Functional grounding: this kind of grounding is related to the proper work of the system like (neutral point grounding, reactors grounding ..etc.) 2. Protective grounding: This kind related to personal protection due electric shocks and its about connecting all conducting of non-current carrier with earth 3. Lighting Protection: Protect the system from lighting discharging any traveling waves or equip the substation with equipments to discharge the lighting as it happen Ground Fault Protection Circuit Breakers Electrical circuits Protected by Normal cit. breakers (15-20) -- 60 mA Cause human Death – and it’s so sensitive for the lowest current . Types Of Circuit breakers: 1st Type: it shutdown the cit. when the passing current about 6 mA . 2ndType: it shutdown the cit. when the passing current about 20 mA . Where Cit. Breakers Connected (Elcb): i. Elcb circuit breaker Connected on the main power line for General protection for all circuits, (Main Protection) But it cause total current break down for any current leak in any following circuits (lighting) ii. Using two cit. breakers , one (normal) for the main lighting board and the other (Elcb) for the main power board . 10 Page Hossam Ahmed Zein
  • 11. ELECTRICAL EARTHLING iii. Could be used for distinct device ( air conditioner ) condition that this device must be earthed. Or for apart of home or some place (Individual Protection) 11 Page Hossam Ahmed Zein
  • 12. ELECTRICAL EARTHLING 12 Page Hossam Ahmed Zein
  • 13. ELECTRICAL EARTHLING Structural lightning protection & Design considerations esign Structural lightning protection design considerations BS 6651 (Protection of structures against lightning) clearly advises strict adherence to the provision of a conventional Lightning Protection System (LPS) - to the total exclusion of any other device or system for which claims of enhanced protection are made. The principle components of a conventional structural lightning protection system, in accordance with BS 6651 are: Air Termination Network 13 Down Conductors Page Hossam Ahmed Zein
  • 14. ELECTRICAL EARTHLING Earth Termination Network Bonding (to prevent side flashing) Other areas that need to be looked at: Corrosion Inspection, testing, records and maintenance Air termination network On high risk structures such as explosive factories, no part of the roof should be more than 2.5m from an air termination conductor. This is generally achieved by applying a 5m x 10m n mesh to the roof. However, for most structures, a mesh of 10m x 20m is considered sufficient, giving a maximum distance from any part of the roof to the nearest conductor of 5m. Air terminations for tall conducting structures The zone of protection does not seem to be applied because of the need to interconnect the down conductors of the tall block to the air termination of the lower block. In such cases it is necessary to connect t the lower air termination up to the lower down conductors to facilitate this inter connection, even though this extension is within the zone of protection of the tower. The 'Zone of Protection' offered by an air termination network is considered to be 45º for heights up to 20m. Above this height, the zone of protection is determined by the 'Rolling Sphere Method'. This involves rolling an imaginary sphere of 60m radius over a structure. The areas touched by the sphere are deemed to require protection. On tall structures, this can obviously include the sides of the building. Zones of protection 14 Page Hossam Ahmed Zein
  • 15. ELECTRICAL EARTHLING Down conductors Down conductor siting and distancing is often dictated by architectural circumstances. There should be one down conductor for every 20m or part thereof of the building perimeter at roof or ground level (whichever is greater). These should be evenly spaced and distances apart of more than 20m avoided if possible. If the building is above 20m in height or of an abnormal risk this distance should be reduced to 10m. They should be routed as directly as possible from the air termination network to the earth termination network to avoid risks of side flashing. Re entrant loops are also to be avoided. Re-entrant BS 6651 recommends that the length of conductor forming the loop should not exceed eight conductor times the width of its open side. BS 6651 allows the use of 'natural conductors' such as rebars and structural steelwork, provided that they are electrically continuous and adequately earthed. Lightning Protection Scheme to BS 6651 using the reinforced concrete within the ction structure for down conductors Inner area requires no conductors as it is within the zone of protection determined by the rolling sphere Earth termination network Each down conductor must have a separate earth termination. Moreover provision should be made in each down conductor, for disconnection from the earth for testing purposes. This is achieved with a test clamp (see below). BS 6651 stipulates that the resistance to earth of the lightning protection system measured at any point, should not exceed 10 ohms. With the test clamp 15 disconnected, the Page resistance of each individual earth should be no Hossam Ahmed Zein
  • 16. ELECTRICAL EARTHLING more than ten times the number of down conductors in the complete system. eg fo a for system with 15 down conductors, the individual earth readings should be no more than 10 x 15 = 150 ohms. Several types of earth electrode are permissible, but by far the most commonly used are deep driven earth rods. BS 6651 states that the combined earth rod length of a system earth should be no less than 9m whilst each individual earth rod should be no less than 1.5m in length. Deep driven Oblong test or earth electrode junction clamp Parallel earth rod electrodes Where ground conditions make deep driving of earth rods impossible, a matrix arrangement of rods coupled to one another by conductors can be used. If possible, the earth rods must be spaced at a distance at least equal to their driven depth. If earth rods cannot be driven in a parallel line a "Crows Foot" configuration can be used, parallel ensuring that the spacing/depth ratio is still maintained. High resistivity soil conditions can be overcome by backfilling earth rods with a suitable medium such as Marconite conductive concrete which effectively increases the diameter of effectively the earth rod and hence its surface area, thus lowering resistance to earth. Spacing of parallel earth rod electrode 16 Page Hossam Ahmed Zein
  • 17. ELECTRICAL EARTHLING Bonding All metal work, including water pipes, gas pipes, handrails, air conditioning units, metal cladding, metal roofs etc, in the vicinity of the LPS must be bonded to it, to avoid the danger of side flashing. For the same reason, the LPS earth should be bonded to the main electrical earth, as well as any other earthing system present i the structure. in Example of side flashing If the lightning protection system on a structure is hit by lightning, then the current flowing through the system and the resistance/impedance offered by the conductor path will determine themagnitude of the potential difference seen by the otential lightning conductors with respect to true earth. The lightning conductors can, instantaneously, have a potential magnitude of megavolts (1,000,000V) with respect to true earth. Typically, at instant of discharge: Potential difference at A = 1,500,000V Potential difference at B = 0V Bonding to prevent side flashing 1 Air termination 2 Down conductor 3 Bond to aerial 4 Bond to vent 5 Bond to re- -bar 6 Bond to metal staircase 7 Bond to metal window frame 8 Bond to vent pipe 9 Bond to steel door/frame 10 Test clamp 11 Indicating plate 12 Main earthing terminal of electrical installation 17 13 Earth termination point Page Hossam Ahmed Zein
  • 18. ELECTRICAL EARTHLING Corrosion BS 6651 contains tables of materials suitable for use in lightning protection system components. Adherence to these requirements is vital to avoid corrosion problems. The correct choice of material and installation design should ensure a life span of 30 years for the earth electrode system. Measurement There are three methods of Earth ground testing methods : 1) Soil Resistivity (using stakes) 18 Why: Soil Resistivity is most necessary when deter-mining the design of the grounding system for new Page installations (green field applications) to meet your ground resistance requirements. Hossam Ahmed Zein
  • 19. ELECTRICAL EARTHLING Lowest Resistance is petter ,if it was high enough could be overcomed with more elaborate ground systems Requirements: replacing the electrode as deeper as possible • where, soil and water exist ( ‫أو‬ ) water table. • where there is a stable temperature, i.e. below the frost line. Depend on: Soil Content of Moisture : The Soil Resistivity decreases significantly due increasing of its content of moisture 19 Page Hossam Ahmed Zein
  • 20. ELECTRICAL EARTHLING Temperature: If temp. of Soil was too high the Soil Resistivity increase & if temp. was too low (subzero) then the moisture will freeze and the Resistivity will increase . Electric Current : If the current was too high then it will produce a heat all over the carrying conductors ,this heat will dry the moisture and increase the resistivity . Depth of Soil : that the content of moisture increases as we get deeper within the ground unless re a rock base. 2) Fall-of-Potential (using stakes) : The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site. Procedure: > Disconnect the earth electrode iii. Using Ohm’s Law (V = IR), a known current i. Two stakes (inner & outer) are placed in is generated by the test device between the soil in a direct line, away from the the outer stake (auxiliary earth stake) and earth electrode with indicated table the earth electrode, while the drop in ii. Connect earth tester to the electrode voltage potential is measured between the inner earth stake and the earth electrode. 3) Selective (using 1 clamp and Auxiliary stakes) The earth electrode of interest does not need to be disconnected from its connection to the site. Much safer and easier way Procedure: > i. a special clamp is placed around the earth electrode, which eliminates the effects of parallel resistances in a grounded system ii. is very similar to the Fall-of-Potential testing, providing all the same measurements. 4) Stake-less [UNILAP GEO X](using 2 clamps only) Comparative advantage: o measure earth ground loop resistances for parallel / multi- grounded systems using only current clamps. o Used inside buildings, on power pylons or anywhere you 20 don’t have access to soil. Page Produce operating safety and Less Dangerous. Hossam Ahmed Zein
  • 21. ELECTRICAL EARTHLING If there is only one path to ground, the Stakeless method will not provide an acceptable value and the Fall-of-Potential test method must be used. Procedure :> i. Two clamps are placed around the earth ground rod or the connecting cable and each are connected to the tester. ii. A known voltage is induced by one clamp, and the current is measured using the second clamp. iii. The tester automatically determines the ground loop resistance at this ground rod. Some Practical Examples: 21 Page Hossam Ahmed Zein
  • 22. ELECTRICAL EARTHLING Cellular sites/microwave and radio towers At most locations there is a -legged tower with each leg individually grounded. These grounds are then connected with a copper cable. Next to the tower is the Cell site building, housing all the transmission equipment. Inside the building there is a halo ground and a MGB, with the halo ground connected to the MGB. The cell site building is grounded at all corners connected to the MGB via a copper cable and the corners are also interconnected via copper wire. There is also a connection between the building ground ring and the tower ground ring. Electrical Substations A substation is a subsidiary station on a transmission and distribution system where voltage is normally transformed from a high value to low value. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low-voltage switchgear, surge protection, controls, and metering. 22 Page Hossam Ahmed Zein
  • 23. ELECTRICAL EARTHLING Measuring of High Voltage Trans. Towers 1 Lightning protection at commercial/industrial sites Most lightning fault current protection systems follow the design of having all four corners of the building grounded and these are usually connected via a copper cable. Depending on the size of the building and the resistance value that it was designed to achieve, the number of ground rods will vary. Stakeless measurement • The ground stakes of the Selective measurement building (lightning protection) First, perform a Stakeless The resistances should be measurement on: 3-pole Fall-of-Potential measured on: measurement • The individual legs of the • Each leg of the tower and all four corners of the tower and the four corners of This measurement should be building (cell sites/towers) the building (cell recorded and measurements sites/towers) should take place at least • Individual ground rods and twice per year. their connections (electrical • All grounding connections substations) (electrical substations) This measurement is the • Both ends of the remote resistance value for the entire • The lines running to the site (remote switching) site. remote site (remote • All four corners of the switching) building (lightning protection) 23 Page Hossam Ahmed Zein
  • 24. ELECTRICAL EARTHLING Remote switching sites (known as slick sites) Where digital line concentrators and other telecommunications equipment is operating. The remote site is typically grounded at either end of the cabinet and then will have a series of ground stakes around the cabinet connected by copper wire. Stackless method Fall of potential 1 2 24 Page Hossam Ahmed Zein