Earthing Arrangements

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Earthing Arrangements

  1. 1. Earthing ArrangementsEarthing Arrangements
  2. 2. In this section we are going to look at : -  The basics of shock  Shock protection  Class 1 & Class 2 equipment  The three common earthing arrangements
  3. 3. Earthing arrangements - always a good talking point!
  4. 4. Definitions from Part 2 BS 7671Definitions from Part 2 BS 7671
  5. 5. Bonding conductor A protective conductor providing equipotential bonding
  6. 6. Where protection against electric shock does not rely solely on basic insulation alone. Exposed-conductive parts being connected to a protective conductor within the fixed wiring of the installation. Class I equipment Class I insulation Single-layer insulation Live part Exposed conductive part
  7. 7. Class II equipment Where protection against electric shock relies on the application of additional or supplementary insulation. There is no provision for the connection of a protective conductor to exposed metalwork. Class II insulation Live part Two layers of insulation Exposed metalwork
  8. 8. Double insulation Double insulation (Class II) - Insulation comprising both basic insulation and supplementary insulation Symbol found on equipment
  9. 9. Earth The conductive mass of Earth, whose electric potential at any point is conventionally taken as zero
  10. 10. Earth Electrode A conductor or group of conductors in intimate contact with, and providing an electrical connection to earth
  11. 11. Earth electrode resistance The resistance of an earth electrode to earth
  12. 12. Earth fault current A fault current which flows to earth
  13. 13. Earth fault loop impedance The impedance of the earth fault current loop starting and ending at the point of earth fault. Symbol ZSymbol Z UnitUnit ΩΩ
  14. 14. The earth fault loop The earth fault loop starting at the point of fault consists of: • The circuit protective conductor (c.p.c.) • Consumers earthing terminal and earthing conductor • For TN systems, the metallic return path • For TT and IT systems the earth return path • The path through the earthed neutral point of the transformer • The transformer winding and phase conductor to point of fault
  15. 15. Earth leakage current A current which flows to earth, or to extraneous conductive parts, in a circuit which is electrically sound. This current may have a capacitive quality including that from the deliberate use of capacitors for noise filtration.
  16. 16. Earthed equipotential zone A zone within which exposed conductive parts and extraneous conductive parts are maintained at substantially the same potential by bonding, such that under fault conditions, the differences in potential simultaneously accessible exposed and extraneous- conductive parts will not cause electric shock.
  17. 17. Earthing Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation
  18. 18. Basic contact (shock) Results from Making contact with parts of a circuit orMaking contact with parts of a circuit or system which are live under normal conditionssystem which are live under normal conditions
  19. 19. Earthing Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation
  20. 20. Extraneous conductive part A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation.
  21. 21. Fault A circuit condition in which current flows through an abnormal or unintended path. This may result from an insulation failure or a bridging of insulation. Conventionally the impedance between live conductors or between live conductors and exposed or extraneous conductive parts at the fault position is considered negligible.
  22. 22. Functional earthing Connection to Earth necessary for proper functioning of electrical equipment Table 51ATable 51A Functional earthing conductors to be coloured cream
  23. 23. Contact of persons or livestock with exposed-conductive parts which have become live under fault conditions. Fault contact
  24. 24. Protective conductors A conductor used for some measure of protection against electric shock and intended for connecting together any of the following parts • Exposed conductive parts • Extraneous-conductive parts • The main earthing terminal • Earth electrode(s) • The earthed point of the source, or an artificial neutral
  25. 25. Protective conductors Earthing conductor main bonding conductor circuit protective conductor
  26. 26. Shock conditionsShock conditions
  27. 27. Electric Shock Contact of persons or livestock with live parts under fault free conditions.
  28. 28. Uo = 230V Electric Shock ouchouch
  29. 29. Protection against Electric Shock Maximum shock voltageMaximum shock voltage No disconnectionNo disconnection
  30. 30. Protection against Electric Shock InsulationInsulation BarriersBarriers EnclosuresEnclosures
  31. 31. Protection against Electric Shock Placing out of reachPlacing out of reach ObstaclesObstacles Protection of a specialist natureProtection of a specialist nature
  32. 32. Uo = 230V Fault (shock) Automatic operation of protective deviceAutomatic operation of protective device Reduced shock riskReduced shock risk OuchOuch againagain
  33. 33. Fault Protection Earthed Equipotential Bonding And automatic Disconnection Supply No longer called this However the same systems apply
  34. 34. Fault Protection It is now simply: - Automatic Disconnection Supply But really it’s EEBADS by another name.
  35. 35. Earthing ArrangementsEarthing Arrangements
  36. 36. TT Earthing Arrangement suppliers cut out suppliers metering earthing conductor from overhead supply to earth electrode consumer unit
  37. 37. 1st Letter - Method of earthing for suppliers network 2nd Letter-Method of earthing at consumers installation T = Direct connection to earth at one or more points T = Direct connection to earth
  38. 38. Frequently used types of earth electrode plate lattice rod Regulation 542-02-01Regulation 542-02-01
  39. 39. Earth electrodes recognised by BS 7671Earth electrodes recognised by BS 7671 The following types of earth electrode are recognised by the Regulations: Regulation 542-02-01  Earth rods or pipes  Earth tapes or wires  Earth plates  Underground structural metalwork embedded in foundations  Welded metal reinforcement of concrete (except pre-stressed concrete) embedded in earth  Lead sheaths and other metal coverings of cables where not precluded by regulation 542-02-05  Other suitable underground metalwork
  40. 40. Installation of earth electrodes When installing earth electrodes the following precautions should be observed Regulation 542-02-02 Remember climatic conditions could affect electrode resistance The type and embedded depth of an earth electrode shall be sufficient to avoid soil drying and freezing
  41. 41. Earth electrode resistance The graph illustrates the relationship between electrode resistance and buried depth for a ‘rod type’ electrode. The deeper the rod, the closer to the water table it becomes, resulting in lower resistance Typical value of resistance of ‘rod type’ electrode buried to a depth of 1 metre (60 Ω approx.)
  42. 42. Reducing earth electrode resistanceReducing earth electrode resistance Under certain circumstances the value of electrode resistance may be excessively high and steps must be taken to reduce its value. The following methods may be adopted: • use of extendable rods • use of additional rods • soil conditioning agents (temporary measure) • electrodes buried to a greater depth
  43. 43. Use of additional rods to reduce resistance As a ‘rule of thumb’, the distance between adjacent earth rods should not be less than the buried depth. Distance (m) Depth (m) Earthing conductor
  44. 44. Problems associated with the TT system Vulnerable to mechanical damage Vulnerable to corrosion High resistance as compared toHigh resistance as compared to TN systemsTN systems
  45. 45. Use of the residual current device Regulation 413-02-19 Preferred method of protection against indirect contact, by means of residual current device.
  46. 46. Calculating touch voltage Maximum permitted touch voltage = 50V unless special location. (max 25V) Regulation 413-02-20
  47. 47. The following condition must be fulfilled: RRA.A.II∆∆nn ≤≤ 50V.50V. Where: RRAA is the sum of the earth electrode and protective conductors connecting it to the exposed-conductive parts I.I.∆∆nn is the current causing automatic operation of the r.c.d.
  48. 48. REMEMBER 50V max, or 25V max
  49. 49. The residual current devicecurrent device LoadLoad ExposedExposed metalworkmetalwork Test resistorTest resistor Test buttonTest button Search coilSearch coil ToroidToroid Operating coilOperating coil
  50. 50. The R.C.D under healthy circuit conditions
  51. 51. The R.C.D under earth fault conditions
  52. 52. Protection and the TT system Remember! The earth fault loop impedance for a TT system may be too high to allow circuit breakers and fuses to operate under phase to earth fault conditions.
  53. 53. Solution R.C.D.
  54. 54. TN-S Earthing Arrangement Separate neutral and earth conductors suppliers cut out suppliers metering
  55. 55. 1st Letter - Method of earthing for suppliers network 2nd Letter - Method of earthing at consumers installation T = Direct connection to earth at one or more points N = Consumers exposed metalwork directly connected to the earthed neutral point of the supply 3rd Letter - Relationship between phase & neutral conductors on suppliers network S = Separate neutral and earth conductors at consumers installation
  56. 56. PES consumer The circuit arrangement for the TN-S system Fig. 7
  57. 57. PES consumer The TN-S system under fault conditions
  58. 58. TN-C-S Earthing Arrangement Also known as PME combined neutral and earth conductors
  59. 59. 1st Letter - Method of earthing for suppliers network 2nd Letter - Method of earthing at consumers installation T = Direct connection to earth at one or more points N = Consumers exposed metalwork directly connected to the earthed neutral point of the supply 3rd Letter - Relationship between phase & neutral conductors on suppliers network C = Combined neutral and earth on suppliers side 4th Letter - Arrangement of earth and neutral conductors at consumers installation S = Separate neutral and earth conductors at consumers installation
  60. 60. PES consumer PEN conductor The circuit arrangement for the TN-C-S system Also known as PME
  61. 61. The TN-C-S system under fault conditions
  62. 62. Earthing ArrangementsEarthing Arrangements We have looked at: - • The basics of shock • Shock protection • Class 1 & Class 2 equipment • The three common earthing arrangements • Have you any questions

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