The document discusses earthing arrangements and protection against electric shock. It covers the basics of shock protection using Class I and Class II equipment. It then summarizes the three main earthing arrangements: TT, TN-S, and TN-C-S. The TT arrangement uses separate earth electrodes at the supply and installation. The TN-S uses a common earth at the supply but separate earth and neutral conductors at the installation. The TN-C-S, also known as PME, uses a combined earth and neutral conductor on the supply side and separate conductors at the installation.

1. Earthing Arrangements

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. Earthing arrangements - always a good talking point!

4. Definitions from Part 2 BS 7671

5. Bonding conductor A protective conductor providing equipotential bonding

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. 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. Double insulation Double insulation (Class II) - Insulation comprising both basic insulation and supplementary insulation Symbol found on equipment

9. Earth The conductive mass of Earth, whose electric potential at any point is conventionally taken as zero

10. Earth Electrode A conductor or group of conductors in intimate contact with, and providing an electrical connection to earth

11. Earth electrode resistance The resistance of an earth electrode to earth

12. Earth fault current A fault current which flows to earth

13. Earth fault loop impedance The impedance of the earth fault current loop starting and ending at the point of earth fault. Symbol Z Unit 

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. 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. 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. Earthing Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation

18. Basic contact (shock) Results from Making contact with parts of a circuit or system which are live under normal conditions

19. Earthing Connection of the exposed conductive parts of an installation to the main earthing terminal of that installation

20. Extraneous conductive part A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation.

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. Functional earthing Connection to Earth necessary for proper functioning of electrical equipment Table 51 Functional earthing conductors to be coloured cream

23. Contact of persons or livestock with exposed-conductive parts which have become live under fault conditions. Fault contact

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. Protective conductors Earthing conductor main bonding conductor circuit protective conductor

26. Shock conditions

27. Electric Shock Contact of persons or livestock with live parts under fault free conditions.

28. Uo = 230V Electric Shock ouch

29. Protection against Electric Shock Maximum shock voltage No disconnection

30. Protection against Electric Shock Insulation Barriers Enclosures

31. Protection against Electric Shock Placing out of reach Obstacles Protection of a specialist nature Regulation 417.2 Regulation 417.3

32. Uo = 230V Fault (shock) Automatic operation of protective device Reduced shock risk Ouch again

33. Fault Protection E arthed E quipotential B onding A nd automatic D isconnection S upply However the same systems apply No longer called this

34. Fault Protection It is now simply: - A utomatic D isconnection S upply But really it’s EEBADS by another name.

35. Earthing Arrangements

36. TT Earthing Arrangement suppliers cut out suppliers metering earthing conductor from overhead supply to earth electrode consumer unit

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. Frequently used types of earth electrode plate lattice rod Regulation 542.2

39. 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 Earth electrodes recognised by BS 7671 The following types of earth electrode are recognised by the Regulations: Regulation 542.2.1

40. Installation of earth electrodes When installing earth electrodes the following precautions should be observed Regulation 542.2.2 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. 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  approx.)

42. Reducing 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. 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. Problems associated with the TT system Vulnerable to mechanical damage Vulnerable to corrosion High resistance as compared to TN systems

45. Providing automatic disconnection for TT systems Under phase to earth fault conditions overcurrent device should cut of supply rapidly. Increase earth path resistance may be sufficiently high so as to prevent automatic disc connection resulting in: Shock Fire and, or

46. Use of the residual current device Regulation 411.5.2 Preferred method of protection against indirect contact, by means of residual current device.

47. Calculating touch voltage Maximum permitted touch voltage = 50V unless special location. (max 25V) Regulation 411.6.2

48. The following condition must be fulfilled: R A I  n  50V. Where: R A is the sum of the earth electrode and protective conductors connecting it to the exposed-conductive parts I  n is the current causing automatic operation of the r.c.d.

49. REMEMBER 50V max, or 25V max

50. The residual current device Load Exposed metalwork Test resistor Test button Search coil Toroid Operating coil

51. The R.C.D under healthy circuit conditions

52. The R.C.D under earth fault conditions

53. 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.

54. Solution R.C.D.

55. TN-S Earthing Arrangement Separate neutral and earth conductors suppliers cut out suppliers metering

56. 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

57. PES consumer The circuit arrangement for the TN-S system Fig. 7

58. The TN-S system under fault conditions PES consumer

59. TN-C-S Earthing Arrangement Also known as PME combined neutral and earth conductors

60. 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

61. PES consumer PEN conductor The circuit arrangement for the TN-C-S system Also known as PME

62. The TN-C-S system under fault conditions

63. Earthing 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?