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
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.
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
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
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.
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.
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.
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
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