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ELECTRICAL WORKS

This document provides information on correctly sizing electrical cables and the proper installation techniques. It covers cable sizes, three-phase supplies, distribution boards, cable types, and safety precautions.

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ELECTRICAL WORKS

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  1. DISCLAIMER Neither WJEC or NGfL accepts any responsibility whatsoever for any injury, however caused, relating to or resulting from this document. By reading this, the reader accepts and agrees that neither WJEC or NGfL are providing training, intentionally or unintentionally for carrying out electrical works. ELECTRICAL WORKS

  2. ELECTRICAL WORKS CABLE SIZES Correctly sized cables should be selected so that the cable meets the power demands of the equipment it serves. Cable sizes are stated in square millimetres, which is the cross-sectional area of the wire inside. The larger this area is, the higher the current, or load, it can carry.

  3. ELECTRICAL WORKS Fire can occur where a cable is too small for the amount of current carried.

  4. ELECTRICAL WORKS Generally, electrical cables consist of three parts, namely:- • Conductors, which carry electricity. • Insulators, which insulate the conductors. • Protective jacket, which protects the insulation and conductors. This applies to cables supplying small electrical devices, such as computers, monitors, domestic appliances etc.

  5. ELECTRICAL WORKS

  6. ELECTRICAL WORKS Large incoming electricity supplies are brought into buildings on three-phase supplies.

  7. ELECTRICAL WORKS ‘Three-phase’ means that three separate circuit conductors bring electricity into a building at the same frequency (also known as a cycle), one-third of a cycle apart. At any one time, a scenario could be:- L1 Brown phase at peak value L2 Black phase at 2/3rds value L3 Grey phase at 1/3rd value Neutral Phase – blue – allows the 3 phase supply to run simultaneously as a single phase supply.

  8. ELECTRICAL WORKS Supply cables are sometimes brought into a building via underground ducts through the foundations. The cables shown left are over 50mm in diameter, and originate from electrical sub-stations.

  9. ELECTRICAL WORKS A main distribution board sends electricity to smaller distribution board that are placed throughout a building. At each stage of distribution, the voltage is reduced until it reaches 240 volts drawing 13 amps of current.

  10. ELECTRICAL WORKS Distribution board before cables are connected.

  11. ELECTRICAL WORKS A local distribution board. The blanks (indicated by the red arrow) provide spare capacity, which have not been used but could supply additional circuits in the future.

  12. ELECTRICAL WORKS A list, or schedule inside each distribution board shows which circuits are served by the distribution board.

  13. ELECTRICAL WORKS Cables may be left exposed, as in the above photograph.

  14. ELECTRICAL WORKS Alternatively, cables may be hidden. In the picture shown above, the cables are concealed in the body of the lamp fitting, and through the spotlight rail.

  15. ELECTRICAL WORKS Different cables are used for different purposes: the above image shows data cables supplying information via a file server and an Information Technology network.

  16. ELECTRICAL WORKS Above – network cables before being pulled, or placed in position. Cables are tied together to form looms to help cable installation.

  17. ELECTRICAL WORKS Above: data cables (orange in colour) drawn into position. Power cables run in the plastic dado trunking above the data cables.

  18. ELECTRICAL WORKS In order to prevent damage, cables are often placed in steel mesh cable trays. These can be fixed to walls or soffits (ceilings) of a building.

  19. ELECTRICAL WORKS Cables can also be contained in surface-mounted plastic ducts.

  20. ELECTRICAL WORKS This photo shows the correct fitting and fixing of cables.

  21. ELECTRICAL WORKS The next two slides show how cables should not be fixed.

  22. ELECTRICAL WORKS Incorrect installation

  23. ELECTRICAL WORKS Incorrect installation

  24. ELECTRICAL WORKS Cables awaiting fixing inside a distribution board.

  25. ELECTRICAL WORKS ALWAYS REMEMBER! Electricity, in the wrong hands, can kill. NEVER start work or touch electrical equipment unless you have been trained and items have been isolated.

  26. ELECTRICAL WORKS DISCLAIMER Neither WJEC or NGfL accepts any responsibility whatsoever for any injury, however caused, relating to or resulting from this document. By reading this, the reader accepts and agrees that neither WJEC or NGfL are providing training, intentionally or unintentionally for carrying out electrical works.

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