1 Cables A large portion of electrical signals are transmitted through solid electrical solid electrical conductors. A wire is a single conductor. A cable is a configuration of two or more wire conductors. The types of cable used depends on the specific application. Stranding - To ensure flexibility and ease of handling, conductors are stranded: a number of small wires twisted together spirally forming a core equivalent to a single wire of the required size.

2 Curent-carrying Capacity of Cables I When a cable carries electric current, there are inevitable current loss. (P = I 2 R) This wasted power is used in heating the cable, causing the temperature of the cable to rise. If the cable is overloaded, the temperature may rise sufficiently to damage or even destroy the insulation. Wiring Regulations fix values of current to be carried by the cable which give a safe temperature rise under normal conditions.

3 Current-carrying Capacity of Cables II The current ratings are to be modified by rating factors in respect of: Ambient temperature Class of excess-current protection Grouping Disposition Type of sheath

4 Conductor Materials Copper and aluminum are the materials commonly used. Copper has higher conductivity than aluminum and take up less space than aluminum for the same current capacity. On the other had, aluminum has about one-third the weight of copper and so will have advantage in some circumstances.

5 Insulation Wire and cables are usually surrounded by some type of electrical insulator. The materials chosen to make this insulation have a high insulation resistance, high mechanical strength and durability. They must be able to operate over a fairly wide temperature range, and withstand oil and corrosive chemcials without deterioration. The most popular insulating materials are PVC and elastomers.

6 PVC Insulators PVC - Polyvinyl Chloride High insulation resistance Practically impervious to chemical action tendency to soften under moderate temperature May be used where the combination of ambient temperature and temperature rise due to load does not exceed 65 o C

7 Some Common Types of Wire and Cables I Hookup wire - Generally consists of a multistrand single conductor surrounded by PVC or polyethylene. Used for connecting elements in ordinary low-frequency circuits. Test-Prod Wire - Very flexible wire surrounded by rubber insulation. Used in test leads of measuring instruments. Rubber insulation provides high insulation-resistance as well as flexibility.

8 Some examples of cable (i) hookup wire(ii) test prod wire(iii) shielded cable (iv) multi-conductor cable(v) coaxial cable PVC-insulated, steel wire armoured, Cross-linked polyethylene insulated, PVC-sheathed cable armoured cable

9 Some Common Types of Wire and Cables II Shielded Cable - Consists of an inner-signal conductor and braided metal sheath surrounding the inner conductor. This type of cable is for carrying low-level signals. The braided sheath is effective in reducing the pick up of interference signals by the inner signal carrying conductor. Multiconductor Cable - Consist of many conductors bundled together in one sheath. Can have any number of conductors as well as different types in the same bundle.

10 Some Common Types of Wire and Cables III Coaxial Cable - Similar to the shield cable in construction, but used to carry high - frequency and pulse type signals. At high frequencies, ordinary single-conductor cable would radiate too much energy from the cable during transmission. Coaxial cables eliminate this problem. Armoured Cable - Larger cables, used for underground work and for large interior power installation, need to be mechanically protected. Jointing of Electrical Conductors Regulation requires that joints in cable conductors and bare conductors shall be electrically and mechanically sounded. A joint which is mechanically sound must not pull out under the normal strains to which it may be subject during installation or its working life.

11 Joints and Jointing To be electrically sound, the joint must have at least as low a resistance as an equal length of the conductor itself, nor must it overheat when carrying current. Jointing methods: Mechanical joint Soldering Welding A joint which is mechanically sound must not pull out under the normal strains to which it may be subject during installation or its working life. To be electrically sound, the joint must have at least as low a resistance as an equal length of the conductor itself, nor must it overheat when carrying current.

12 Preparing the Cable The insulation is removed by cutting with a sharp knife. The blade should be held at a narrow angle to the wire and used with a paring action.

13 Soldering Methods The correct method of soldering is to heat the wire to be soldered to such a temperature that it will melt solder put into contact with it. Soldering cannot be satisfactory accomplished by simply dropping blobs of melted solder on the cold wire, Some examples of joint Britannia joint - can tolerate considerable tension Scarf joint - joint is smaller, but cannot tolerate tensile stress Straight twist joint - The wires are twisted tightly round each other in opposite directions, each turn of the wire closely up to the next.

14 Some examples of joint (i) Britannai joint (ii) Scarf joint (iii) Straight twist joint

15 Semipermanent Connections Semipermanent connections between electrical elements are usually made by screwing on various types of terminals to binding posts or terminal blocks. Some examples are shown below. Separable Connections Use whenever circuits or interments are designed to be readily joined or connected to other electrical components. The class of components used to provide separable connections are called connectors. Connectors usually have two halves. One of the halves are called plugs, pins, or male ends. The corresponding halves are known as receptacles, jacks, sockets, or female ends.

16 Some examples of Separable Connections Cable connectors: (a) coaxial connector (screw type); (b) coaxial connector (bayonet type); ( c) pin-contact-type connector; (d) rack-and-panel connector.

17 Some examples of Semi-permanent Connections

18 Classes of Insulating Material