Electrical Cables PDF

Learning Outcomes State the function of a cable State the three main parts of a cable Describe the specific roles of the three main parts of a cable State the common materials used to make conductors and insulators Identify the common sizes of cable used in residential premises 1....

Learning Outcomes State the function of a cable State the three main parts of a cable Describe the specific roles of the three main parts of a cable State the common materials used to make conductors and insulators Identify the common sizes of cable used in residential premises 1.8.1 Conductor A conductor is any material that allows a current to flow through easily. Conductors for everyday use should be: of low electrical resistance; mechanically strong and flexible; and relatively cheap. Silver has the lowest resistivity and thus better conductivity compared to copper and aluminium. However, it is too expensive to install cables made of silver. Hence, the most used material for cables is copper, as it is mechanically stronger and has better conductivity than aluminium. Unit 1.8 | Electrical Cables 58  1.8.2 Insulator An insulator is any material that has high electrical resistance. Its function is to confine a current to the conductor, minimising the risk of electric shock and fire. Many materials can be used to insulate cable conductors, accessories and equipment. The type of insulation used depends on the supply voltage it is subjected to and its operating environment. Different insulating materials are used for different applications (Table 1.8-1). Table 1.8-1: Common insulating materials and applications Material Application Bakelite Accessories Ceramic Support for switchgears Polyvinyl Chloride (PVC) Cable insulation or sheaths Mica Insulation for heating elements Rubber Cable insulation Paper Cable insulation and capacitors 1.8.3 Cable A cable consists of at least two parts: a conductor an insulator Fig. 1.8-1: Cable conductor and insulator The conductor is normally made of copper and aluminium. The insulator can be made of: polyvinyl chloride (PVC); vulcanised rubber (VIR); mineral insulation (MI); or cross-linked polyethylene (XLPE). A cable is always named after the insulation used. Hence, the PVC cable got its name because its insulator is made of PVC. Unit 1.8 | Electrical Cables 59  1.8.4 Mechanical Protection Some cables have an additional sheath and/or armour. These are provided to prevent mechanical damage to the cables during installation and throughout their subsequent operation. Fig. 1.8-2: Parts of a cable Sheathing involves covering the cable insulation with materials such as PVC, copper and aluminium. Apart from giving mechanical protection, the sheath also prevents moisture from damaging the insulation of the cable. Armouring involves wrapping the cable with metal in the form of steel wire or metal tape. Very often, cables are sheathed and armoured to give complete protection (Fig. 1.8-3). Fig. 1.8-3: PVC cable with steel-wire armour 1.8.5 Stranding Stranding refers to the conductor being divided into a number of smaller wires that are twisted together in a spiral fashion, forming a core equivalent to a single wire of the required size (Fig. 1.8-4). Fig. 1.8-4: Cable stranding Conductors are often stranded to ensure flexibility and ease of handling. Unit 1.8 | Electrical Cables 60  1.8.6 Cable Size The size of a cable can be specified by: the number of strands and the diameter of each strand (e.g., 7/0.67, where “7” refers to seven strands of wire and “0.67” refers to each strand of wire having a diameter of 0.67 mm); and the nominal cross-sectional area of its conductor (e.g., 2.5 mm2). A cable is normally indicated by the cross-sectional area of its conductor. Some common cable sizes for residential wiring are 1.5 mm2, 2.5 mm2, 4 mm2, 6 mm2 and 10 mm2. The size of a cable will determine how much current it can carry. 1.8.7 Identification To ensure ease of connection, every conductor should be identifiable at its terminations and preferably throughout its length (Table 1.8-2). Table 1.8-2: Standard colour identification for cables Alphanumeric Function Colour Number Phase of Single-Phase Circuit L Brown Neutral of Single-Phase or Three-Phase Circuit N Blue Phase One of Three-Phase Circuit L1 Brown Phase Two of Three-Phase Circuit L2 Black Phase Three of Three-Phase Circuit L3 Grey Protective (Earth) Conductor – Green and yellow Unit 1.8 | Electrical Cables 61