Properties of conductors and insulators

All materials are made up of atoms and electrons. What makes them different materials is the way in which the atoms and electrons are arranged and how strongly the electrons are attracted to the atoms. 

• A conductor is a material, usually a metal, in which the electrons are loosely bound to the central nucleus. These electrons can easily become ‘free electrons’ which allows heat and electricity to pass easily through the material. 
• An insulator is a material, usually a non-metal, in which the electrons are very firmly bound to the nucleus and, therefore, will not allow heat or electricity to pass through it. 

Let us now define the terms and properties of some of the materials used in the electrical industry. 

• Ferrous A word used to describe all metals in which the main constituent is iron. The word ‘ferrous’ comes from the Latin word Ferrum, meaning iron. Ferrous metals have magnetic properties. Cast iron, wrought iron, and steel are all ferrous metals. 
• Non-ferrous Metals that do not contain iron are called non-ferrous. They are non-magnetic and resist rusting. Copper, aluminium, tin, lead, zinc, and brass are examples of non-ferrous metals. 
• Alloy An alloy is a mixture of two or more metals. Brass is an alloy of copper and zinc, usually in the ratio 70–30% or 60–40%. 
• Corrosion The destruction of a metal by chemical action. Most corrosion takes place when a metal is in contact with moisture (see also mild steel and zinc). 
• Thermoplastic polymers These may be repeatedly warmed and cooled without appreciable changes occurring in the properties of the material. They are good insulators but give off toxic fumes when burned. They have a flexible quality when operated up to a maximum temperature of 70°C but should not be fl exed when the air temperature is near 0°C, otherwise, they may crack. 
• Polyvinylchloride (PVC) used for cable insulation is a thermoplastic polymer.
• Thermosetting polymers Once heated and formed, products made from thermosetting polymers are fixed rigidly. Plug tops, socket outlets and switch plates are made from this material. 
• Rubber is a tough elastic substance made from the sap of tropical plants. It is a good insulator but degrades and becomes brittle when exposed to sunlight. 
• Synthetic rubber is manufactured, as opposed to being produced naturally. Synthetic or artificial rubber is carefully manufactured to have all the good qualities of natural rubber – flexibility, good insulation, and suitability for use over a wide range of temperatures. 
• Silicon rubber Introducing organic compounds into synthetic rubber produces a good insulating material that is flexible over a wide range of temperatures and which retains its insulating properties even when burned. These properties make it ideal for cables used in fi re alarm installations such as FP200 cables. 
• Magnesium oxide The conductors of mineral-insulated metal-sheathed (MICC) cables are insulated with compressed magnesium oxide, a white chalk-like substance that is heat resistant and a good insulator, and lasts for many years. The magnesium oxide insulation, copper conductors and sheath, often additionally manufactured with various external sheaths to provide further protection from corrosion and weather, produce a cable designed for long-life and high-temperature installations. However, the magnesium oxide is very hygroscopic, which means that it attracts moisture and, therefore, the cable must be terminated with a special moisture-excluding seal.

Copper

Copper is extracted from an ore that is mined in South Africa, North America, Australia, and Chile. For electrical purposes it is refi ned to about 98.8% pure copper, the impurities being extracted from the ore by smelting and electrolysis. It is a very good conductor, is non-magnetic and offers considerable resistance to atmospheric corrosion. Copper toughens with work, but maybe annealed, or softened, by heating to a dull red before quenching.

Copper forms the largest portion of the alloy brass and is used in the manufacture of electrical cables, domestic heating systems, refrigerator tubes, and vehicle radiators. An attractive soft reddish-brown metal, copper is easily worked and is also used to manufacture decorative articles and jewelry. 

Aluminium

Aluminium is a grey-white metal obtained from the mineral bauxite which is found in the United States, Germany and the Russian Federation. It is a very good conductor, is non-magnetic, offers very good resistance to atmospheric corrosion and is notable for its extreme softness and lightness. It is used in the manufacture of power cables. The overhead cables of the National Grid are made of an aluminium conductor reinforced by a core of steel. Copper conductors would be too heavy to support themselves between the pylons. Lightness and resistance to corrosion make aluminium an ideal metal for the manufacture of cooking pots and food containers. 

Aluminium alloys retain the corrosion resistance properties of pure aluminium with an increase in strength. The alloys are cast into cylinder heads and gearboxes for motor cars, and switch-boxes and luminaires for electrical installations. Special processes and fluxes have now been developed which allow aluminium to be welded and soldered.

Brass

Brass is a non-ferrous alloy of copper and zinc which is easily cast. Because it is harder than copper or aluminium it is easily machined. It is a good conductor and is highly resistant to corrosion. For these reasons, it is often used in the electrical and plumbing trades. Taps, valves, pipes, electrical terminals, plug top pins and terminal glands for steel wire armour (SWA) and MI cables are some of the many applications. 

Brass is an attractive yellow metal which is also used for decorative household articles and jewellery. The combined properties of being an attractive metal which is highly resistant to corrosion make it a popular metal for ships’ furnishings.

Cast steel

Cast steel is also called tool steel or high-carbon steel. It is an alloy of iron and carbon which is melted in airtight crucibles and then poured into moulds to form ingots. These ingots are then rolled or pressed into various shapes from which the finished products are made. Cast steel can be hardened and tempered and is therefore ideal for manufacturing tools. Hammerheads, pliers, wire cutters, chisels, files and many machine parts are also made from cast steel.

Mild steel

Mild steel is also an alloy of iron and carbon but contains much less carbon than cast steel. It can be fi led, drilled or sawn quite easily and maybe bent when hot or cold, but repeated cold bending may cause it to fracture. In moist conditions, corrosion takes place rapidly unless the metal is protected. Mild steel is the most widely used metal in the world, having considerable strength and rigidity without being brittle. Ships, bridges, girders, motor car bodies, bicycles, nails, screws, conduit, trunking, tray and SWA are all made of mild steel.

Zinc

Zinc is a non-ferrous metal which is used mainly to protect steel against corrosion and in making the alloy brass. Mild steel coated with zinc is sometimes called galvanized steel, and this coating considerably improves steel’s resistance to corrosion. Conduit, trunking, tray, SWA, outside luminaires and electricity pylons are made of galvanized steel.