Page 191 - WCS - Electrical

WORKSHOP CALCULATION & SCIENCE - CITS

Economical factors

• Low cost.
• Easy availability.
• Easy to manufacture.
Classification of conductors
Conductors

Metallic Electrolytic Gaseous
The best conductors are metallic. The commonly used conductors in electrical appliances and machines are
described hereunder.

Silver
It is a soft and extremely malleable metal. Even though it is the best conductor, its use is limited because of its
high cost.

Copper
It is a very good conductor. It is mealleable and ductile, and also has high resistance to corrosion by liquids.
Therefore, it is widely used for wires, cables, overhead conductors, busbars and conducting parts of various
electrical appliances.
Aluminium
It is a metal light in weight. It is also ductile, malleable and a good conductor of electricity. Nowadays, it is more
widely used (since it is cheaper than copper) for wires and cables. All aluminium conductors (AAC) and aluminium
conductors (steel reinforced) (ACSR) are used in overhead and transmission lines. (More details on copper and
aluminium are furnished under the topic ‘non-ferrous metals and alloys as applicable to electrical trades’).
Resistance wires
These are conductors with very high resistance for specific applications like filaments of incandescent lamps,
heating elements etc. The following are a few examples:
1 Tungsten 2 Nichrome 3 Eureka
4 German silver 5 Manganin 6 Platinum
7 Mercury 8 Carbon 9 sBrass.

The resistance values of the metallic resistances will increase with increase in temperature.
insulators
Description
These are the materials which offer very high resistance to the flow of current and make current flow very negligible
or nil. These materials have very high resistance - usually of many megohms (1 megohm = 106 ohms) per
centimetre cube. The insulators should also posseses high dielectric strength. This means that the insulating
material should not break down or puncture even on application of a high voltage (or high electrical pressure) to
a given thickness.
Properties of insulators

The main requirements of a good insulating material are:
• high specific resistance (many megohms/cm cube) to reduce the leakage currents to a negligible value
• good dielectric strength i.e. high value of breakdown voltage (expressed in kilovolts per mm)
• good mechanical strength, in tension or compression (It must resist the stresses set up during erection and
under working conditions.)

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CITS : WCS - Electrical - Exercise 18