Page 129 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Types of Power wiring

Objectives: At the end of this lesson you shall be able to:
• explain the types of electrical wiring and their application
• state the advantages and disadvantages of each types

Many wiring systems are developed to meet the safety requirements, economy of cost, easy maintenance and
trouble shooting. A particular system can be chosen according to technical requirements but the system needs
to be approved by the local electricity authorities. The following are the fundamental requirements for any wiring
system. They are:

i For safety, switches should control the live phase wire. The second terminal of the switch called as half wire
should be connected to the appliance or socket through the wire. The neutral can be connected directly to the
appliance, socket or lamp.
ii For safety, fuses should be placed in the live/phase wire only.
iii To supply the rated voltage, parallel connections should be given to all lamps and appliances.
Types of wiring system: There are three types of wiring systems used for tapping supply from mains to the
different branches. They are as follows.
1 Tree system
2 Ring main system
3 Distribution board system

Tree system: In this system, copper or aluminium strips in the form of bus bars are used to connect the main
supply to the raising mains (Fig1). This system is suitable for multi-story buildings and the bus bar trunking space
is provided in the building at a convenient location and at load centres for the purpose of economy.
At each floor the running main is connected to the sub-main board through proper cable terminations. If there are
more than one flat in each floor the individual main switches for the flat get their supply from the sub-main board
through a distribution network which may include an energy meter for each flat.
However the system adopted within the flat will be the distribution board system.
Advantages

1 The length of the cables required for installation will become less. Hence, the cost is less.
2 This system is suitable for high rise buildings.
Disadvantages
1 The voltage across the appliances which are at the farthest end of the tree system may be less when compared
to the one connected to the nearest end if the bus bars size is not of sufficient size.
2 As fuses are located at different places, fault location becomes troublesome.
Ring main system: This system consists of two pairs of cables of size 4 or 6sq.mm which run through the rooms
and are brought back to the main or sub-board (Fig 2&3). Tappings are taken for sockets or ceiling roses from the
pair of cables through fuses and controlling switches. There may be saving of copper used because the current
can be fed from both sides. As this system requires special sockets or plugs with fuses it becomes costly; and
hence rarely used in India.
As per IEE regulations one ring circuit has to be there for every 100 sq metres of the floor area or part thereof.
The number of power plugs fed from branch lines (spurs) should not exceed two and the total current should not
exceed 30 amps. Protection for individual power plug can be provided by having built-in-fuses with the individual
power plugs or by having MCB type switch and socket arrangement.

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CITS : Power - Electrician & Wireman - Lesson 20-25