Bs bab 1

ELECTRICAL INSTALLATION IN BUILDINGS

General Objective

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: To understand electrical installation in buildings.

Specific Objectives :
At the end of this unit you should be able to :
•

differentiate the differences between single and three phase electrical supply

•

compare single and three phase electrical supply

•

understand types of electrical wiring

•

understand equipment in electrical wiring

•

understand and use electrical symbols in buildings


1.0

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INTRODUCTION
The importance of a good, safe residential wiring job is sometimes taken too

lightly. Poorly installed wiring can be very hazardous to the owner or renter as well as to
his personal property. A good and safe wiring installation will be a source of satisfaction
not only to the electrician, but to the owner or renter as well.

Electricity is too often taken as a matter-of-fact item when, in fact, it is one of our
most important servants. We use it without giving it a second thought, and we are
inclined to completely ignore the wiring installation as long as the lights come on when
we push a switch.

Today, nearly everything around and in the home depends on electricity. This
includes housekeeping chores, entertainment, hobbies, heating, cooling and the list is
endless. The average person spends the major portion of his time in his home. Most of his


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worldly goods are there; his loved ones are there, so every precaution should be taken to
ensure that safe and adequate electrical wiring is installed. Planning for the future growth
of electrical usage should also be done. This will involve only a small amount of extra
money if done during construction, but if the customer should later find the wiring
inadequate for his needs, then many dollars will be required to bring it up to date.

This can be seen from the tremendous increase in electrical consumption since
1928, when only one or two branch circuits were adequate. In the home of today, twenty
or more circuits are not uncommon. Electrical usage has far exceeded the predictions of
the electric utility companies.

Oooo…..Now I know, why
electricity is so important in our
lives.


1.1

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Differences Between Single And Three Phase Electrical
Supply.

What is a single-phase electric supply?
The supply cable to small buildings such as domestic premises is brought into the
building through a trench and left in a position near the entrance ready for the installation
of meters and fuse gear. If the meter and fuse gear is to be sited on the inside wall a 100
mm diameter duct is required for the cable.

The meter and fuse gear may, however, be sited on the outside wall. A more
recent method for the supply intake to a small building has been the use of an external
metal meter cabinet read from outside the premises and thus prevents the difficulty of
obtaining access to meters when the building is unoccupied.

The Building Research Establishment (BRE) has developed a combined service
intake component for use in domestic premises. The unit consists of a precast concrete
block which has provision for the entry of electric, water, gas, telephone and television
services. If the electric meter and fuses are housed in the same cupboard as the gas meter,
a fire-resisting partition should separate the two meters.


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What is a three phase electric supply?
Due to the higher electrical load large buildings such as hospitals,
factories and office blocks, will require a three phase supply. The loading in
some large buildings may be too high for the local low or medium voltage
system to provide and a private sub-station must therefore be installed, fed
from the high voltage cables from the Electricity Companies’ nearest
switching station. Sub-stations are often required for factories and hospitals.

The electrical installation in a large building is similar to a small
building, but is divided into sections. There may be one main intake panel
incorporating large fused switches or circuit – breakers, each of which
control a feeder cable to subsidiary distribution panels. These are smaller
version of the main intake panel and they control distribution boards for
each sub-section.


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Three phase electrical power use
Three phase electricity is used to run many different types of machines and
equipment today. Although single phase electricity is more prevalent today, three phase is
still chosen as the power of choice for many different types of applications. It is often
chosen because of its smoother more powerful nature which allows machines to last
many years longer and perform functions, such as instant reversing, that are not as
appropriate to single or two phase.
1. Uses from appliances
Three phase power has many uses from appliances such as refrigerators, to welders,
transformers, technical applications, ovens, computers and just about anything else you
can imagine being powered by electricity. Three phase by its very nature is a much more
smooth form of electricity than single phase or two phase power. It is this more consistent
electrical power that allows machines that use it to run more efficiently and last many
years longer than their relative machines running on the other phases. Some applications
are able to work with three phase power in ways that would not work on single phase at
all.


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Economic justification
Many properties are, for economic reasons, only connected to one of the three
phases of utility power available at the nearest distribution transformer. A single phase
supply line was the cheaper option when the premises were built. Similarly, large
buildings with 3 phase supplied might still have locations where it is uneconomic or
inconvenient to run 3 phase wiring. If three-phase equipment is to be used, a simple
comparison can be made: compare the cost for providing three-phase supply (e.g.
distribution transformer, power lines, monthly line charges) with the price of a single to
three-phase converter.

Example of applications
Woodworking workshops
a.

Metalworking workshops

b.

Vehicle servicing

c.

Farming

d.

Large buildings


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Single phase to three phase conversion (3 phase converters)
Installation
Install a single-phase industrial wall switch-socket combination. Connect your
new converter. Now you have three-phase power for all your electric motor and machine
tool requirements.
Motor speeds are constant as with a three-phase supply. Starting motors draw
high currents. A Booster E will produce these currents in order to maintain output
voltages. This will draw high input currents for a short time. On the single phase supply
side

it

pays

to

install

heavy

cables.

This

will

minimize

voltage

drop.

Installation instructions and service schematics are provided with each converter. A
simple block diagram and circuit schematic is shown below.

Figure 1.1 Single phase to three phase conversion

Figure 1.2 Single phase to three phase conversion


1.2

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Types of Electrical Wiring

1.2.1 Open Wiring

Figure 1.3 : open wiring

1.2.2 Hidden Wiring

Figure 1.5 : hidden wiring

Figure 1.4 : open wiring


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1.2.3 Conduit Wiring

We have completed our lesson on the
types of electrical wiring. The next
topic is equipment that are used in
electrical wiring. Are you ready? Let’s
go.

Figure 1.6 : Conduit Wiring


1.3

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How An Equipment Is Used In Electrical Wiring

1.3.1 Electrical Circulation
A current which may start at zero, increases to reach maximum, falls away to zero, and
then increases to an equal but opposite maximum and falls away again to zero.

1.3.2 Electrical Insulation
A non-conducting material such as rubber, plastic, porcelain, glass or mica, surrounding
or supporting a conductor. For example :
Copper conduit
Copper conduit is more resistant to corrosion than steel conduit and is therefore
useful for outdoor use and for burying in concrete when water is liable to gain access to
the conduit during installation. It has excellent electrical conductivity and provides good
earthlings, while its very high thermal conductivity tends to reduce condensation. The
smooth bore of the copper tube permits the wires to be drawn through easily, both
initially or after years of service.

Compression, capillary and bronze welded joints may be used, and inspection
bends and junction boxes are cast in copper alloy, such as gunmetal. The tube can be bent
by a machine or a spring.


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Aluminium conduit
Aluminium conduit is a welded aluminium tube which is prepared for jointing by
driving a pre-threaded steel tube into the end of the conduit. It can be installed with
earthing requirements required by the 16th edition of the IEE Wiring Regulations. The
system of jointing enables the conduit to be used with standard fittings, and the lightness
in weight of the tube offers a reduction in transport and installation costs. The steel
inserts which have tapered shanks, are knurled so as to ensure mechanical and electrical
connections when fitted into the tube.

To prevent corrosion the steel inserts are sheradised. The conduit may be used for
practically every type of installation, and in many respects it is equivalent to galvanized
steel. As with copper, it has excellent electrical conductivity and provides good earthing,
while its very high thermal conductivity tends to reduce condensation.

Aluminium conduit may be buried in the concrete or plaster, provided they
remain dry after setting is complete. If the concrete or plaster is expected to remain damp
for long periods, the conduit should be wrapped in adhesive PVC tape. The conduit may
be easily bent by a machine and like copper has a smooth bore which enables wires to be
drawn through easily, both initially or after years of service.


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Plastic conduit
PVC is used which may be unplasticised for rigid conduit and plasticised for
flexible tubes. The conduit may be obtained as heavy or light gauge and in round or oval
sections. Heavy-gauge conduit may be jointed by screwed threads and light-gauge
conduit by solvent welding. Injection-moulded PVC fittings are used and the range of
fittings is wide enough for a conduit installation to be completely plastic and therefore all
insulated. To ensure reliable and earthing continuity in an all-plastic installation, it is
necessary to use a continuous earth wire.

PVC is generally unaffected by water, acids, oils and soils and can be safely
burried in concrete, plaster or lime. It is light in weight, smooth in bore, quick to install
and will not ignite. If the conduit is held in a flame for long periods it carbonises, but the
carbon created is non-conductive. The coefficient of expansion is high (0.0003 5/°C), and
if the conduit is installed in straight runs for lengths in excess of 6 m the use of expansion
is recommended. Smaller diameter conduit may be bent cold by mear’s of a bending
spring and larger-diameter tube bent by heating and inserting a special rubber core. As
plastic tends to 'spring back' it is necessary for the initial bend to be more acute than is
finally required and the conduit secured as soon as possible afterwards. It is
recommended that the conduit should be used in temperatures between 70 °C and —10
°C.


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1.3.3 Lightening Conductor
Every sub-circuit which originates from the lighting distribution fuse board is
generally limited to a total load of 1000W and requires 5 A fuses and switches. In
large buildings 15 A fuses and wiring are sometimes used, due to the higher total
load on the circuit

1.3.4 Earth Circuit
A connection with the ground made in such a manner that an immediate and safe
discharge of electrical energy is ensured.

1.3.5 Fuse
A safety device consisting of a short length of relatively fine wire in a suitable
holder. The device is inserted in the live conductor at a suitable point and if the
current exceeds the design value, the fuse wire melts and thus prevents damage to
the circuit it protects.


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.

1.

List 3 examples that show the
application of three phase.electric
supply.

2.

What is the difference between single
phase and three phase electric supply?


1. The example is :
Woodworking workshops
a.

Metalworking workshops

b.

Vehicle servicing

2.

Three-phase electric supply is for large
buildings but single-phase supply is for
small buildings.

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1.4

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Graphic manual symbols in electrical installation in
buildings.

What is the symbol in electrical
installment in buildings?

One-way switch

Two-way switch


Discharge lamp

Lighting column

Wall lamp

Fuse

General guidelines

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Filament lamp

Pull or pendant switch

Circuit-breaker

Neutral link


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Please follow these general guidelines pertaining to electrical safety:
1.

Turn off power and unplug from the wall before working on electric or
electronic circuits, except when absolutely necessary.

2.

Do not use extension cords as a permanent power source.

3.

Do not work on electrical equipment in a wet area or when touching an object
that may provide a hazardous earth ground path.

4.

Turn off power and unplug equipment before checking or replacing fuses.
Locate and correct the cause of a blown fuse or tripped circuit breaker before
replacing the fuse or resetting the circuit breaker.

5.

Replace defective cords and plugs. Inspect cabling for defects such as frayed
wiring, loose connections, or cracked insulation.

6.

Remove metal jewelry, watches, rings, etc., before working on electrical
circuits.

7.

Always check the electrical ratings of equipment you use and be sure you use
that equipment within its ratings.

8.

Never overload circuits.

9.

Never leave unprotected systems unattended

10.

Never place containers of liquid on electrical systems.

11.

Never defeat the purpose of a fuse or circuit breaker.


12.

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Never install a fuse of higher amperage rating than that specially listed for
your circuit.

13.

Make sure equipment chassis or cabinets are grounded. Never cut off or defeat
the ground connection on a plug.

14.

Safely discharge capacitors in equipment before working on the circuits.

If you don’t want to be like this, make sure
questions regarding operation, maintenance,
or

safety

of

electrical

or

electronic

equipment should be directed to your lab
supervisor or an appropriate electronics
expert.


C3001 / UNIT 1 / 21

1.

List 5 general guidelines for electrical
safety.

2.

Sketch the symbol of a discharge lamp


1. The guidelines are :
a. Never leave unprotected systems unattended
b. Never place containers of liquid on electrical systems.
c. Never defeat the purpose of a fuse or circuit breaker.
d.


e. Never overload circuits
2.

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1.

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Electrical supply in large buildings should be :
a) Single phase
b) Three phase

2.

Give 1 reason why we need electricity ?

3.

The graphic symbols below are used for:

a.

b.

………………………..
4.

……………………….

Complete the sentences :
a. ……………and …………before working on electric or electronic circuits.
b.

Do not use………… as a permanent power source.

c. Remove……………….. before working on electrical circuits.
d. Do not work on electrical equipment in a………or…………….. an object that
may provide a hazardous earth ground path.
e. Always check the………………. of equipment you use and be sure you use


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1. Three phase is better to use for electrical supply in large building.
2. Electricity is important because it is a part of our common necessity.
3. a. wall lamp
b.
4

Fuse

The sentences is :
a.

Turn off power and unplug from the wall before working on electric or electronic
circuits.

b.


c.

Remove metal jewelry, watches, rings before working on electrical circuits.

d.

Do not work on electrical equipment in a wet area or when touching an object that
may provide a hazardous earth ground path.

e.

Always check the electrical ratings of equipment you use and be sure you use that
equipment within its ratings.

1.

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