Chapter 1 4

Electrical installation
1. What is installation?
An installing or being installed
Something installed; specif., A complete mechanical apparatus fixed in position for use: a
heating installation
Any military post, camp, base, etc.
A work of art requiring construction or elaborate setting up at its exhibition site
An installation typically makes use of a variety of media and often includes nontraditional
media, as projected images or taped sounds
The definition of an installation is the act of putting something in, a device that stays in
one place, a military base, or an art piece that often involves building and different types of
materials.
2. What is electrical installation?
• The electrical installation is an assembly of components that allows you to reliably and safely use
electrical power around your home.
12/16/2019 Prepared by Haymanot T. (Lecturer ) 1

Chapter 1: Illumination
Contents
Lighting
Lighting terminologies and laws
Artificial light source and their types
Lighting scheme and their types
Illumination design and calculation
Prepared By:
Haymanot Takele (Lecturer)

Lighting
A form of radiate energy which is radiated or sent from a source in the form
of wave
Is part of whole family of electromagnetic wave
Light in general is an electromagnetic radiation such as radio wave, x-ray etc.

Lighting terminologies and laws
Luminous intensity: symbol, I ; unit, candela (cd)
This is a measure of the power of a light source and is sometimes referred to
as brightness
Luminous flux: symbol, F ; unit, lumen (lm)
This is a measure of the flow or amount of light emitted from a source.
Illuminance: symbol, E ; unit, lux (lx) or lm/m2
This is a measure of the amount of light falling on a surface. It is also referred
to as illumination.
Luminous efficacy: symbol, K ; unit, lumen per watt (lm/W)
This is the ratio of luminous flux to electrical power input. It could be thought
of as the ‘ efficiency ’ of the light source.

Coefficient of utilization (CU): no unit
The amount of useful light reaching a working plane will depend on
The lamp output,
The reflectors and/or diffusers used,
Position of lamp,
Color of walls and ceilings, etc.
The lighting designer will combine all of these considerations and
determine a figure to use in his or her lighting calculations.

Maintenance factor (MF): no unit
• The total flux emitted by the source and reflector may be reduced due to
deposition of dust and dirt upon their surfaces.
• In order to allow for the collection of dirt on a lamp and also ageing, both of
which cause loss of light, a maintenance factor is used.
 Example
A new 80W fluorescent lamp with a lumen output of 5700lm is installed. After
about 3 or 4 months this output would have fallen and settled at around 5200lm.
By what amount the light output has decreased?
𝑀𝐹 =
𝑓𝑎𝑙𝑙𝑒𝑛 𝑎𝑛𝑑 𝑠𝑒𝑡𝑡𝑒𝑙𝑒𝑑 𝑜𝑢𝑡𝑝𝑢𝑡
𝑖𝑛𝑡𝑖𝑎𝑙 𝑙𝑢𝑚𝑒𝑛 𝑜𝑢𝑡𝑝𝑢𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑎𝑚𝑝
=
5200
5700
= 0.9
This value, 0.9, is the maintenance factor and should not fall below 0.8. This is
ensured by regular cleaning of the lamps.

Lighting laws
1. Inverse-square law:
 When using this law, the distance used in the measurement is
from the light source to a point directly below it.
If the surface are moved to a position twice the distance from its original
position then the illumination received on that surface is quarter of its
former value.
When a lamp is suspended above a surface, the illuminance at a point
below the lamp can be calculated:
Illumination on a surface.
𝐼𝑙𝑙𝑢𝑚𝑖𝑛𝑎𝑛𝑐𝑒 𝐸 𝑙𝑢𝑥 =
𝑙𝑢𝑚𝑖𝑛𝑜𝑢𝑠 𝑖𝑛𝑡𝑒𝑛𝑠𝑖𝑡𝑦 (𝑐𝑑)
𝑑2
→ 𝐸 =
𝐼
𝑑2

𝐸 𝑥 =
𝐼 ∗ 𝑐𝑜𝑠3
𝜃
𝑑2
Where
d= distance from the surface
I= illumination
 When using this law, the distance used is from the light source
measured at an angle to the point at which the lux value is required.
 Illumination varies as cosine of the angle between the normal to the
surface and direction of incident light.
 When a lamp is suspended above a horizontal surface, the illuminance
(E) at any point below the surface can be calculated.
2. Cosine rule

Example
1. A 250W sodium-vapour street lamp emits a light of 22500cd and is situated 5m
above the road. Calculate the illuminance
(a) Directly below the lamp
(b) At a horizontal distance along the road of 6m
Solution
(a) Directly below the lamp by inverse square law
It can be seen that the illuminance at A is given by
𝐸𝐴 =
𝐼
𝑑2
=
22500𝑐𝑑
(5𝑚)2
= 900𝑙𝑥
(b) At a horizontal distance along the road of 6m by cosine rule
The illuminance at B is calculated as follows.
Since the angle θ is not known, it can be found most simply by trigonometry:
𝑡𝑎𝑛𝜃 =
𝐴𝐵
𝑑
=
6
5
= 1.2
𝜃 = 𝑡𝑎𝑛−1 1.2 = 50.20 ∴𝜃 = 50.20,
𝑐𝑜𝑠50.20=0.64
𝐸 𝐵 =
𝐼𝑐𝑜𝑠3 𝜃
𝑑2
=
22500𝑐𝑑∗0.643
52
= 236𝑙𝑥

Source of light and their types
1. Natural source
2. Human made or artificial light source

1. Natural source
Light occurs in nature
oSunlight,
oMoonlight, and
oStarlight are the most important sources of light to life.
But because of their need for additional light, humans have learned to create
light as well.
Understanding the fundamental difference between natural and man-made
light is the beginning of understanding light sources.
Natural light sources occur within nature and are beyond the control of
people.

2. Human made or artificial light source
Man-made light sources can be
controlled by people, more or less
when and in the amount wanted.
These includes:
Wood, Oil and Gas flame,
Electric lamps,
Photochemical reactions, and
Various reactions, such as
explosives.
Due to their obvious advantages in
terms of
Availability,
Safety,
Cleanliness, and
Remote energy generation,
Electric lamps have displaced
almost all other man-made sources
for lighting of the built environment.
However, because man-made sources consume natural resources, natural light sources
should be used to the greatest extent possible

• Generally electric lamps are classified in to two:-
1. Incandescent lamps
2. Discharge lamps
Types of artificial light sources

Incandescent lamp
• An incandescent lamp consists of a glass globe completely evacuated or gas
filled and a fine metal wire called filament .
• The filament is of modern lamp is made of tungsten because this material
has high melting point (34000c) and can be manufactured in the form of
suitably thin wire.
• There are other materials which can be used for the filament carbon,
osmium and tantalum.
• These materials have high melting point.

• There are two types of incandescent lamps:-
 Vacuum filled lamps :-
 are lamps where air is evacuated from the glass bulb just to prevent
oxidation of the filament.
 They are evacuated b/c tungsten has a property to react with the oxygen
in the air and quickly evaporate.
 Gas filled lamps:-
 the glass bulb is filled with inert gases.
 Filling the bulb with an inert gas slows bulb blackening, which is caused
by condensation of evaporated tungsten particles on the inner bulb wall.
 Operates up to around 25000c.
 In gas-filled lamps, the bulb is so bright that it is given an opaque
coating internally.

• Incandescent lamps are usually sold by wattage.
• But a watt is not a measure of light-it is a measure of power consumed.
• It is a measure of how much electricity the lamp uses during its operation.
• Lumen tells how much light a lamp emits.
• The efficiency of incandescent lamps is
10-15 lm/w.
a 25W IL produces about 250 to 375 lm.
 The average lifetime of incandescent lamps is about 2000 hours when
operating at rated voltage but they achieve this longer life by reduction in
light output and efficiency.

• This reduction is because the filament evaporation continues
throughout the life time even if the globe is filled with inert
gases.
• An incandescent lamp gives out light at all frequencies including
DC.
• Incandescent lamps suffer from these disadvantages:-
• low efficiency and overheating.
• coloured light.

Discharge lamps
• In electric discharge lamps, light is produced by the passage of an
electric current through a vapor or gas.
• When an electric current is passed through certain gases visible light
is produced.
• Gases are normally pure conductors especially at atmospheric
pressure, but applications of suitable voltage called, ignition voltage,
across the two electrodes can result in a discharge through the gas,
which is accompanied by electromagnetic radiation.

• The wavelength of the radiation depends upon the gas, its pressure
and the metal vapor used in the lamp.
• The colour of the light emitted depends upon the type of gas used.
• The colour obtained from some of the gases and vapours
commonly employed are listed in the table below

• Electric discharge lamps can in general be classified as cold cathode and
hot cathode types.
Cold cathode lamps
• In some type of discharge lamp the electrodes are not heated.
• These types are therefore known as cold cathode lamps, an example of this
being the ordinary neon tube.
• Uses a high voltage (3.5KV) for its operation
• They are familiar as fluorescent tubes with 25mm in diameter, either
• Straight,
• Curved, or
• Bent
to take a certain form for general lighting purpose.
• The electrodes of these lamps are not preheated. e.g. Neon lamps

Hot cathode lamps
• In this type of discharge lamps the electrodes are heated, as this reduces
the voltage required to strike and maintain discharge.
• The hot cathodes are usually in the form of short filament which may be
heated either by passing a heating current through it or by discharge
current itself.
• Hot cathode lamps are produced as
• Sodium vapour lamps,
• High-pressure mercury vapour lamps, and
• Fluorescent lamps (low pressure mercury vapour lamps).

Sodium vapor lamps
• Is a double glass container, the inner glass tube filled with neon and
argon gas and some sodium drops.
• When the supply is switched on, the lamp would not start as the
supply voltage is too low to start the discharge.
• The leak transformer is connected across the mains produces a
starting voltage of about 400v.
• Then the neon argon gas starts the discharge;
• This produces initial heat;
• At this stage a red light is emitted b/c of neon gas, and afterwards
the sodium starts vaporizes and the discharge continues causing the
colour of the discharge to change from red to yellow.

• This lamps have an efficiency around 150lm/w.

High pressure mercury vapor lamps
• It consists of a quartz tube containing mercury at high pressure and a little argon
gas to assist starting.
• There are two main electrodes and auxiliary electrode connected through a high
resistance.
• The auxiliary electrode is used to start the discharge.
• A choke is provided to limit a current to a safe value.
• A capacitor is connected in parallel to the lamp to improve its power factor.
• The initial discharge takes place in the argon gas between the auxiliary (starting)
electrode and main electrode close to it.
• This causes the main electrode to heat up and the main discharge between the
main electrodes takes place.

• The high pressure mercury vapour lamp
has an efficiency of about 40-50lm/W
they are manufactured in 250 W and
400W ratings for use on 220-250v a.c.
supply mains.
• Their application is mainly for
industrial and street lighting,
commercial and display lighting.
Typical high-pressure mercury vapour lamps:
(a) basic circuit (b) modern mercury vapour lamp.

Fluorescent lamp (Low pressure mercury vapour lamps)
• This lamps work on the phenomenon called florescence.
• Certain materials, such as calcium halo phosphate, emit visible light whenever
they absorb ultra-violet light. This phenomenon is known as fluorescence.
• Florescent lamps contain a glass tube filled with mercury vapor and inert gas.
• Is provided with two electrodes coated with electron emissive material.
• When current flows the electrons hit the mercury atom energy is transferred to the
mercury electrons pushing them in to higher orbit around the atom.
C
AC supply capacitor
Inductor or
choke
Lamp
electrode
Starter switch

• When current flows the electrons hit the mercury atom energy is transferred
to the mercury electrons pushing them in to higher orbit around the atom.
• When these electrons fall back to their original orbit they release this
energy in the form of ultraviolet radiation.
Operating principle
 When the supply switched on the ckt is complete via choke
 The element coated with oxide become warm and the oxide coating emits
some electron and the gas ionizes at the end of the tube.
 The starter contact ( bimetallic+ heater or bimetallic+ he gas) separates away
to the current pass through them and the choke is open circuited.
 This causes high voltage ( v=l di/dt) to appear across the breaking contact and
an energy is released in the form of arc and this high voltage appear across the
end of the tube.
 When the gas fully ionized the choke limit the current to safe value.
 The application includes lighting of shops, homes, factories, streets, ships,
transport (buses and trains),

•Read and compare the types of lamps with their
working principle including equivalent circuit
diagram and practical applications
Assignment

Lighting Schemes
• The Planning of lighting in a house is most important aspect of interior decoration
and designing of illumination system in modern houses.
• The intensity of illumination in various portions of rooms or buildings is based on
purpose for which the light is required.
• For instance more light is required in study rooms as compared to bed room.

The following classification is done in the building as:
Direct Lighting:
 In this scheme the light is directly made to fall on the
working plane.
 If proper reflector is used more than 90% of the total light
flux can be made to on working plane. see figure below

 This scheme causes hard shadows and glare is by large, not
preferred for indoor residential lighting.
 It is mainly used for industrial a general outdoor lighting.

Semi-Direct:
• in this scheme, semi-direct reflectors are used. As a result (60-90)% of the total
flux is made to fall on the working plane and the remaining light is used to
illuminate the ceiling and watts.
• This is more suited with high ceiling where high level of distributed illumination is
desirable. see figure below:

Semi-Indirect Lighting:
• It produces very soft lighting system.
• The light flux 60% to 90% is thrown upward to the ceiling for reflection and the
remaining light reaches the working plane directly except for some absorption by
the reflector.
• it provides soft shadows and glare free lighting scheme.
• It is adopted for indoor light decoration purpose.

Indirect Lighting Scheme:
• In this scheme, 90% to 100% of total light is thrown upward to the ceiling for
diffused reflection by using inverted or bowl reflectors.
• The light thus thrown towards ceiling is reflected back on working plane.
• The ceiling thus acts as an indirect light source and glare is reduced to minimum.
• It is particularly used where shadows are to be necessarily removed, see figure
below:

Methods of lightning calculation
• In order to estimate the number and the type of light fittings required
to suit a particular environment,
• it is necessary to know what level of luminance is required,
• the area to be illuminated, the maintenance factor and the coefficient
of utilization,
• and the efficiency of the lamps to be used.
• Two method
• Watts per square metre method
• Lumen or Light flux method

• Applicable for rough calculations.
• It consists in making an allowance of watts per square meter of area to
be illuminated accordingly to the illumination desired on the assumption
of the average figure of an overall efficiency of the system.
• According to NEC 220-3(d) this figure is about 3 watt per ft2.
• A = 30ft X 50ft= 1500ft2 (house size ,80w fluorescent lamp)
• Total wattage required = 1500 X 3w
• = 4500w  4.5Kw
• No of lamps required = 4.5Kw/80w = 56.1
•  56 lamps - each 80w
• Current carrying capacity = 4.5Kw/220V = 20.5A
Watts per Square Meter method

It is the most advisable method to be used.
Lumens' reaching the working plane is calculated as:
𝜙 =
1.25∗𝐸∗𝐴
𝐶𝑓
or 𝜙 =
𝑛𝑃𝜙 𝐿 𝜂 𝐵
𝐷𝐹Where
E - Illumination level
P - Wattage of each lamp
n - Number of lamps required
1.25 - Design values for new installations: 1.25 En
A - Working surface in m2
L - Luminous flux of one lamp in lm
B - Utilisation factor
Lumen or Light flux or efficiency method

Example
1. A work area at bench level is to be illuminated to a value of 300lx, using 85W single fluorescent fittings having an efficacy of 80lm/W.
The work area is 10mX8m, the MF is 0.8 and the CU is 0.6.
(a) Calculate the number of fittings required.
Solution
Total lumens (F) required
=
𝐸 𝑙𝑥 ∗𝐴𝑟𝑒𝑎
𝑀𝐹∗𝐶𝑈
=
300∗10∗8
0.8∗0.6
= 50,000𝑙𝑚
Since the efficacy is 80lm/W:
𝑇𝑜𝑡𝑎𝑙 𝑤𝑎𝑡𝑡𝑎𝑔𝑒 =
𝑇𝑜𝑡𝑎𝑙 𝑙𝑢𝑚𝑒𝑛𝑠 𝐹 𝑟𝑒𝑞𝑢𝑖𝑟𝑒𝑑
𝜂 ൗ𝑙𝑚
𝑊
=
50,000𝑙𝑚
80 ൗ𝑙𝑚
𝑊
= 625𝑊
As each lamp is 85W:
Number of lam𝑝𝑠 =
𝑡𝑜𝑡𝑎𝑙 𝑤𝑎𝑡𝑡𝑎𝑔𝑒
𝑟𝑎𝑡𝑖𝑛𝑔 𝑜𝑓 𝑡ℎ𝑒 𝑙𝑎𝑚𝑝
=
625
85
= 8

Lighting design and calculations
Example 1.1 The illuminance (E) on the working plane in Fig. below is 500 lux. The
reflectance is 50%, calculate the luminance of the working plane.
L = E x R(p.u.)
= 500 x .5 = 250 Apostilbs
= 250 / 3.14 = 80 cd/m2
Note: 1cd/m2
= 3.14 Apostilb
= 3.14 lm/m2

Example 1.2. A point light source has an intensity of 1,000 candela and the light
falls perpendicularly on a surface. Calculate the illuminance on the surface if its
distance from the surface is:
a) Two meters,
b) Four meters and
c) Six meters

Example 1.3 A point light source has an intensity of 2,000 candela in all
directions and is mounted 4 meters above a surface.
Calculate the illuminance on
i. the surface directly underneath (Ea) and
ii. at a distance of 3 meters to the side (Eb).

Example 1.4 A walkway is illuminated by three SON 250W lamps each having a
luminous intensity of 4750 candela in all directions below the horizontal. Each lamp is
installed at a height of 6m and the distance between them is 16 meters. Calculate the
illuminance contributed by each lamp
(A) Directly underneath,
8 meters from the base,
16 meters from the base,
32 meters from the base.
(B) the total illuminance at:
(i) the base of each lamp post,
(ii) midway between the base of each lamp post.
(C) sketch an illuminance profile on a straight line joining the base of each lamp post.

• Let the illuminance at A, B, C and D be Ea, Eb, etc., respectively.

C) Sketch an illuminance profile on a straight line joining the base of each
lamp post.

1. It is proposed to illuminate an electronic workshop of dimensions 9 x 8
x3 m to an illuminance of 550 lx at the bench level. The specification calls for
luminaires having one 1500 mm 65 W fluorescent natural tube with an initial
output of 3700 lumens. Determine the number of luminaires required for
this installation when the UF and MF are 0.9 and 0.8, respectively. The
number of luminaire required (N)
Therefore 15 luminaires will be required to illuminate this workshop to a level of 550 lx.

• Other factors that may be taken into consideration when using the lumen
method are :
1. Room Index: this includes
Room dimensions:
(i) Length (a)
(ii) Width (b)
(iii ) Height (h)
Useful Height - Hk. This can be calculated as:
hk = h- hd or hk= h-hd-hv ………. (1)
where :
hk = useful height,
h = room height
hd = height of working area, usually taken as : 0.85 m
hv = height of illumination unit hanging from the ceiling ,
measured in (m).
The reflection factor – ρ
 This means the light reflected from ceilings, walls and floors which and
depends on the colors, type of floor and ceilings
 The reflection coefficients ρ can be used to determine the utilization factor UF
for any luminaire from the manufacturers catalogues when the room index is
calculated.12/16/2019 Prepared by Haymanot T. (Lecturer ) 46

Example It is proposed to illuminate a class room of dimensions 6 x 8 x 2.85 m
to an illuminance (E) of 400 lx at the bench level.
The specification calls for luminaires having one 1050 mm 40 W fluorescent
natural tube with an initial output of 3200 lumens with white metal base and
prismatic plastic diffuser.
Determine the number of luminaires required for this installation when the MF is
0.7, respectively. The reflection coefficients are: (C= 0.70, W= 0.3, F=0.2)
Solution
From the room dimension we can calculate the
room index ( k )
assuming the working table height is 0.85 m.
Hence, hk = 2.85-0.85 = 2m

Since 15 luminaire are large number that can be installed in the ceiling,
so we suggest to use luminare with 2x40 W fluorescent lamps with prismatic
diffuser.
Hence, the number of luminaire required will be,
Luminaires distribution:
Distance between two adjacent luminaire s is
Distance between the luminaire and its adjacent wall = (½ to ⅓)x(room height):
Note: Usually we take the factor ½ when the dimensions of the room are such
that the ratio of the length to the width is less than 1.6, otherwise we take the
factor of ⅓ .12/16/2019 Prepared by Haymanot T. (Lecturer ) 48

Luminaires distribution on room ceiling

Space: height ratio (shr)
• This is the ratio of space between luminaires (S) to their height above the
working plane (Hm).
• Manufacturers will specify a recommended SHR for each of their luminaires.
Ensuring that luminaires are spaced within the recommended value will mean
an acceptable variation in illuminance across the working plane.
• This is expressed in terms of the Uniformity Ratio.

• Example A factory area is 40m long, 20m wide and is 8m high. Point source
luminaires are suspended 1.5 meters below ceiling level. The working plane is
1 meter high. Calculate the minimum number of luminaires which must be
installed to conform with a recommended SHR of 1.5 : 1.
 This means that the minimum number to
conform with SHR. requirement is 3 rows with
5 luminaires per row.
 More than this number can be used if
desired for reasons such as balance, effect,
control or ease of installation.
 Assuming that three rows of five luminaires is
suitable, the actual spacing is determined as
follows:

Chapter 2: Electrical installations in consumer premises
Contents
Components and accessories
Electrical installation materials
Electrical regulations and standards
Residential installation design
Commercial installation design
Residential design drawings
Commercial installation design drawings

WIRING MATERIALS AND ACCESSORIES
In order to assemble properly and intelligently the great
number of available
• Electrical devices,
• Fittings,
• Materials and
• Equipment to form a complete wiring system,
 we must understand the basic principles regarding them.

Wire and Cable
The term wire and cable are used more or less synonymously in house
wiring .
Bare conductors, whether single or stranded together are termed as wire .
Conductors covered with insulation are termed as cables.
The necessary requirements of cable are that it should conduct
electricity efficiently, cheaply and safely .

This should neither be small so as to have a large
internal voltage drop nor be too large so as to cost too
much.
Its insulation should be such as to prevent leakage
of current in unwanted direction and thus to
minimize risk of fire and shock.

A Cable consists of three parts
A) The conductor or core :- the metal Wire or strand of Wires
carrying the current.
B) The insulation or dielectric :- a covering of insulating material to
avoid leakage current from the conductor.
C) The protective covering :- for protection of insulation from
mechanical damage.

Conductor Materials used in Cables
 Copper and Aluminum are the materials used as conductors
Copper:- Though Silver is the best conductor but due to its higher cost it is hardly
used any where.
• The next best conductor is, copper.
• It is cheaper comparatively
It is mechanically strong, hard, extremely tough, durable and ductile.
It is highly resistive to corrosion, Oxidation and pitting.

2. Aluminum
 Aluminum is frequently used in place of copper for
bare electric cables used for long distance power
distribution.
The electrical conductivity of aluminum is about 60%
of that of copper (2.87x10-8 ohm-m at 20 0c) .
So for the same resistance for a given length, the
aluminum required be 1.61 times that of copper in
volume and 1.26 times of that of copper in diameter.

The only application of aluminum cables for wiring
in buildings is
• for a continuous bus- bar system of distribution,
• used sometimes in blocks of flats or office buildings
for rising mains and sub-mains of larger sectional
area.

Insulating materials
The insulation material used in electric cable must posses following
properties:
i. High resistivity
ii. High flexibility
iii. Non- inflammability
iv. High resistivity to moisture, acids or alkalies qualities.
 So the type of insulating material used depends upon the service for
which the cable is required

The various types of insulting materials used in cables are
i) Rubber:
 Rubber may be natural or synthetic.
 Its relative permittivity is between 2 and 3 and its dielectric
strength is 30KV/mm.
 Though it posses high insulting qualities,
 It absorbs moisture readily ,
 softens when heated to a temperature of 60 oC to 70 oC ,
 swells under the action of mineral oils and ages when
exposed to light .
 Hence pure rubber cannot be used as insulating material.

PVC: Polyvinyl chloride is a man made thermo- plastic which is
tough, Incombustible and chemically un reactive.
 Its chief drawback is that it softens at temperatures above
80oC. It does not deteriorate with age
 PVC insulated cables are usually employed for low and
medium voltage domestic and industrial lights and power
installations
iii) Vulcanized India Rubber. It is prepared by mixing India rubber
with Minerals such as sulphur, zinc red lead, etc.
 The copper conductors used in this cable are tinned to protect
them from corrosive action of rubber on copper.

 The use of VIR cables is limited to low voltage distribution
and internal wiring as paper insulated cables have largely
superseded them.
iv. Impregnated Paper. It is quite cheap, has low capacitance,
high dielectric Strength (30KV/mm) and high insulation
resistivity(10 Mohm-cm) .
 The main advantage of paper insulated cables is that a cable
of given size can be worked out at a higher current density
than a VIR cable

Types of Cables used in Internal Wiring
 The wires employed for internal wiring of buildings may be
divided into different groups according to:-
 The type of conductor
The number of cores
 The voltage grading and
The type of insulation used.
 According to the number of cores, the cables may be divides
into the classes known as:- single core, twin core, twin core
with ECC (earth continuity conductor )cables etc.
 According to voltage grading the cables may be divided into
two classes:
250/440 volt and 650/1100 volt cables.

Cables classified according to the type of insulation are:-
Vulcanized Indian Rubber (VIR) Cables :
 VIR cables are available in 250/440 volt as well as in 650/1100
volt grades and used for general conduit wiring.
Lead Sheathed Cables :
 These cables are also available in 250/440 volt grade and are used
for internal wiring where climatic condition has moisture.

 The lead sheathed cable is a vulcanized rubber insulated
conductor covered with a continuous sheath of lead.
 The sheath provides very good protection against the
absorption of moisture and sufficient protection against
mechanical injury and can be used without casing or
conduit system.
 It is available as a single core, flat twin core , flat three
core and flat twin core with an earth continuity
conductor.

3. PVC Cables: These cables are available in 250/440 volt and
650/1100 volt grades and are used in concealed wiring system.
Since PVC is harder than rubber, PVC cables do not require
cotton taping and braiding over it for mechanical and moisture
protection.
4. Weather Proof Cables:
• These cables are used for outdoor wiring and for power supply.
• These cables are not affected by heat or sun or rain.
• These cables are either PVC insulated or vulcanized rubber insulated
conductors being suitably taped (only in case of vulcanized rubber
insulated cable) braided and then compounded with weather resisting
material.
• These cables are available in 250/440 volt and 650/110 volt grades.

5. Flexible Cords and Cables:
 A flexible cord consists of wires either silk or cotton or plastic covered.
 Plastic cover is more popular as it is available in various pleasing colors.
 Flexibility and strength is by using conductors having large number of
strands.
 Most stranded conductors are built upon a single central conductor.
 Surrounding this conductor are layers of wires in a numerical progression of
6 in the first layer, 12 in the second layer, 18 in third layer, and so on .
 The number of wires contained in most common conductors are to be found in
the progression 7, 19,37,61.127 Stranded conductors are used in both fixed
wiring cable and flexible cords, the latter being flexible cables not exceeding
4mm2.
 Conductors for fixed wiring up to 25mm2 have seven strands; for example a
6mm2 conductor has seven strands each of 1.04mm diameter (7/1.04).

Colors of Conductors
 The wiring regulations require that all conductors have to be
identified by some means to identify their function.
 For example, according to the British wiring regulation, the phase
conductor of three-phase system are colored red, yellow and blue
with the neutral colored black.
 Protective colors are identified by green/yellow.

Function Colour identification of
core of rubber or PVC
insulated cable.
Earthing White
Live of a.c single-phase
circuit
Green
Neutral of a.c single-phase or
3ᶲ circuit
Black
Phase R of 3ᶲ a.c. circuit Green
Phase S of 3ᶲ a.c. circuit Yellow
Phase T of 3ᶲ a.c. circuit Red

General Specification of cables
 The complete specification of a cable will give the following
information:
i) The size of the cable
ii) The type of conductor used in cables (copper or aluminum).
iii) Number of cores that the cable consists of (i.e. single core,
twin core, three core, twin core with ECC etc.
iv) Voltage grade
v) Type of insulation, taping, braiding and compounding.

Conduits
 The commonest method of installing cables is to draw them into a
conduit.
 The conduit can be steel or plastic.
 Steel conduit is made in both light gauge and heavy gauge, of which
heavy gauge is much more frequently used.
 In general conduits can be classified as:
i ). Light gauge steel-plain (unscrewed) conduit
ii). Heavy gauge steel-screwed conduit
iii). Flexible conduit
iv). PVC conduit

Light Gauge Steel Conduit:
• This type of conduit is used with special grip fittings.
• It is available with an external diameter of 12mm , 16mm,
19mm, 25mm,31 mm,38mm and 50mm.
• In general light gauge is the cheapest and quickest of the
conduit installations but should be used where the location is
dry and there is little possibility of mechanical damage

Heavy Gauge screwed Steel Conduit:
 Though it is very expensive, this type of conduit
provides a permanent installation with a maximum of
protection for the cables.
 The joints into fittings are by means of screw threads
which provide mechanical strength and good electrical
continuity .
 These are available in approximately 3 meter lengths
and are threaded at the two ends

Flexible steel Conduit
 This usually consists of light galvanized steel strip spirally wound
and, to some extent, interlocked, so as to form a tube.
 It is made in size from 19mm to 50mm internal diameter and in
two grades: non-water tight and water tight.
 It can be made with an external covering of PVC sleeving.
 One of the most common uses of flexible conduit is for protecting
the final connections to motors.
 it has the additional advantage of reducing the transmission of
vibration.
 However ,the flexible conduit is costlier than the rigid conduit.

PVC Conduit:
 This type of conduit wiring is finding wide applications in internal wiring
because it is light in weight, shock proof, anti-termite, self extinguishing
and fire resistant, acid and alkaline resistant.
 Such conduits can be used for surface, recessed or concealed type wirings.
 Conduits may be joined by the screwed or plain type of couplers (sockets)
depending upon whether conduits are of the screwed type or plain type.
 In long runs of conduits, inspection type couplers are provided.
 Fixing method shall be the same except that in this case spacing shall be at
every 60cm instead of 1.0m in case of metallic conduit.

Conduit Accessories and Fittings
 Conduit Couplers:
 Conduit is available in lengths from 3m to 5m and for straight runs of greater length,
Couplers are used to join two lengths of conduit.
 Bends, Elbows and Tees:
 In general conduit fittings include bends, elbows and tees.
 Bends are usually used for change in direction of conduit. these should never be sharp.
The minimum allowable radius of curvature is 2.5 times the outside diameter of the
conduit.

Solid elbows and tees should be used only at the end of the
conduit run( e.g. close behind a light fitting or accessory).
The detachable lid provided in inspection type tees and
elbows facilitate pulling of cables.
 Conduit Boxes:
Conduit boxes are used in surface conduit wiring as well
as covered conduit wiring. The conduit boxes are of
different designs.

These serve the following purposes.
1. For providing connections to light, fan and other points.
 The conduit boxes serving the purpose are known as outlet boxes because
conduit terminate at the boxes.
 These boxes may have entry either from side or from back or from sides.
2. For pulling of cables into the conduits.
The boxes serving this purpose are known as inspection boxes. These are
provided after every 30m length of straight run.
3. For housing junction of cables.
• The conduit boxes serving this purpose are known as junction boxes,

Lighting Accessories and Fittings
Switches
 the most familiar switch is that used to control lighting circuits. Most
are rated at 5/6A, but rating at 15A are also available.
Three types of switches are available;
Single pole
Two-way and four-way(intermediate), each for the control of a
practical circuit arrangement.

Switches
A switch is used to make or break an electrical circuit.
It is used to switch ‘on’ or ‘off ’ the supply of electricity to an appliance.
There are various switches such as
Surface switch
ﬂush switch
Ceiling switch
Pull switch
Push button switch
Bed switch

Surface switch It is mounted on wooden boards fixed on the surface of a wall.
 It is of three types
o One-way switch
o Two-way switch
o Intermediate switch
o One-way switch: It is used to control single circuits and lamp
One way switch

Two-way switch: It is used to divert the ﬂow of current to either of two
directions. The two-way switch can also be used to control one lamp
from two different places as in the case of staircase wiring
Two way switch

Intermediate switch: It is used to control a lamp from more than two
locations

Flush switch
It used for decorative purpose
Bed switch
As the name indicates, it is used to switch ‘on’ the light from any place,
other than switch board or from near the bed.
This switch is connected through a ﬂexible wire
Bed switch
Flush switch

Cont…
Switches for water-heaters are of the double-pole type and rated to carry
20A. Other ratings for the double-pole switches.
Dimmer switches are used to allow control of level lighting from a
luminaries, water tight switches are designed for out door use while
splash-proof switches are found in situations where water is present, such
as in shower rooms.

Most switches tend to be from moulded plastic.
Metal-clad versions are also available for industrial use.
Switches are of two types, known as surface switches(or tumbler
switches) and flush switches ( concealed switches).

lamp holders
 These are designed for
quick removal and
replacement of the lamp and
yet they must hold the lamp in firm metallic contact to prevent
overheating.
There are three main sizes of lamp holders;
 the Bayonet-cap(B.C)
 the medium Edison screw(E.S) and
 the Goliath screw(G.E.S)

For ordinary tungsten filament lamps up to 200W the lamp caps and thus
the lam holders are B.C,
up to 300W the caps are E.S, and
above 300w they are G.E.S.
In any case where a lamp is to be installed the appropriate size and type of
holder must be fitted.
 Lamp holders may be either the insulated type of Bakelite or the brass types
with porcelain interior.

Cont…Plugs and socket-outlets
These are used to enable portable apparatus to be connected to the fixed
wiring and comprises two or three contact tubes and terminals.
The plug is the movable part connected to the apparatus by flexible wire,
and comprises two or three contact pins to fit into the contact tubes.

Fuses
•A fuse is a component that automatically breaks an electrical connection if the current
increases beyond a certain value
- circuit fuses “burn open” in the case of an overcurrent condition and thereby protects
the circuit
A fuse element consists essentially of a piece of copper or tin-lead alloy wire which
will melt when carrying a predetermined current.
This element with contacts, carrier, and base is called a fuse.
It is placed in series with the circuit to be protected, and automatically breaks the
circuit when overloaded.
FUSES
Protective Devices - Fuses
- Circuit breakers

In general, the regulations regarding fuses require that fuses shall be
accessible, and shall be fitted either on the front of a switch-board or in
protecting cases.
In most cases of installation work, the fuses are fitted in a distribution board.
The time for blowing out of a fuse depends upon the magnitude of excess
current
The larger the fault current the more rapidly the fuse blows.

Types of fuses
There are three main types of fuses;
i. The re-wirable
ii. The cartridge (holder) and
iii. The HRC (high breaking capacity) fuse; the latter is a
development of the cartridge type.

Three terms are used in connection with fuses:
 current rating -> this is the maximum current that a fuse will carry
indefinitely without undue (excessive) deterioration of the fuse- element.
 fusing current -> this is the minimum current that will ‘blow’ the fuse
 Fusing factor -> this is the ratio of the minimum fusing current to the
current rating.
 Fusing factor = minimum fusing current/ current rating >= 1

96
Circuit Breakers
•A circuit breaker also operates on an overcurrent condition to protect a
circuit, but they can be reset and be used again
- circuit breakers “trip” open to break the current path of the circuit
It is a device designed
To open and close a circuit by non- automatic means, and
To open the circuit automatically on a predetermined over –current
without injury to itself when properly applied within its rating.
Three Phase
Circuit
Breaker
Single
Phase
Breaker

So a circuit breaker is combination device composed of a manual switch and an
over-current device.
 A circuit breaker has several advantages over any type of fuse.
a) In the event of a fault or overload, all the poles are simultaneously disconnected
from the supply.
b) overload and time-lags are capable of adjustment within limits.
c) the circuit can be closed again quickly onto the fault safely.
 Essentially a circuit breaker consists of a carefully calibrated bimetallic strip.

As current flows through the strip, heat is created and the strip bends.
If enough current flows through the strip, it bends enough to release a trip
that opens the contacts, interrupting the circuit just as it is interrupted when a
fuse blows or a switch is opened.
Circuit breakers are rated in amperes just as fuses are rated.
 Standard ratings: both fuse and circuit breakers are available in standard
rating of 6,10,16,20,25,35,50,63,80,100,125,160,224, 250,300 and large sizes

In addition to the bimetallic strip that operates by heat, most
breakers have a magnetic arrangement that open the breaker
instantly in case of short circuit.
A circuit breaker can be considered a switch that open itself in
case of overload.

Distribution Board
A distribution board is an assemblage of parts, including one or more fuses
or circuit breakers, arranged for the distribution of electrical energy to final
circuits or to other sub-distribution boards.
It consists of a case inside which is a frame holding a number of fuse or CB
carriers.

Behind the frame or sometimes alongside or above it, is a bus-bar to
which the incoming sub-main is connected.
From the bus-bar there is connection provided to one side of each fuse
way(circuit breaker).
Each final sub-circuit is then connected by the installer to the outgoing
terminal of the fuse ways.

• A second bus-bar is provided to which the incoming neutral and the neutral
of the outgoing circuits are connected.
The standard distribution boards usually have either 4,6,8,12 or 24 fuse
ways.
Both single-phase and three-phase are available.
It is not necessary to utilize all the available fuse ways on a board, and in
fact it is very desirable to leave several spare ways on each board for
future extensions.

Cont………Distribution board

Electrical Trades
Electrician’s Tools Of The Trade
Learning Objectives
Identify the basic hand tools used in the electrical trades.
Select the essential tools for each specific job.
Maintain and use these tools safely.
List factors to consider when purchasing hand tools.

The Electrician’s Tool Pouch
The Electrician’s tool pouch (pocket) is essential in that it helps kept electrical tools
organized.
The tool pouch allows the right tools at hand which makes the job more efficient.
The electrician must have proper tools for the job.
Tools must be maintain and kept in good working condition.
Certain tools are essential, and without them the electrician should not attempt to do
any type of wiring.
Listed are the basic hand tools that are essential to electrical wiring.
Prepared by Haymanot T. (Lecturer ) 10512/16/2019

Electrical Specific Hand Cutting Tools
Prepared by Haymanot T. (Lecturer ) 106
Diagonal pliers (dykes)
• Cutting small conductors
• Cutting conductors in limited spaces
Lineman’s pliers (side cutters)
• Cutting large conductors
• Forming loops on large conductors
• Pulling and holding large conductors
Wire strippers
• Stripping insulation from conductors
• Cutting small conductors
• Crimping wire lugs
Needle-nose pliers
• Forming loops on small conductors
• Cutting and stripping small
conductors
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Electrical Specific Hand Tools
Tap tool
• Equipping drill holes with bolt threads
• Re tapping damaged threads
• Determining bolt size
Center punch
• Making center tap in wood or metal for
drilling
Flat-blade screwdriver
• Installing and removing slot-head screws
Phillips screwdriver
• Installing and removing phillips-head
screws
12/16/2019

Slip-joint pliers
• Holding couplings and conductors
• Tightening couplings and conductors
Magnetic torpedo level
• Leveling conduit and equipment
Keyhole saw
• Cutting holes in plasterboard for circuit
boxes
Conduit reamer
• Reaming burrs from cut conduits and EMT
12/16/2019

Hacksaw
• Cutting large conductors and cables
• Cutting conduit, metal, or bolts
Steel measuring tape
• Measuring conduit and cable
Adjustable wrenches
• Used for turning bolts, nuts, and small
pipe fittings
Nut driver
• Installing and removing nuts and bolts
• Tightening and loosening nuts and long
bolts
12/16/2019

Electrician's Essential Tools
Electrician’s hammer
• Diving and pulling nails
• Opening wooden crates and breaking
plasterboard
Circuit tester
• Checking circuits for power
• Checking fuses and breakers
Electrician’s knife
• Opening paper cartons
• Stripping cables and large conductors
Hex key set (Allen wrenches)
• Installing and removing Allen
screws
12/16/2019

Tools Used for Specialty Work
Steel fish tape and reel
• Pulling conductors through conduit
• Pulling cables through insulated walls
Conduit bender
• Bending conduit for conductor installation
Rotating speed screwdrivers
• Used for trim work, installing
switch and receptacles
Sheet metal Cutters
• Used for cutting and trimming sheet metal
12/16/2019

Rules for care of hand tools
There are many more hand and power tools that electricians will use in residential and
commercial wiring.
All tools should be used only for the purpose intended.
It is the electrician’s responsibility to keep his or her tools sharp, clean, and lubricated.
A well maintained tool has a longer life and is safer than an improperly maintained tool.
Repair tools when possible, but discard worn or damaged tools.
12/16/2019

Factors For Purchasing Tools
Factors to consider when purchasing tools are size, design, and quality.
Always purchase the correct sized tools for the work to be done.
Tools should be designed specifically for electrical work. Ex: Insulated handles,
hammers with straight claws.
The purchase of quality tools last longer which saves replacement cost.
12/16/2019

Remember
Select the right tool for the job.
Keep tools clean, lubricated, and in good working condition.
Purchase good quality tools they are safer and will last longer.
Repair tools when possible, but discard worn or damaged tools.
Prepared by Haymanot T. (Lecturer ) 11412/16/2019

Designing an Electrical Installation
 Those responsible for the design of electrical installation of
whatever size must obtain and study very carefully the
requirements of IEE regulations for electrical installation.
 The regulations give the designers degree of freedom in the
practical detailed arrangements to be adopted in any particular
installation.

In some cases the experience and knowledge of the designer
will be called in to play to arrive at the best or
Most economical arrangement and this will encompass the
practical application of installation techniques as well as the
ability to apply the theoretical aspects of the work.

Assessment of general characteristics
Before any detailed planning can be carried out, it is necessary to
assess the characteristics of the proposed scheme.
This applies whether the installation is:
- New one
- An extension to an existing one
- Rewiring an existing building
The assessment required is broad one and some of the aspects to
be considered are described below:
 Purpose and Intended use of the building and type of construction

The Construction and use of the building will indicate the type of
equipment to be installed
Environmental conditions: Environmental conditions include:
 ambient temperature,
 altitude,
 presence of water, dust, corrosion, lighting and wind hazards.
With large commercial premises fire rescues must be dealt.
The maximum current demand: It is necessary to estimate the
maximum current demand.
Diversity may be taken into account.

Electrical
Points
Installed
Utilization
Two lamps
points(incandescent)
One may not be
used
One fluorescent tube May be used
Two fan points One may not be
used
Two 5-ampere socket
outlet
One may not be
used
One 15-ampere socket
outlet
May or may not
be used
Diversity
The electricity consumed in any
residential building is never hundred
percent of the installed capacity.
There may be some electrical points,
which remain unused even during peak
load periods (when there is high
demand of power).
Some electrical points in the building
are installed keeping future
requirements in view and some points
are used only during special occasions.

For instance in drawing room if there are three lighting points,
two fan points, two five ampere socket outlets, and one 15 ampere
outlet. Let look, which points, can be used at a time even during
peak load hours.
Now assuming, some points may be used and some remain
unused.
However it is applicable for domestic installation only.
 Some commercial building or business establishments
definitely use 100 % of the installed capacity during peak load
hours but not for 24 hours

• The percentage between installed capacity and utilization
capacity is not applicable in the case of power wring in industries
where hundred percent of installed motors are used for the whole
day.
• Places where utilization percentage is not applicable while
designing the electrical system for the following:
-For street lighting
-Three phase power wiring for industries
-Power wiring for agricultural sector

Generally diversity is the current, which is likely to flow in a
circuit, compared with the sum of current ratings of all the current
consuming appliances connected to that load.
The application of diversity has advantages:
Reducing size of conductors and
Associated protective devices

Maintainability
consideration must be given the frequency and to the quality of
maintenance that the installation can reasonably be expected to
receive.
The designer must ensure that
Periodical inspection testing and maintenance can readily be
carried out and
The effectiveness of the protective measures and
The reliability of the equipment are appropriate to the intended life
of installation.
 It is necessary to ensure as far as possible that all parts of the
installation which require maintenance remain accessible.

Diversity [Demand] Factors
• Diversity [Demand] factor is the ratio of the maximum demand of a load to the total
connected load . This factor is also called factor of power utilization.
• The service equipment and conductors do not need to have an ampere rating equal to
the total ampere ratings of all the individual branch circuits. It is unlikely that every
circuit in the installation is loaded to its maximum capacity at the same time .
• Therefore demand factor have been established, based on many tests and past
experiences, that represent the maximum part of various types of loads that are likely to
be in use at any one time .

As every single load or group of loads in a circuit are not operating simultaneously, and normally
working under partial load, the power demand factor is always less than 1.0..
The DF for various loads are given below:
Type of load DF estimate
1. Lighting Circuits 0.7-0.9
2. Heating loads
2.1. Water Heaters 0.2-.3
2.2. Ovens/stoves 0.2
2.3. Electric Iron 0.3
3. Motor Loads 0.7-0.9
4. Office equipment 0.3-0.5
5. General purpose SOs 0.2-0.5

Systems of supply:
A number of aspects of design will depend upon the system of
supply in use at location concerned .
 Five types of supply are stated by IEE regulations on earthing
and testing of installation portion
Circuit Design:
Having proposed main requirements of IEE regulations for
electrical installation,
Now it is proposed to go into the more practical aspects of
installation design.

Service Entrance and Branch Circuits
Service Entrance
Power is transmitted from generating station or substation to the
place of public utility by means of transmission lines and there
from power is further distributed by means of distributors or
distributing lines.
The consumers are supplied with power by taking connections
(tapping's) from distributing lines.
The conductors and equipment used for delivering electric
energy from the supply system to the wiring system of the
premises is called the service.

Service lines are of two types
Overhead service lines and Underground Cable Service Lines
In overhead-line distribution for premises, the service cables are
connected to the line conductors by means of mechanical
connectors called line-taps .
Conductors to the premises are always insulated, and are in most
instances pvc-insulated.

The service cables are taken to insulators mounted on D-irons, cleated
to the walls of the house, and then run to the supply-intake position.
Use of underground cable is usually made for service connection
when the power to be supplied to the consumer is large (say above
25kw).
Usually a two-core, pvc-insulated steel-wire armored and pvc-
sheathed cable is used as under ground cable.

The junction which this cable makes with the street-main is
contained in a tee-box generally buried under the pavement just
outside the premises or fitted on the pole.
The service cable conductors are joined to two of the main cable
cores: one to the neutral and the other to one of the phase conductors.
The connectors are either soldered using the usual cable tee-joint or
by crimping.

Any installation must be provided with control and protective
equipment.
The service conductors terminate in a main fuse cut out and a
connector-block for the neutral conductor.
The supply cut outs are connected to the energy meter.
The cut outs and energy meters are usually in the same board called
meter board.
The cut-outs are sealed to prevent tampering by unauthorized
persons.
From the meter the installation main cables are taken to the main
switch or switch fuse.

The consumer's main switch must be of the double-pole, linked
blade type which will isolate the complete installation from the
supply when the switch is operated.
 If the supply is single-phase, or three-phase and neutral, then all
three , or four poles will be broken.
The main switch can be a switch unit or a switched fuse
depending on the size of the installation.
In larger types of installations, a CB is used, which acts not only
as a main switch but offers the necessary protection against fault
currents.

Branch Circuits
The branch circuits are supplied power from the distribution board.
DBs contain circuit protective devices like BS 1361 cartridge fuses or
MCBs.
 In domestic installations, the DB is combined with the main switch and is
known as the consumer unit. They vary in capacity from 4-way to 12-way
units.
Main cables are those which carry the total current of the installation.
Sub-main cables carry current to sections of large installations to SMDB.
12/16/2019
Prepared by Haymanot T.
133

A final circuit feeds one type of circuit and is not split up to
feed another circuit.
In a domestic installation a supply is often required for a
building which is detached from the main building, such as a
garage.
In this case a final circuit in the consumer unit feeds a cable
taken into that building which must be terminated in a SMDB.
It is a requirement of the regulations that every detached
building is provided with its own means of isolation

A final circuit can range from a pair of 1.5mm2 cables feeding a
light to a very heavy three-core cable feeding a large motor from
a CB or switch at the main DB.
Each circuit should have its own protective fuse or CB.
The rating of the protective device must not be less than the
designed load current of the circuit and , also,
That rating should not exceed the current-carrying capacity of the
lowest-rated conductor in a circuit.

The final circuits include:
lighting circuits
socket outlet circuits (general purpose)
socket outlet circuit for water heater
socket outlets for cookers
Power outlets feeding a motor
Bell circuits
General purpose socket outlet circuits and socket outlet circuits supplying 3kw water-heaters
are usually rated at 16A. lighting circuits are rated at 10A.

For socket outlet circuits feeding cookers , a 20/25A CB is used for
protection.
General purpose socket outlet circuits can be connected in ring or radial.
 The number of SOs to be included in one circuit can be known from tables
of EEPCO's regulation.
The current rating of cables feeding a motor is based on the full-load
current taken by the motor.

More than one motor may be connected to a 16A final circuit,
provided that the full-load rating of the motors doesn't exceed the
rating of the smallest cable in the circuit.
If, however, the rating of the motors exceed 16A, then the circuit
must supply one motor only.

Location of electrical points in house wiring
The service line which includes weather proof cable support wire , pole clamp and other
equipment should reach from the nearest pole up to energy meter .
The size of the service cable should be sufficient enough to withstand the existing electrical
load requirements.
1.Energy meter
The energy meter should be installed at a place which is easy accessible to the consumer as
well as to the meter reader .
The height of the meter should be 1.75 meter above floor .

The meter should be installed in a covered verandah such that the rain
showers at an angle don’t damage the meter .
Its location should be in front verandah so that privacy of the owner is not
disturbed as the meter reader will be visiting for meter reading every
month .
The other suitable place where there is no verandah is outside the wall ,
providing protective covering.

2.Main switch
Its purpose is to isolate the supply to the building .
 It is normally installed very close to the energy meter and should be
readily accessible to the consumer.
 The fuses are also provided inside main switch to interrupt the supply due
to short circuit current that may occur.

3. Distribution board :
The supply is given to main switch and then to main distribution board for
the purpose of distribution of electricity to various portions of the house
through sub circuits .
Every sub circuit is protected by its individual fuse or circuit breaker such
that if one fuse or circuit breaker makes the circuit off , the entire room is
not plunged into darkness and the other circuit shall maintain the supply to
other parts .

4. socket outlet
3 pin , 5A , socket outlets are used for general purpose
15 A socket outlets are used for higher loads ( such as e electric mitad ,
stove , water heater etc) .
The location of socket outlets should be such that its utility is most
convenient.
If socket outlets in residential buildings are close to floor , the children
may get shock , for offices buildings , socket outlets are generally installed
close to floor .
 In bath rooms , 15 A socket outlets should be provided 2 meter above
floor.

5. Lighting points
The lights should be so placed that these are most convenient in their utility
and control .
For instance , when person has to go up stair ,instead of moving into
darkness , he/she can switch on the light from starting point and switch off at
the end .
The number of lighting pints are determined from the size of room or hall .
The main requirements that lighting points installed should provide uniform
illumination and minimum glare.
The ceiling fans shouldn’t be installed in kitchen, bathrooms , toilets and
small stores . The exhaust fan should be installed in big cook housed about
half meters below ceiling , it should be installed in kitchen.

7. Switch boards :
 The switch boards should be convenient to operate and adequately
located .
The switch board (box) must be provided inside a room close to the
entry door so that there is no difficulty in switching on the light during
night time .
The height of switch board should be about 1.3 meters above floor .

8. Earth wire installation:
Earthing means , the direct connection of all the metal non current carrying
parts of electrical equipment such as metallic frame work , electric motor
body , main switch , distribution board , earth terminal of socket outlet ,
metallic covering of cable and conduit pipes etc .
The earth plate is buried in the ground which have a good electrical
connection to the surrounding earth .
This is all done to avoid electrics shock .

The supply authority provides ear thing to its meter . Beyond the meter ,
ear thing is the responsibility of the owner of the house .
The owner should make arrangement of his own earthing system with a n
adequate electrode.
It is necessary to take into account the following procedures:
a) Sub - division and number of circuit
b) Designed circuit current
c) Nominal current of protective device
d) Application of correction factors
e) Size of cables
f) Voltage drop calculation

Installation Design
Design Procedure
Having determined all the necessary details, we can decide on an installation
method, the type of cable, and how we will protect against electric shock and
overcurrent’s.
We would now be ready to begin the calculation part of the design procedure.
Basically there are eight stages in such a procedure.
These are the same whatever the type of installation, be it a cooker circuit or a
submain cable feeding a distribution board in a factory.12/16/2019Prepared by Haymanot T. (Lecturer )148

Here then, are the eight basic steps in a simplified form:
Determine the design current Ib.
Select the rating of the protection In.
Select the relevant correction factors (CFs).
Divide In by the relevant CFs to give tabulated cable current-
carrying capacity It.
Choose a cable size to suit It.
Check the voltage drop.
Check for shock risk constraints.
Check for thermal constraints.

Design Current Ib
This is defined as ‘the magnitude of the current to be carried by a circuit in normal
service’,
And is either determined directly from manufacturers’ details or calculated using the
following formulae:
Single phase:
Three phase:
Where: P = power in watts
V = phase to neutral voltage in volts
VL = phase to phase voltage in volts
PF = power factor
eff = efficiency

Nominal Setting of Protection
Having determined Ib we must now select the nominal setting of the protection In such
that In Ib.
This value may be taken from IEE regulations, EBCS-10 or from manufacturer’s data.
Note that
Re wirable fuses have a fusing factor between 1.8 and 2.0 (usually 1.8),
Cartridge fuses have a fusing factor between 1.25 and 1.75 and
HRC fuses have a fusing factor of 1.25 (maximum) and
Circuit breakers are designed to operate at no more than 1.5 times their rating
Fusing factor = Fusing current
Current rating

Design Current : It is sated under cable size selection portion.
• For fluorescent lamps due to the choke , starting current is very high and the cable chosen should
consider this effect .
Ib = Total power x 1.8
Rated voltage
• Then select the rating of protective device
( In  Ib)
-Calculate the total designed load
-Calculate the expected maximum load

I-Using the actual load of lighting and power
 Lighting load = rating x number of lamps
General purpose socket outlet
= current rating x voltage rating x number of socket
 Specific loads simply add power rating
- Total sum will give the design load
• Since due to diversity there will be expected maximum load.
• Diversity factor for different circuits will be as follows:
- For lighting circuits Dive. Factor (DF) = 0.6 --- 0.8
- For general purpose sockets DF = 0.3
- For specific loads DF = 1

• So expected load will be
= Lighting x DF + GPS x DF + specific loads
Ib =
PF = Power factor and
V = supply voltage
PFxV
loadExpected

II. Calculate the design load from circuit breaker rating
- Total lighting load
= number of lamps x Circuit breaker rating
- Total general purpose socket load
= number of sockets X CB rating X V
- Specific load = simply add
- Total design load = the sum of loads as stated
Ib = expected load
V x PF

Ex:- In a house the following equipments are installed
Light points =5
Fan points = 2
Plugs = 2
Let us assume that the wattage of each lamp is 60W,that of fan point 60W and
that of plug 100w.
Wattage of light points 60 x 5 = 300 w
Wattage of fan points 60 x 2 = 120 w
Wattage of plug points 100 x 2 = 200 w
Total wattage = 620 w
The line current =total wattage / supply voltage =620/230 =2.7A

Correction Factors
When a cable carries its full-load current it can become warm.
This is no problem unless its temperature rises further due to other influences, in
which case the insulation could be damaged by overheating.
These other influences are:
• High ambient temperature;
• Cables grouped together closely;
• Un cleared over currents; and
• Contact with thermal insulation.

Determination of conductor size
Before making an estimate it is necessary to find out the size of wire, cable or aluminum
conductor.
The following essential points are to be considered while calculating the size:
A. Current carrying capacity
B. Voltage drop
C. Minimum permissible size

A. Current Carrying Capacity
• The value of current in any circuit will be more as compared to sub-
circuits.
• In sub-circuits as the load decreases ,the current is also reduced
• Thus it is necessary to divide the services in to groups in accordance
with the amount of current which will flow through them .
• Afterwards, size of wire in each group is determined .

B. Voltage Drop
• Voltage drop is there as when the current flows through the wiring and the same
should be as low as permissible and economical .
• The voltage drop can be determined by Ohm’s law .As the resistance is inversely
proportional to area, so the voltage drop will be less if the area of wire is more.
• The voltage drop is usually very small magnitude and will not have much effect for
small domestic wiring .
• For multistoried buildings, factories and industries, the voltage drop is required to
be ascertained .

• If the voltage drop is much , the house hold appliance and motors will not work.
• It should be noted that the maximum voltage drop should not be more than as given below:
(a) Lighting circuit. In any circuit :
(I). At 200 volt supply, voltage drop should not be more than 5V.
(II). At 210 Volt supply, voltage drop should not be more than 5.1V.
(III). At 220 volts supply, voltage drop should not be more than 5.4 V.
(IV). At 230 volts supply, voltage drop should not be more than 5.6 V.
(V). At 240 volts supply, voltage drop should not be more than 5.8 V.
(VI). At 250 volts supply, voltage drop should not be more than 6.0 V.
 The permissible voltage drop in a lighting circuit is 2% of the supply voltage plus one
volt.
• `

(b). Industrial loads.
• The maximum voltage drop at the extreme end equipment or motor should not be more
than 5% of the declared supply voltage .
• The following table shows the various sizes of wires ,current rating and voltage
drop if loaded fully are given. Considering the load in amperes and voltage
drop ,suitable size of wire with required insulation is selected.
• Table below shows Current ratings and voltage drop for vulcanized rubber
PVC or polythene insulated or tough Rubber PVC lead sheathed single core
aluminum wires or cables.

Size of conductor 2 cable d.c. or single
–phase a.c.
3 or 4 cables of
balanced 3 phase
4 cable d.c.
Normal
area
sq.mm
Number and
diameter of
wire in mm
Current
ratings in
amperes
Approx.
length of
run for 1
volt-drop
in meters
Current
ratings in
amperes
Approx.
length of
run for 1
volt-drop
in meters
Current
ratings in
amperes
Approx.
length of
run for 1
volt-drop
in meters
1.5 1/1.40 10 2.3 9 2.9 9 2.5
2.5 1/1.8 15 2.5 12 3.6 11 3.4
4.0 1/2.24 20 2.9 17 3.9 15 4.1
6.0 1/2.80 27 3.4 24 4.3 21 4.3
10.0 1/3.55 34 4.3 31 5.4 27 5.4
16.0 7/1.70 43 5.4 38 35 6.8
25.0 7/2.24 59 6.8 54 8.5 48 8.5
35.0 7/2.50 69 7.2 62 9.3 55 9.0
50.0 7/3.0 91 7.9 82 10.1 69 10.0

Examples
1. The main circuit wire in a house is required to carry a current of 45 amperes when
connected to single phase a.c. supply. Determine the size of wire If the length of the
circuit is 40 meters.
solution :-
Assuming the declared voltage =230 V
Drop =
Referring to table above , if the size of conductor selected is 25.0 sq.mm which can
carry 59 amperes , the voltage drop at 59 amperes rating will be as follows
For 1v drop ,the length of wire is 6.8m and for 40m it is equal to 40/6.8volts.
volts
x
6.51
100
2302


• For a current rating of 59A , the voltage drop is 40/6.8 v and the voltage drop at 45A will
be equal to
• 4.48 v voltage drop is within the permissible limit .
Thus the wire having size as 25 sq.mm (7/24) is suitable.
Example 2 :- A room is to be wired for single phase a.c. supply directly taken from mains
which has declared voltage of 200 volts. The length of wire from the main switch to light
and plug points is 30 meters .
Solution :- permissible voltage drop =
vx 48.4
59
45
8.6
40

volts
x
51
100
2200


• Referring to the above table, minimum size of wire 1.5 sq.mm(1/1.40) should be in a
position to carry 5 amps safely.
• Now it will be seen that there will be a drop of 1 volt after every 2.3 meters for 10
amperes loading.
Voltage drop at 10 amps =
Hence this is not permissible.
• Now considering the next higher size 2.5 sq.mm (1/1.80) wire and consulting table
above
volts
3.2
30
voltsx 52.6
10
5
3.2
30


Which is within permissible limit.
This wire having size 2.5 sq.mm (1/1.80) is suitable.
5.2
30
voltsx 4
15
5
5.2
30


Typical arrangement of a sing
phase domestic DB
12 6A Light & fan
11 6A Light & fan
10 30A Socket
9 30A Socket
8 10A Water pump
7 10A WH
6 30A Cooker
5 20A WH
4 20A WH
3 20A AC
2 20A AC
1 20A AC12/16/2019 Prepared by Haymanot T. (Lecturer ) 168
3 x 4mm sq.
3 x 4mm sq.
3 x 4mm sq.
3 x 4mm sq.
3 x 4mm sq.
3 x 1.5 mm sq.
3 x 2.5 mm sq.
3 x 2.5 mm sq.
3 x 1.5 mm sq.
3 x 2.5 Ring
3 x 2.5 Ring
3 x 1.5 mm sq.
AC = Air conditioner
WH = water heater
WM=washing machine

Electrical Drawings
• It includes:
- Electrical plans
- Separate plan for different floors
• Circuit diagrams
• Separate diagrams prepared for different distribution boards
• Required notes
• Legend showing symbols and abbreviations
• Fixture schedule

Electrical Installation Floor Plans
• The plan shows the location and type of :
• switches
• sockets
• bell points
• telephone points
• lighting fixtures and different electrical devices.
• It is generally traced from the floor plan and reflected
ceiling plan.

Includes :
• fixtures and equipment location
• Lay out of lighting fixtures
• Lay out of switches : identification and type with appropriate
Symbols
• Lay out of sockets and interconnecting cables.
• Location of specific electric devices and their power, points
like electric mitad , heater cooker etc.
• Location of distribution board ( main and subs)

Circuit Diagrams :
Definition : These are simply diagrams tabular in their form which are showing
the electric circuit system of a building.
Purpose : It provides the number and type of circuit in particular distribution
board.
• It enables the electrician to determine the cable size and its current rating.
Includes : The number of circuit and their description such as cross – sectional
area and number of cables , current rating of circuits ,type of distribution board.

CKT. DIAG.

NB Number of lamps , 10 to 15 lamps each up to 100 w can be in one lighting circuit . If
more number of lamps add another circuit.
 For general purpose socket outlets 5 A each , use 5 to 8 sockets in one circuit if there is
more add another circuit.
• Keep reserve circuit breaker in any distribution board .
• Make all external socket outlets weather proof.
 Provide three phase supply for main distribution board from EEPCO line.
 Telephone system has its own line , separated from the electric supply line. TV , tape
recorder and other appliances may get power from general purpose socket outlets.

Lighting Requirements
Area Lighting Requirement
Living Room Ceiling outlet or wall brackets controlled by switches at entrance and switches on
walls of living room.
Dinning Room Ceiling outlet or wall bracket controlled by switches at entrance.
Kitchen Ceiling out let controlled by switches at entrance and lighting out over the sink
controlled by switches on wall
Bed- Rooms Lighting outlet (ceiling or wall) controlled by switches at entrance and near bed.
Bath-Rooms Shower(toilet) Ceiling outlet and wall outlet over mirror controlled by switches at entrance.
Hall or Corridor Ceiling outlet controlled switch in each separate hall area .
Balconies Entrances
Verandah
Out door lighting at each entrance controlled by switches.
Recreation Room Ceiling outlet for each 14m2 of floor space
Laundry Ceiling outlet controlled by wall switches at entrance.
Store Garage Ceiling outlet controlled by switches at entrance.

Power Requirement For Different Rooms In Residential Building
Area No. Remark
Living Room 3 Installed at every 4 meter of usable room
Dinning Room 2 Installed at every 4 meter of usable room
Kitchen 3 One general purpose OL and two specific POs
Bed- Rooms 3 For TV , tape recorder & head light
Bath-Rooms
Shower (toilet)
2 One general purpose side of mirror and one S P. for WH
Hall or Corridor 1 One for each corridor
Balconies
Entrances
Verandah
1
1
1
One weather proof general purpose socket OL
Laundry 1 On separate circuit
Store Garage 1 On separate circuit

Contents
1. Earthing and method of earthing
2. Grounding system
3. Testing of electrical installation
Chapter 3: Grounding system and testing electrical installation

What is earthing?
The term Earthing or Grounding simply means connecting the electrical
system / equipment to the ground by means of a suitable conductor.
The whole of the world may be considered as a vast conductor which is
at reference (zero) potential.
In the UK it is referred to this as 'earth' whilst in the USA it is called
'ground'.
1. Earthing and method of earthing

People are usually more or less in contact with earth, so if other parts which
are open to touch become charged at a different voltage from earth a shock
hazard exists.
The process of earthing is to connect all these parts which could become
charged to the general mass of earth, to provide a path for fault currents and
to hold the parts as close as possible to earth potential.
In simple theory this will prevent a potential difference between earth
and earthed parts, as well as permitting the flow of fault current which
will cause the operation of the protective systems.

• The standard method of tying the electrical supply system to earth is to
make a direct connection between the two.
• This is usually carried out at the supply transformer, where the neutral
conductor (often the star point of a three-phase supply) is connected to earth
using an earth electrode. Figure below shows such a connection.

Types of earthing systems
The first letter indicates the type of supply earthing.
T -indicates that one or more points of the Supply are
directly earthed (for example, the earthed neutral at the
transformer).
I -indicates either that the supply system is not earthed
at all, or that the earthing includes a deliberately-inserted
impedance, the purpose of which is to limit fault current.
TT system
TN-S system
TN-C-S
system
TN-C
system
IT system

The second letter indicates the earthing arrangement in
the installation.
T - all exposed conductive metalwork is connected
directly to earth.
N - all exposed conductive metalwork is connected
directly to an earthed supply conductor provided by the
Electricity Supply Company.
The third and fourth letters indicate the arrangement of
the earthed supply conductor system.
S - neutral and earth conductor systems are quite separate.
C - neutral and earth are combined into a single conductor.
Types of earthing systems
TT system
TN-S system
TN-C-S
system
TN-C
system
IT system

1. TT systems: This arrangement covers installations not provided with an earth terminal by
the Electricity Supply Company.
• Thus it is the method employed by most (usually rural) installations fed by an overhead
supply.
• Neutral and earth (protective) conductors must be kept quite separate throughout the
installation, with the final earth terminal connected to an earth electrode by means of an
earthing conductor
• Effective earth connection is sometimes difficult. Because of this, socket outlet circuits must
be protected by a residual current device (RCD) with an operating current of 30 mA .

2. TN-S system: This is probably the most usual earthing system in the world,
with the Electricity Supply Company providing an earth terminal at the incoming
mains position.
• This earth terminal is connected by the supply protective conductor (PE) back
to the star point (neutral) of the secondary winding of the supply transformer,
which is also connected at that point to an earth electrode. The earth conductor
usually takes the form of the armour and sheath (if applicable) of the
underground supply cable.
TN-S earthing system

3. TN-C-S system: In this system, the installation is TN-S, with separate neutral
and protective conductors. The supply, however, uses a common conductor for
both the neutral and the earth.
• This combined and neutral system is sometimes called the 'protective and
neutral conductor' (PEN) the 'combined neutral and earth‘ conductor (CNE).
The system, which is shown In Fig.below is most usually protective multiple
earth (PME) system.

4. TN-C system:
• This installation is unusual, because combined neutral and earth wiring is
used in both the supply and within the installation itself.
• Where used, the installation will usually be the earthed concentric system,
which can only be installed under the special conditions (mostly used in
France)

5. IT system:
• The installation arrangements in the IT system are the same for those of the TT
system . However, the supply earthing is totally different.
• The IT system can have an unearthed supply, or one which is not solidly
earthed but is connected to earth through a current limiting impedance.
• IT system is shown in Fig. below.

• The path followed by fault current as the result of a low impedance
occurring between the phase conductor and earthed metal is called the earth
fault loop. Current is driven through the loop impedance by the supply voltage.
• The extent of the earth fault loop for a TT system is shown in below
and is made up of the following labelled parts.
Principle of earthing system

12/16/2019 190
l. - the phase conductor from the transformer to the
installation
2-the protective device(s) in the installation
3-the installation phase conductors
from the intake position to the fault
4. - the fault itself (usually assumed to have zero
impedance)
5. - the protective conductor system
6. - the main earthing terminal
7. - the earthing conductor
8. - the installation earth electrode
9. - the general mass of earth
10. - the Supply Company's earth electrode
11. - the Supply Company's earthing conductor
12-the secondary winding of the supply transformerPrepared by Haymanot T. (Lecturer )

12-the secondary winding of the supply transformer
For a TN-S system (where the Electricity Supply
Company provides an earth terminal), items 8 to 10
are replaced by the PE (protective) conductor, which
usually takes the form of the armouring (and sheath if
there is one) of the underground supply cable.
For a TN-C-S system (protective multiple earthing)
items 8 to 11 are replaced by the combined neutral
and earth conductor.
For a TN-C system (earthed concentric wiring), items
5 to 11 are replaced by the combined neutral and
earth wiring of both the installation and of the
supply.12/16/2019 Prepared by Haymanot T. (Lecturer ) 191

• It is readily apparent that the impedance of the loop will probably be a good deal
higher for the TT system, where the loop includes the resistance of two earth
electrodes as well as an earth path, than for the other methods where the
complete loop consists of metallic conductors. Earthing system components
(6,7 and 8)
Earthing system has three main components
• Earthing conductors
• Earthing electrodes
• Inspection points (earthing well)

1- Earthing conductors
• The earthing conductor is commonly called the earthing lead.
• It joins the installation earthing terminal to the earth electrode or to the
earth terminal provided by the Electricity Supply Company.
• It is a vital link in the protective system, so care must be taken to see that its
integrity will be preserved at all times.

2- Earth electrodes
 The principle of earthing is to consider the general mass of earth as a reference
(zero) potential.
 Thus, everything connected directly to it will be at this zero potential.
 The purpose of the earth electrode is to connect to the general mass of earth

Calculation of earthing resistance for one electrode driven at the earth Equation
used to calculate earthing resistance is:
Where
ρ = earth resistivity in ohm.m
l = length of the electrode (m)
d= diameter of the electrode in (m)
Example 1 : calculate the earthing resistance of an earthing electrode of length
3m and its diameter is 2 cm driven in an earth of 60 Ω.m resistivity.
Solution

• This is very large value. To reduce this resistance we can put another rode
(electrode) at distance D in parallel with the first rode. Hence the total earthing
resistance RII will be:
• Example 2: For example 1 above calculate
the earthing resistances when two similar electrode
are put in parallel.
Solution:
From example 1 RI=19.4 Ω
α = ( ρ / 2π D RI ) = ( 60/ 2x3.14x3x19.4 ) = 0.16
RII = {( 1+ 0.16) /2 } (19.4) = 11.25 Ω
For standard building, it is found that the best earthing system is to use thee rods
connected in triangular form as shown in Fig. above, in this case the earthing resistance
will be reduce to RIII= RI/3.12/16/2019 Prepared by Haymanot T. (Lecturer ) 196

• For any number of rods in parallel, we can calculate the earthing resistance
from the following equation and table:
Req = [ RI / No. of rods.] x F
Where F is a multiplying factor that can be taken from table-1.
The resistivity (ρ) in . )Ω . m( for various types of soils are given table-2.
Table-1
(m. Ω )ρ Type of soil
30 – 5 1. Swamps and soil submerged in water
100 – 20 2. Muddy soil
500– 50 3. Mud-sand
3000– 200 4. Silicone sand.
3000 – 1500 5. Rocky ground.
Table-2

3- Inspection points (Earthing well)
For protection of the earthing rod and earthing conductors and also for
maintenance and inspection purposes an earth well is constructed/
Earthing conductors, as well as protective and bonding conductors, must be
protected against corrosion.
Probably the most common type of corrosion is electrolytic, which is an
electro-chemical effect between two different metals when a current passes
between them whilst they are in contact with each other and with a weak acid.
 The acid is likely to be any moisture which has become contaminated with
chemicals carried in the air or in the ground.

 A main earth terminal or bar must be
provided for each installation to collect
and connect together all protective and
bonding conductors.
 It must be possible to disconnect the
earthing conductor from this terminal
for test purposes, but only by the use of
a tool.
 This requirement is intended to prevent
un authorized or unknowing removal of
protection.

• Where the final connection to the earth electrode or earthing terminal is
made there must be a clear and permanent label Safety Electrical
Connection - Do not remove
• With the increasing use of underground
supplies and of protective multiple earthing
(PME) it is becoming more common for the
consumer to be provided with an earth
terminal rather than having to make contact
with earth using an earth electrode.

The advantages of earthing
• The practice of earthing is widespread, but not all countries in the world use it.
There is certainly a high cost involved, so there must be some advantages.
• In fact there are two. They are:
1. - The whole electrical system is tied to the potential of the general mass of
earth and cannot 'float' at another potential.
2. - By connecting earth to metalwork not intended to carry current (an extraneous
conductive part or a an exposed conductive part) by using a protective conductor, a
path is provided for fault current which can be detected and, if necessary, broken.

The disadvantages of earthing
• The two disadvantages are:
1. - Cost: the provision of a complete system of protective conductors, earth
electrodes, etc. is very expensive.
2. - Possible safety hazard: It has been argued that complete isolation from
earth will prevent shock due to indirect contact because there is no path for the
shock current to return to the circuit if the supply earth connection is not made

The objective of a grounding system are:
To provide safety to personnel during normal and fault conditions by limiting
step and touch potential.
To assure correct operation of electrical/electronic devices.
To prevent damage to electrical/electronic apparatus.
To dissipate lightning strokes.
To stabilize voltage during transient conditions and to minimize the
probability of flashover during transients.
To divert stray RF energy from sensitive audio, video, control, and computer
equipment.
2. Grounding system

A safe grounding design has two objectives:
1. To provide means to carry electric currents into the earth
under normal and fault conditions without exceeding any
operating and equipment limits or adversely affecting continuity
of service.
2. To assure that a person in the vicinity of grounded facilities is not
exposed to the danger of critical electric shock.

The three main types are:
Equipment grounding (safety)
System grounding
Lightning/surge grounding
Types of grounding

Types of Grounding Systems
• Many different types available
• Choice depends on local conditions and
required function
• Simplest form is a single stake
• Mostly used for:
• Lightning protection
• Stand alone structures
• Back-up for utility ground
Earth / Ground Basics

Ground rod group
• Typically for lightning protection on larger structures or protection
around potential hotspots such as substations.
• A group of connected rods will have a more complicated interaction,
typically configurations like the fig below is present around
substation sites and sensitive buildings.

Ground plate• For areas where there is rock (or other poor
conducting material) fairly close to the
surface ground plates are preferred as they
are more effective
• A ground mesh consists of network
of bars connected together, this
system is often used at larger sites
such as electrical substations.
Ground mesh

Soil Characteristics
• Soil type. Soil resistivity varies widely depending on soil type, from as low
as 1 Ohmmeter for moist loamy topsoil to almost 10,000 Ohm-meters for
surface limestone.
• Moisture content is one of the controlling factors in earth resistance
because electrical conduction in soil is essentially electrolytic.

Properties of Earthing System
• The electrical properties of earthing system depend essentially on
two parameters:
• Earthing resistance
• Configuration of the earth electrode.
• The earthing resistance has two components:
• Dissipation resistance RD, ( the resistance of the earth between the earth
electrode and the reference earth)
• Resistance RL of the metal parts of the earth electrode and of the earthing
conductor. Normally rl<< rd
• Thus, usually the earthing resistance = the dissipation resistance rd

Earthing Systems
1. System earth,
2. Static earth,
3. Maintenance earth,
4. Equipment earth,
5. Electronic earth,
6. Lightning Protection earth.

System Earth
• Refers to the point in an electrical circuit that is connected to earth.
• Typically at the electrical neutral
• Provides a low impedance path for fault currents improving ground fault
coordination
• Ensures longer insulation life of motors, transformers and other system
components.
• Intended to carry low frequency or fault currents back to protective devices.
• Primarily a current consideration

Circuit and system grounding
There are three fundamental purposes for grounding an electrical system:
i. To limit excessive voltage from lightning, line surges, and crossovers
with higher voltage lines.
ii. To keep conductor enclosures and noncurrent-carrying metal
enclosures and equipment at zero potential to ground.
iii. To facilitate the opening of overcurrent protection devices in case of
insulation failures because of faults, short circuits, etc.

Static Earth
• Connection made between a piece of equipment and earth for the
purpose of draining off static electricity charges
• prevents flash over potential being reached.
• utilized in dry materials handling, flammable liquid pumps and
delivery equipment, plastic piping, and explosive storage
facilities.

Maintenance Earth
• Utilized for safe work practices, and is a
temporary earth.
• Equipotential personal grounding consists of one
set of jumpers connecting all phases together and
to both the neutral and a cluster bar mounted
below the worker’s feet.

Electronic and Computer Earth
• Earthing for electronic equipment is a special case in which the equipment
earth and the system earth are combined.
• Electronic equipment earthing systems must not only provide a means of
stabilizing input voltage levels, but also act as the zero voltage reference
point.
• Earthing systems for modern electronics installation must be able to provide
effective earthing and bonding functions well into the high frequency (MHz)
range.

Lightning Protection Earth
• Earthing requirements are dependent upon the structure, equipment
to be protected, and the level of lightning protection required.
• Intended to provide a path to dissipate high energy into the soil.
• Primarily an energy consideration and may be accomplished by
having an equal potential grid under the area to be protected.

Configuration of Earth Electrodes
• Hemi-Sphere Type
• Plate Type
• Rod Type
• Slip or conductor Type

Hemi-Sphere Type Electrodes
r
r
Re


2


Plate Type Electrodes
• Circular Plate







A
Re

4
Area=A

Rod/ Pipe Type Electrodes














1
2
ln
2
ln
2
r
l
l
R
or
r
l
l
R
e
e




l
r

Slip or Conductor Type Electrodes



















 

t
l
l
R
l
l
R
For
r
rl
l
R
e
e
e







2
2
ln
2
depthat tBarriedTape
ln
hwith widtearth tape
2
2
ln
conductorsearthFor
I

• The purpose of resistivity measurements is to quantify the effectiveness of the
earth where a grounding system will be installed.
• Differing earth materials will affect the effectiveness of the grounding system.
• The capability of different earth materials to conduct current can be
quantified by the value E (resistivity in W.m).
• Resistivity measurements should be made prior to installing a grounding
system, the values measured will have an effect on the design of the
grounding system.
Ground Testing Methods
Resistivity Measurement

• Resistivity measurements are performed by using a four wire method.
Used to determine which kind of earthing should be used, so before
placing earth stakes
Resistivity Measurement ( Wenner method)

From the indicated resistance value RE, the soil resistivity is calculated
according to the equation :
E = 2  . a . RE
E ...... mean value of soil resistivity (W.m)
RE ...... measured resistance (W)
a ...... probe distance (m)
Resistivity Measurement

Resistance of driven rods:
• The Ground Resistance (R) of a single rod, of diameter (d) and driven length
(i) driven vertically into the soil of resistivity (ρ), can be calculated as follows:
where: ρ Soil Resistivity in m
l Buried Length of the electrode in m
d Diameter of the electrode in m
The rod is assumed as carrying current uniformly along its rod.
• Examples
(a) 20mm rod of 3m length and Soil resistivity 50 Ω-m .....R=16.1 Ω
(b) 25mm rod of 2m length and Soil resistivity 30 Ω-m .....R=13.0 Ω












 1
8
ln
2 d
l
l
R



• The resistance of a single rod is not sufficiently low.
• A number of rods are connected in parallel.
• They should be driven far apart as possible to minimize the overlap
among their areas of influence.
• It is necessary to determine the net reduction in the total resistance
by connecting rods in parallel.
• The rod is replaced by a hemispherical electrode having the same
resistance.

Rod Electrodes in Parallel
• If the desired ground resistance cannot be achieved with one ground
electrode, the overall resistance can be reduced by connecting a number
of electrodes in parallel.
• These are called “arrays of rod electrodes”.
• The combined resistance is a function of the number and configuration
of electrodes, the separation between them, their dimensions and soil
resistivity.
• Rods in parallel should be spaced at least twice their length to utilize the
full benefit of the additional rods.

• If the separation of the electrodes is much larger than their lengths
and only a few electrodes are in parallel, then the resultant ground
resistance can be calculated using the ordinary equation for
resistances in parallel.
• In practice, the effective ground resistance will usually be higher
than this.
• Typically, a 4 spike array may provide an improvement of about 2.5
to 3 times.
• An 8 spike array will typically give an improvement of may be 5 to 6
times.

Methods of decreasing ground resistance
• Decreasing the ground resistance of a grounding system in high resistivity soil
is often a difficult task.
• Recently, some new methods have been proposed to decrease ground
resistance.
1-chemical rods
• Chemical rods are electrodes with holes along their length, filled with mineral
salts.
• The specially formulated mineral salts are evenly distributed along the entire
length of the electrode.
• The rod absorbs moisture from both air and soil.
• Continuous conditioning of a large area insures an ultra-low-resistance ground
which is more effective than a conventional electrode.

• If the conductive salts are running low, the rod can be recharged with a refill kit.
• These rods are available in vertical and horizontal configurations.
• They may be used in rocky soils, freezing climates, dry deserts, or tropical rain
forests.
• They provide stable protection for many years.

Disadvantages are:
• Chemicals concentrated around electrodes will cause corrosion
• Chemicals leach through the soil and dissipate
• Scheduled replacement may be required
• May be prohibited because they may contaminate the water table

• Concern for public safety and the increasing complexity of fixed electrical
installations place extra responsibility on electrical test engineers.
• It is therefore important to have suitable test tools for carrying out the
stringent tests imposed by the International Electro-technical Commission
(IEC) and the European Committee for Electro-technical Standardization
(CENELEC).
3. Testing of electrical installation

• The visual inspection is carried out first, to confirm that permanently-wired
electrical equipment is compliant with the safety requirements and not
visibly damaged, and that fire barriers, protective, monitoring, isolating and
switching devices and the relevant documentation are present.
• Electrical testing may commence after this inspection

• It states that verification of the installation shall be carried out in the
following sequence:
1. Visual inspection
2. Testing of the following:
• Continuity of protective conductors;
• Insulation resistance;
• Protection by separation of circuits;
• ﬂoor and wall resistance;
• Automatic disconnection of supply;
• Polarity;
• Functional performance;
 In addition to this the following tests
are under consideration:
• Electric strength test;
• Voltage drop.
 To test the protective measures as
described above, IEC 60364.6.61
refers to the IEC / EN 61557.

• The purpose of the electrical installation condition report and testing is to
provide, as far as is reasonably practicable:
• For the safety of persons and livestock.
• Protection against damage by fire and heat.
• Confirmation that the installation is not damaged or deteriorated.
• The identification of the installation defects and non-compliance with the
Wiring Regulations.
Purpose of testing

Contents
• Electrical installation and cost estimation
• Contract document preparation
Chapter 4: Contracting electrical constrictions

Purpose of Estimating
Establish the bid price of a product for a quotation or contract.
Verify quotations submitted by suppliers.
Ascertain whether a proposed product can be manufactured and marketed
profitably.
Provide data for make-versus-buy decisions.
Help determine the most economical method, process, or material for
manufacturing a product.
Provide a temporary standard for production efficiency and guide operating
costs at the beginning of a project.
Help in evaluating design proposals.
Cost Estimating

To simplify the cost calculation, the cost components are broken down into the
following categories:
• Installation Costs
- costs to obtain and install all equipment involved in conceptual designs
including construction fixed overhead costs
- Cost for construction overhead for installation can be optionally be calculated
separately from equipment finding costs and then tabulated as long as this is
done consistently throughout the estimation process
• Design and Engineering Costs
- costs for detailed A&E firm design plans, and engineering involved in
installation, testing and final implementation of conceptual design.
- Estimated to be 25% of the total installation costs12/16/2019 Prepared by Haymanot T. (Lecturer ) 242
Elements of cost estimation

• Contingency Costs
- Additional 25% contingency added to account for unexpected costs
associated with the conceptual design such as
- base or
- city planning related costs.
• Overall Construction Costs
- Sum of installation,
- design and engineering as well as contingency costs.
- Approximately 1.5X of installation costs
- If any of these costs are more directly known, for example design costs
are $350K, from an estimate from an A&E firm, these costs can be
substituted for this formula used to estimate these costs.

• Determination of the required quantity of material
- Receptacles and light points are counted from the CAD file; or,
- making all devices BLOCKS in auto CAD design and loading from
CAD to excel.
- From distribution board scheduling tables.
Preparation of bill of quantities and specifications
Process of preparation
- Material selection like from catalog, …
- Material quantity determination
- Gathering information of selected material unit price, delivery cost, etc
- Determining unit price considering profit and other coast
Cost Estimation

• Data Required on the bill of quantities:
- List of Materials required with defined quantity and specification
- Unite Price of materials
- Total price per material type
- Total price of all materials
- Service Cost
i. Professional cost
ii. Labor cost

Cost estimation
• Estimates may be prepared manually or using spreadsheet or other computer
packages
• Estimate can be made based on device installation basis. Like switch, receptacle,
distribution board, fan outlet, etc
• Work Breakdown Structure (WBS)
- is a system that allows an estimate of total project construction and labor costs by
summing the costs of all tasks or items to be purchased (WBS elements) to yield an
estimate of the total project cost.
- Typically a WBS element will contain both Materials and Services (M&S) and
internal (collaborating institutional) labor.
• M&S is typically something that one purchases.
• Labor is people being paid to do a job.

• A good cost estimate requires that those making the estimate do not “miss”
elements that should be there.
• Contingency will be included in the cost estimating process to cover
cost/schedule risks. Through a consensus process, an upper and lower bound was
determined as a percent change from the original estimated cost.
• Use checklist to insure that all the WBS elements that should be in the estimate
are really there for your subsystem
• Watch out for duplication:
- There are several areas where it will be easy to double count the cost and
manpower resource requirements.
• Transportation Costs:
- the estimated cost of a component must include delivery costs to the site.
- This includes any special shipping containers, handling fixtures, etc. in addition to
the actual cost paid to a transport company

• SPECIAL CONDITIONS
- scope of works
- procurement & cost of tender
documents & drawings
- completion period
- penalty for delay
- submission of tenders
• GENERAL CONDITIONS
- general responsibility of the contractor
- variation in quantities
- defects after completion
Contract agreement: conditions of contract
• OTHER CONDITIONS
-submission of tenders
-Scope of Works
-Regulations and Standards
-Wiring System
• CABLE MANAGEMENT

WHAT IS TENDER?
Tender is an offer which incorporates the sum of
- Money,
- Time and
- Other conditions
Required to carry out the contract obligations in order to
- Complete a project or
- A part of it consisting of specified works.

What is tendering?
Will be started after the design complete and
project budget must be approved
The process that is used to obtain offers leading to a
contract between:
client contractor
client consultant
main contractor sub-contractor
12/16/2019

251
Stages of a project
• Stage 1: inception & feasibility
- identifying project objectives /
- set up project brief
• Stage 2: design
- develop further project brief /
- develop concept & schematic design /
- Undertake detailed design & detailing /
- prepare cost plan & budget
12/16/2019 Prepared by Haymanot T. (Lecturer )
Stage 4: construction
-site possession & mobilization /
-undertake procurement fabrication,
installation, construction /
-carry out contract admin
Stage 5: handover & maintenance
-undertake all testing, commissioning &
training /
-obtain CF /
-handover project to owner /
-carry out necessary maintenance /
-Defect rectification / etc.
Stage 3: tendering
-issue tender documents /
-prepare & submit tender /
-evaluation of tender /
-award tenders /
-prepare contract documents

Purpose of tendering process
• To select a suitable contractor at a time appropriate to the situation of the
project.
• To obtain from the contractor selected at the proper time, an acceptable
tender or offer upon which a contract can be let.
Difference between a contract and a tender
 A tender formally means an invitation to trade under the terms of offer.
 A contract is the term used for when the parties have reached agreement.
12/16/2019

Tendering process – PRINCIPAL ACTIVITIES
1. Selection of the most preferable or appropriate contract
obtaining method.
2. Development of tender list
3. Preparation of tender documentation
4. Invitation / calling off tender
5. Preparation and submission of tenders
6. Receipt and opening of tenders
7. Evaluation of tenders
8. Acceptance of tender award or award of contract
12/16/2019

254
Stage 1: Selection of the most preferable or appropriate contract
procurement (obtaining) method.
• Traditional contract procurement route (way)
• Longer time – lead to critical work scheduling
• Less errors – more precise
• Fast-track contract procurement route
• Shorter time
• Consequences – may be lead to critical error, variation.
Stage 2: Development of tender list
• Potential tenderers can directly apply to the employer for future works.
• Employer generate list based on previous dealings
• The professional team generate their own list and recommend to the
employer.

255
Stage 3: Preparation of tender documentation – typical contents
Letter of invitation
Checklist for submission of
tender
Articles of agreement
Conditions of contract
Form of tender
Tender specifications
 Bills of quantities
 Tender drawings
 Sample copy of letter of acceptance
 Sample copy of bang guarantee for
performance bond
 Schedule of day work rates
 Sample copy bank guarantee for
advance payment

Document Preparation and Approval
Signing of this document by the Design Manager indicates that:
• The Project Manager and Service Delivery Representative have been consulted.
• Where the job involves more than one engineering discipline, the specification
has been reviewed by people competent in each discipline.
• Funding for the scope of this Specification is secured and availability
confirmed by the Project Manager.
 Prepared by : ____________ Date : ____ position: design manager
Approved by :_____________ Date :____ position: principal’s representative
Contractors shall accept this specification only if the Principal’s Representative
has approved it.
Approval may be either by signature above on a hard copy or by issue of this
specification as an email attachment from the Principal’s Representative

Chapter 1 4

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