ELECTRICAL INSTALLATION TECHNOLOGY I
ELECTRICAL FINAL CIRCUITS
Definition
Final circuit - a Circuit which supplies Appliances (normally via socket-outlets or other types of
connectors)
Types of final circuits

Ring Circuit: a Circuit which is wired from a single Protective Device, being run
through an area to be supplied (via appropriate socket-outlets, connectors, etc) and
returning back to the same Protective Device, thus forming an electrically
continuous loop;
-
Ring circuit can be defined as a final sub-circuit in which the current carrying and earth
continuity conductors are connected in the form of a loop, both ends of which are
connected to a single way in a distribution fuse board or its equivalent.
A spur of a ring circuit shall be a branch cable having conductors of a cross-sectional area not
smaller than that of the conductors forming the ring.
The main I.E.E Regulations relating to the ring are as follows.
1. Cable size: minimum twin 2.5mm2 and earth p.v.c or t.r.s.
2. Maximum number of socket outlets allowed: unlimited number in floor area under
100m2, but spurs may not be more than half the socket outlets on the ring circuit,
including stationary appliances.
3. Fused 13A plugs to be used at socket outlets supplying portable appliances.
4. Fixed appliances must be protected by a local fuse, i.e. a fused spur box.
5. A 30A fuse should be used to protect the ring circuit.
6. All socket outlets in any one room must be connected to the same phase.
7. Apparatus permanently connected to the ring circuit without a fused plug or socket outlet
must be protected by a local fuse or circuit-breaker with a rating not exceeding 15A. the
apparatus must have an adjacent controlling switch.
The purpose of ring circuit is
a) To minimize the trailing flexes.
b) To take advantage of the fact that all outlets in a domestic installation are not operated
simultaneously. This is known as diversity in installation.
Domestic ring circuit

Radial Circuit: a Circuit which is wired in a ‘radial’ or ‘branch’ configuration,
emanating from a Protective Device, to the area to be supplied;

Spur Circuit: a Circuit which is wired in a ‘radial’ or ‘branch’ configuration from
any point on a Ring Circuit;

Distribution Circuit: a Circuit connecting between Distribution Boards (may also
be referred to as a ‘sub-Circuit’).
Sequence of control
It is essential that consumer’s supply should be effectively controlled and also that all switchgear
should be accessible.
Note. All conductors and apparatus must be of sufficient size and power for the work they are
called upon to do, and so constructed, installed and protected as to prevent danger.
The main switchgear in an installation must contain;
a) Means of isolating the supply.
b) Protection against excess current.
c) Means of cutting-off the current if a serious earth fault occurs.
Sequence of control equipment
The earth leakage circuit breaker is used where it is difficult to get a good earth path.
Domestic installations are usually supplied from a 16mm2 twin armoured cable. The figure
below shows line drawing of a typical sequence of supply control layout.
It contains the following;
1.
2.
3.
4.
The Supply Authority’s sealing chamber for the termination of the armoured cable.
The Supply Authority’s fuse and neutral block.
The Supply Authority’s energy meter (kWh meter).
Consumer’s control unit.
Consumer terminals
This is the point of connection of the consumer’s conductors to the supply authority’s meter. All
equipment before this point belongs to the supply authority.
Consumer terminals – the termination of electric conductors situated upon any consumers
premises ad belonging to him, at which the supply of energy is delivered from the service lines.
Every consumer’s installation must be controlled and protected by the following switchgear;
- A means of isolation e.g. linked switch.
- Excess current protection e.g. fuses or circuit breaker.
- Earth leakage protection.
Consumer’s control unit
The consumer control unit is made up of the following;
a)
b)
c)
d)
Main switch (60A) which isolates both the phase and the neutral conductors.
One 30A fuse for the cooker unit.
One 30A fuse for the 13A ring circuit (capable of taking two 7/0.8 in cables).
One or two 5A fuses for lighting circuits.
Loading of final sub circuits
The assumed current demand from points is as follows;
15A socket outlet
5A socket outlet
2A socket outlet
Lighting outlet
-
-
15A
5A
at least ½A
minimum 100W
Only one phase of a supply should preferably be brought into a multi-gang switch
box. Where more than one phase is used there must be a rigid screen or barrier
separating the phases, and a clearly visible notice warning of the maximum voltage
present. This notice should be placed outside the switch.
All final sub-circuits must be electrically separate (there must be no bunching of
neutral conductors). All neutral conductors must be connected at the distribution
board in the same order as the line conductors.
Layout diagrams for final circuits
A layout diagram shows exactly where the accessories or appliances will be placed in an
installation.
Layout diagram for a bungalow
Cooker Control Unit
It consists of a double-pole switch feeding the cooker and an independent 13A socket outlet. It is
essential that the ECC supplying the unit should be effectively connected.
The cooker is supplied from a separate way in the consumer’s control unit and wired with
10mm2 twin and earth p.v.c cable. It is fused at 30A which is sufficient to protect a maximum of
9kW (3-plate cooker).
Wiring diagrams
These diagrams shows how the connections from the supply cable to the consumer unit,
accessories and appliances will be done.
Lighting circuits
Loop in method
This method of wiring is universally used in wiring lamps and other appliances connected in
parallel so that each of the appliances can be controlled individually.
When a connection is required at a light or switch, the feed conductor is looped in by bringing it
directly to the terminal and then carrying it forward again to the next point to be fed.
The switch and light feeds are carried round the circuit in a series of loops from one point to
another until the last on the circuit is reached. The phase or line conductors are looped either in
switch board or box and neutrals are looped either in switch board or from light or fan. Line or
phase should never be looped from light or fan.
Advantages off Loop-In method of wiring
i.
It doesn’t require joint boxes and so money is saved.
ii. No joint is concealed beneath floors or in roof spaces.
iii. Fault location is made easy as the points/joints are made only at the outlets so that they
are accessible.
Disadvantages
i. Length of wire or cables required is more and voltage drop and copper losses are more.
ii. Loop-in switches and lamp holders are usually difficult to install.
Use of ceiling roses
There are two main types of ceiling roses
a) The three plate pattern
It is used to economize in wire and minimize the number of joint boxes used in the installation.
b) The two plate pattern
Note: Ceiling roses must not be used on circuits operating above 250V and no more than one
flexible cord is permitted from any one ceiling rose. The earthing terminal of every ceiling rose
must be connected to earth continuity conductor of the final sub-circuit.
One way switching
One switch serving a number of lights. The conductors are looped between the lamps.
Two way switching
Used for stairs and corridors
Two way and intermediate switching
Dim-bright switching
The lamps are connected in series for dim operation and in parallel for bright.
Use of joint boxes
In this method of wiring, constructions to appliances are made through joints. These joints are
made in joint boxes by means of suitable connectors or joints cutouts. This method of wiring
does not consume too much cable size. This method is suitable for temporary installations and it
is cheap.
Accessories and equipment
Definitions
1. Accessory - a device, other than current-using equipment, associated with an Electrical
Installation.
2. Switch plug – a combination of a switch and a plug attached to a flexible cord with an
electric appliance.
3. Socket outlet – a socket for an electric plug that is fixed to a wall and connected to an
electricity supply.
4. Distribution board – an assembly designed for housing isolation switches and Protective
Devices and for connecting multiple Circuits, including their associated neutral and Earth
Conductors.
5. Protective Device: a device installed at the start of a Circuit which will automatically
disconnect the input of electricity in the event of a fault or overload occurring on that
Circuit. Such devices include, but are not limited to, fuses, fuse links, miniature circuitbreakers (MCB), moulded case circuit breakers (MCCB), earth leakage circuit-breakers
(ELCB), and residual current devices (RCD).
6. Joint box: an enclosure that protects a connection of two or more wires carrying
electrical current.
Testing and Inspection
The main purpose for testing an installation is to detect faults before dangerous situations arise.
Factors which an installation must be protected against are;
a)
b)
c)
d)
e)
Earth leakage
Danger of electric shock
Excess current
Moisture and
Corrosion
Earth Tests carried out on an installation.
a) Verification of polarity
b) Insulation resistance tests
c) Ring circuit continuity tests
When to carry out the tests
-
On new installations
On additions to existing installations
-
Periodically on existing installations.
a) Verification of Polarity Test
The purpose of this test is to check that the phase conductor is taken through the fuse and the
switch to the appliance; and the neutral wire is earthed at the supply Authority’s substation.
But the neutral is not necessarily dead in a system in which the load is unevenly distributed
between the three phases (an unbalanced system); it is possibly to get a shock from the neutral.
The neutral must not be broken by a fuse or switch.
Preparations for Verification of Polarity test
1. Supply OFF
2. Lamps and appliances out
3. All switches OFF; neutral links IN; Fuses OUT
Test instrument: Ohmmeter or Bell set.
Method
Test for continuity between: A and B; C and D; E and F.
Reading: Zero on Ohmmeter or continuity with bell set.
Note: this should not be carried out on a live or phase indication. The insulation test (e.g. the
megger) should not be used as the smallest reading is about 10,000Ω.
b) Tests for effectiveness of earthing
The reasons for carrying out earthing tests are as follows:
i.
ii.
iii.
To measure the resistance of earth continuity conductor (the metal conduit, trunking and
metal sheath or the special continuity conductor which connects the earth lead to those
parts of an installation which require to be earthed).
To check that the earth continuity conductor is capable of carrying heavy leakage
currents.
To ensure that the earth electrode (e.g. buried copper plate) is effectively connected to the
general mass of earth.
NOTE: The consumer’s earthing terminal must be bonded to the metal-work of public services
(e.g. gas and water pipes) on the consumer premises at the nearest point of entry on the
consumer’s side.
c) Earth Insulation Resistance Test
The purpose is to warn of the existence of possible leakages to earth and to pin-point the actual
leakage.
Preparation for Earth Insulation Resistance Test
i.
ii.
iii.
iv.
v.
Supply OFF.
Fuses IN, neutral links IN.
All switches ON.
All lamps IN.
Connect all poles together.
Test instrument: a hand driven d.c generator which should be capable of supplying a d.c voltage
not less than twice the voltage normally supplied to the circuit.
The voltage need not exceed 500V for low-voltage circuits (50V to 1000V)
The test should be carried out at the nearest possible point to the supply Authority’s equipment.
Method
i.
ii.
Connect one wire of insulation tester to earthed metalwork (case of main switch,
trunking, conduit, etc).
Connect other wire to phase (or phases) and neutral in turn.
Reading: The expected reading will depend upon the size of the installation but should be less
than one mega ohm.
d) Between Poles Test
The purpose of this test is to make sure that there are no short circuit or low-resistance
connections between the ‘live’ conductors in the installation.
Preparation
i.
ii.
iii.
Supply OFF.
Lamps OUT; appliances OFF.
All switches ON; all fuses and neutral links IN.
Test Instrument: insulation resistance tester (merger). The test should be carried out at the
nearest possible point to the Supply Authority’s equipment.
Method
Connect insulation resistance tester between phase and neutral.
Readings: the minimum readings required are similar to those for the earth insulation resistance
test on the same installation.
e) Earth Fault Loop Impedance Test
The earth fault loop is the path which the leakage current will take back to the supply
transformer when there is an earth leakage in an installation.
The purpose of this test is to show that the earth fault loop is capable of carrying heavy leakage
currents so that protective gear (fuses) will operate when leakages occur between the line
conductor and the earthed metalwork of the installation
This test must be carried out on new or largely modified installations where earth-leakage
protection relies on the operation of fuses or excess current circuit-breakers.
-
The leakage current flows from the faulty conductor into the earth continuity conductor.
It then flows along the earth continuity conductor to the earthing lead.
The earthing lead carries the current to the earth electrode.
The leakage current now takes the shortest path back to the earthed neutral of the supply
transformer.
Apparatus: Line-earth loop tester.
Circuit of megger line earth-loop tester
Method
The line-earth loop tester operating on full mains voltage, passes a short duration current of
approximately 20A from the line conductor through the consumer’s earth continuity conductor
and the earth return to the neutral of the supply transformer. This measures the value of the loop
in ohms.
Readings
The minimum permissible reading depends on the operating conditions but the two main factors
are:
-
Operating current of fuse or circuit breaker protecting circuit.
Supply voltage.
Factors determining resistance of earth fault loop are;
-
The continuity of the metallic circuit up to the earth electrode.
The resistance of the body of earth surrounding the earth electrode.
f) Ring Continuity Test
This test is carried at the point of connection in the distribution fuse-board prior to the
completion and connection of the ring circuit conductors.
Tests to be carried out are given below;
i.
ii.
iii.
Verification of polarity – check that phase wire is switched and fused. Reading on
ohmmeter or bell set – zero or continuity.
Earthing tests – to ensure that the metalwork of the installation is effectively
connected to the general mass of earth.
a) Testing earth continuity conductor. Maximum reading, ½ Ω.
b) Testing earth fault loop impedance by current injection. Reading determined by
setting of protective equipment.
c) Testing effectiveness of earth electrode. Reading determined by setting of
protective equipment.
Insulation tests –
a) Between poles
b) Earth insulation resistance (between all conductors and earth )
The minimum permissible values for both tests: 1MΩ or 0.5MΩ (where appliances are
connected).
iv.
Ring circuit should be tested with ohmmeter or bell set for continuity of ring.
ELECTRICAL POWER SUPPLY
Authorities of power production
i.
ii.
iii.
Kenya power and lighting company
Kenya Power owns and operates most of the electricity transmission and distribution
system in the country and sells electricity to over 6,761,090 million by end of June
2018.
The Company’s key mandate is to plan for sufficient electricity generation and
transmission capacity to meet demand; building and maintaining the power
distribution and transmission network and retailing of electricity to its customers.
Kenya Electricity Generating Company (KenGen)
It manages and develops all public power electricity generating facilities. It sells
electricity in bulk to Kenya Power. KenGen is the main player in electricity
generation accounting for 1,238MW (76%) of installed electricity generation capacity
for the national transmission grid as at 30th June 2013.
Kenya electricity transmission company (KETRACO) is mandated to construct
new transmission lines with government funding to accelerate infrastructure
development. KETRACO was incorporated in December 2008, is a fully owned State
Corporation and a Special Purpose Vehicle to plan, design, construct, own, operate
and maintain new high voltage (132kV and above)electricity transmission grid and
regional inter-connectors.
iv.
Geothermal Development Company (GDC) is tasked with developing steam fields
to reduce upstream power development risks so as to promote rapid development of
geothermal electric power. GDC is a fully owned Government Special Purpose
Vehicle (SPV) that undertakes surface exploration of geothermal fields, explorations,
appraisals, drilling, steam production and entering into steam sales agreements with
investors in the geothermal electricity generation.
v.
The
Energy
Regulatory
Commission
(ERC)
ERC is an independent agency responsible for regulation of the energy sector
agencies, oversight, coordination preparation of Least Cost Power Development Plans
(LCPDP), and monitoring and enforcement of sector regulations.
vi.
The
Energy
Tribunal
The Energy Tribunal is an independent legal entity that arbitrates disputes between
parties in the sector.
vii.
Rural
Electrification
Authority
(REA)
REA is a government wholly owned entity, charged with implementing the Rural
Electrification Programme. It came into operation in July 2007.
viii.
Kenya Nuclear Electricity Board (KNEB)
KNEB is charged with spearheading and fast tracking development of nuclear
electricity generation to enhance production of affordable and reliable electricity.
ix.
Independent Power Producers (IPPs)
IPPs are private investors in the power sector involved in generation either on a large
scale or for the development of renewable energy under the Feed-in–Tariff (FiT)
Policy.
Source: http://energy.go.ke/?page_id=528, https://kplc.co.ke/content/item/14/kenya-power
Power supply systems
Generating stations
A generating station employs a prime mover coupled to an alternator for the production of
electric power. The prime mover converts energy from some other form of mechanical energy.
The alternator converts mechanical energy of the prime mover into electrical energy which is
transmitted and distributed with the help of conductors to various customers.
The electric power is generated at 11kV then stepped up to 132 kV, 200kV or more for
transmission.
Typical layout
Primary transmission
The electric power at 132kV is transmitted by 3phase, 3wire overhead system to the outskirts of
the city. This forms the primary transmission.
Secondary transmission
The primary transmission line terminates at the receiving station (RS) which usually lies at the
outskirts of the city. The voltage is reduced to 33kV by step-down transformers, then transmitted
by 3phase, 3wire overhead system to various substations (SS) located at strategic points in the
city.
Primary distribution
The secondary transmission line terminates at the sub-station (SS) where voltage is reduced from
33kV to 11kV 3phase 3 wire. The 11kV line runs along the important road sides of the city. Big
consumers (having demand more than 50 kW) are generally supplied power at 11kV for further
handling with their own substations.
Secondary distribution
The electric power from primary distribution line (11kV) is delivered to distribution sub-stations
(DS). They are located near consumers’ localities and step-down voltage to 400V 3phase, 4 wire
for secondary distribution. The voltage between phases is 400V and between any phase and
neutral is 230V. The single phase domestic loads are connected between one phase and the
neutral, whereas 3-phase 400V motor loads are connected across 3-phase lines directly.
Types of generating stations
i.
Hydro-electric power station
This is a generating station which utilizes the potential energy of water at a high level for the
generation of electrical energy. They are generally located on hilly areas where dams can be built
conveniently and large water reservoirs can be obtained. Water head is created by constructing a
dam across a river or lake. This water is led to a water turbine which captures the energy in the
falling water and changes the hydraulic energy into mechanical energy at the turbine shaft. The
turbine drives the alternator which converts mechanical energy into electrical energy.
Advantages
a) Requires no fuel as water is used for the generation of electrical energy
b) It is quite, neat and clean as no smoke or ash is produced
c) Requires very small running charges because water is the source of energy which is
available free of cost.
d) It is comparatively simple in construction and requires less maintenance.
e) It does not require a long starting time like a steam power station.
f) It is robust and has longer life.
g) They serve many purposes i.e. help in irrigation and controlling floods.
Disadvantages
a) Involves high capital cost due to construction of dam
b) There is uncertainty about the availability of huge amounts of water due to dependence
on the weather conditions
c) Skilled and experienced hands are required to build the plant
d) Requires high costs of transmission lines as the plant is located in hilly areas which are
quite away from the consumers
ii.
Steam/thermal power station
This is a generating station which converts heat energy of coal combustion into electrical energy.
Steam is produced in the boiler by utilizing the heat of coal combustion. The steam is then
expanded in the prime mover (steam turbine) and is condensed in a condenser to be fed into the
boiler again. The steam turbine drives the alternator which converts mechanical energy of the
turbine into electrical energy. This type of power station is suitable where coal and water are
available in abundance and a large amount of electric power is to be generated.
Advantages
a) The fuel (coal) used is quite cheap.
b) Less initial cost as compared to other generating stations
c) Can be installed at any place irrespective of the existence of coal. The coal can be
transported to the site of the plant
d) It require less space as compared to the hydro-electric power station.
e) Cos of generation is lesser than that of diesel power station.
Disadvantages
a) Pollutes the atmosphere due to the production of large amount of smoke and fumes
b) Costlier in running cost as compared to hydro-electric plant.
iii.
Diesel power station
This is a generating station in which diesel engine is used as the prime mover for the generation
of electrical energy.
The diesel burns inside the engine and the products of this combustion acts as the ‘working fluid’
to produce mechanical energy. The diesel engine drives the alternator which converts mechanical
energy into electrical energy.
The generation cost is considerably high due to high price of diesel, therefore, they are only used
to produce small power.
This plants are also used as standby sets for continuity of supply to important points i.e.
hospitals, radio stations, cinema halls, and telephone exchanges.
Advantages
a)
b)
c)
d)
e)
f)
g)
The design and layout of the plant is quite simple.
Occupies less space as the number and size of the auxiliaries is small.
Can be located at any place.
Can be started quickly and can pick up load in a short time.
There are no standby losses.
Requires less quantity of water for cooling.
The overall cost is much less than that of steam power station of the same capacity.
h) The thermal efficiency of the plant is higher than that of a steam power station.
i) Requires less operating staff.
Disadvantages
a)
b)
c)
d)
e)
iv.
The plant has higher running charges.
The plant doesn’t work satisfactorily under overload conditions for a longer period.
The plant can only generate small power.
The cost of lubrication is generally high.
The maintenance charges are generally high.
Nuclear power station
This is a generating station in which nuclear energy is converted into electrical energy.
Heavy elements such as Uranium (U235) or Thorium (Th232) are subjected to nuclear fission in a
special apparatus known as a reactor. The heat thus released is utilized in raising steam at high
temperature and pressure. The steam runs the steam turbine which converts steam energy into
mechanical energy. The turbine drives the alternator which converts mechanical energy into
electrical energy.
The most important feature of a nuclear station is that huge amount of electrical energy can be
produced from a relatively small amount of nuclear fuel as compared to other conventional types
of power stations.
Advantages
a) The amount of fuel required is quite small. Therefore, there is considerable saving in the
cost of fuel transportation.
b) A nuclear power plant requires less space as compared to any other type of the same size.
c) It has low running charges as a small amount of fuel is used for producing bulk electrical
energy.
d) This type of plant is very economical for producing bulk electric power.
e) It can be located near the load centers because it doesn’t require large quantities of water
and need not be near coal mines. Therefore, the cost of primary distribution is reduced.
f) There are large deposits of nuclear fuels available all over the world. Therefore, such
plants can ensure continued supply of electrical power for thousands of years.
g) It ensures reliability of operation
Disadvantages
a)
b)
c)
d)
The fuel used is expensive and is difficult to recover.
The capital cost on a nuclear plant is very high as compared to other types of plants
The erection and commissioning of the plant requires greater technical know-how.
The fission by-products are generally radioactive and may cause a dangerous amount of
radioactive pollution.
e) Maintenance charges are high due to lack of standardization.
f) They are not well suited for varying loads as the reactor does not respond to load
fluctuations efficiently.
g) The disposal of the by-products, which are radioactive, is a big problem. They have either
to be disposed of in a deep trench or in a sea away from sea shore.
v.
Gas turbine power plant
This generating station employs gas turbine as the prime mover for the generation of electrical
energy. Air is used as the working fluid.
The air is compressed by the compressor and is led o the combustion chamber where heat is
added to air, thus raising its temperature. The hot and high pressure air from the combustion
chamber is the passed to the gas turbine where it expands and does the mechanical work. The gas
turbine drives the alternator which converts mechanical energy into electrical energy.
Advantages
a) Simple in design compared to steam power station since no boilers and their auxiliaries
are required.
b) Much smaller in size compared to steam power station of the same capacity.
c) The initial and operating costs are much lower than that of equivalent steam power
station.
d) Requires less water as no condenser is used.
e) Maintenance charges are quite small
f) Gas turbines are much simple in construction and operation than steam turbines.
g) Can be started quickly from cold conditions
h) There are no standby losses.
Disadvantages
a) Before starting the turbine, the compressor has to be operated for which power is required
from an external source.
b) Since greater part of power developed by the turbine is used in driving the compressor,
the net output is low.
c) The overall efficiency is low (20%) because the exhaust gases from the turbine contains
sufficient heat.
d) The temperature of combustion chamber is quite high (3000oF) so that its life is reduced.
vi.
Geothermal power plant
Geothermal power stations are similar to other steam turbine in that heat from a fuel source (in
geothermal case, the Earth's core) is used to heat water or another working fluid. The working
fluid is then used to turn a turbine of a generator, thereby producing electricity. The fluid is then
cooled and returned to the heat source.
Dry steam power stations
Dry steam stations are the simplest and oldest design. This type of power station is not found
very often, because it requires a resource that produces dry steam, but is the most efficient, with
the simplest facilities. In these sites, there may be liquid water present in the reservoir, but no
water is produced to the surface, only steam. Dry Steam Power directly uses geothermal steam of
150 °C or greater to turn turbines. As the turbine rotates it powers a generator which then
produces electricity and adds to the power field. Then, the steam is emitted to a condenser. Here
the steam turns back into a liquid which then cools the water. After the water is cooled it flows
down a pipe that conducts the condensate back into deep wells, where it can be reheated and
produced again.
Flash steam power stations
Flash steam stations pull deep, high-pressure hot water into lower-pressure tanks and use the
resulting flashed steam to drive turbines. They require fluid temperatures of at least 180 °C,
usually more. This is the most common type of station in operation today. Flash steam plants use
geothermal reservoirs of water with temperatures greater than 360 °F (182 °C). The hot water
flows up through wells in the ground under its own pressure. As it flows upward, the pressure
decreases and some of the hot water boils into steam. The steam is then separated from the water
and used to power a turbine/generator. Any leftover water and condensed steam may be injected
back into the reservoir, making this a potentially sustainable resource.
Binary cycle power stations
Binary cycle power stations are the most recent development, and can accept fluid temperatures
as low as 57 °C. The moderately hot geothermal water is passed by a secondary fluid with a
much lower boiling point than water. This causes the secondary fluid to flash vaporize, which
then drives the turbines. This is the most common type of geothermal electricity station being
constructed today. Both Organic Rankine and Kalina cycles are used. The thermal efficiency of
this type of station is typically about 10–13%.
Source: https://en.wikipedia.org/wiki/Geothermal_power
Advantages
a) Comparatively ecologically clean. Unlike coal-fired power plants, geothermal ones use a
renewable heat source with a constant supply. The amount of greenhouse gas from
geothermal power plants is only 5% in the contrary with coal-fired power plants.
b) More energy. Geothermal power stations have great capacity – they can gravely help in
meeting the demand for energy that grows every year, both in developed and developing
countries.
c) Stable prices. Simple power plants depend on fuel, so the cost of their electricity is varying,
based on the market price of fuel. Since geothermal power plants do not use fuel, they do not
need to take into account its cost, and they can offer their customers stable electricity costs.
d) Low operating costs. Geothermal installations require minimal maintenance compared to
conventional power plants. As a result, they are reliable and cheap in operation.
e) Renewable and sustainable source. Geothermal energy will never end, unlike nonrenewable energy sources. As long as the earth supports our lives, geothermal energy will
exist and geothermal power will work.
f) Permanent power supply. Unlike other renewable energy sources, geothermal one can
provide a constant supply of energy – 24 hours a day, 7 days a week, 365 days a year,
regardless of external factors. For example, solar panels can produce electricity only during
the day, and wind turbines produce energy only with sufficient wind.
g) Small area. They occupy less space than their coal, oil and gas equivalents. Although they
will reach far below the earth’s surface, their area will be negligible.
h) Low noise work. There is a little noise in the production of geothermal energy. The main
source of noise is the fans that are in the cooling systems. To reduce its level, engineers can
install in the generator shops materials with high damping properties. It helps to reduce noise
pollution.
i) Energy security. Using local geothermal resources, the need to supply sources from other
countries reduces, which, in turn, lowers dependence on external influences and helps to
increase our energy security.
Disadvantages
a)
b)
c)
d)
e)
Ecological problem. High environmental consumption of fresh water can be a loss for
the environment, which will ultimately lead to its deficit. Liquids extracted from the earth
during drilling contain a large number of toxic chemicals (including arsenic and
mercury), as well as greenhouse gases (such as hydrogen sulfide, carbon dioxide,
methane, ammonia and radon). If they are incorrectly disposed or treated, they can get
into the atmosphere or leak into groundwater and damage the environment and human
health.
Geographical limits. Geothermal activity is the highest along the tectonic fault lines in
the earth’s crust. Exactly in these places the geothermal energy has the greatest potential.
The drawback is that only few countries can use geothermal resources. Therefore, while
having a look at their geographical peculiarities, such countries are the main producers of
geothermal energy: the USA, Iceland, Kenya, Indonesia, the Philippines, Mexico
Seismic instability. There are reasons to believe that geothermal structures have caused
underground shakings in different parts of the world. Despite the fact that seismic activity
is often insignificant, it can lead to building damage, injuries and death.
Expensive construction. Geothermal power plants require significant investments.
Although they have low operating costs, the cost of their construction may be much
higher than coal, oil and gas plants. Much of these expenses concerns the exploration and
drilling of geothermal energy resources. What is more, geothermal power plants require
specially developed heating and cooling systems, as well as other equipment that can
withstand high temperatures.
Possible exhaustion. Studies show that without careful management, geothermal tanks
can be exhausted. In such cases, the geothermal power plant will become unnecessary
until the tank is restored. The only inexhaustible option is to get geothermal energy
directly from the magma, but this technology is still in the process of development. This
option is worth investing at least because the magma will exist billions of years.
Source: https://avenston.com/en/articles/geothermal-pp-pros-cons/
vii.
Wind power plant
This generating station employs wind turbine as the prime mover for the generation of electrical
energy.
Winds turns the propeller-like blades of the wind turbine around a rotor. These blades drive the
wind turbine which converts wind energy into mechanical energy. The wind turbine (which is a
set of gears) drives the alternator which converts mechanical energy into electrical energy.
Advantages


Wind energy is a green energy source. Harnessing wind energy does not pollute the
environment nearly as much as fossil fuels, coal and nuclear power do.
As mentioned in the introduction of this article, the potential of wind power is
absolutely incredible. Several independent research teams have reached the same
conclusions: The worldwide potential of wind power is more than 400 TW (terawatts)





Wind energy is a renewable source of energy. Wind is naturally occurring and there is
no way we can empty the energy resources. Wind energy actually originates from the
nuclear fusion processes that take place on the sun.
Space efficient. The wind turbines can’t be placed too close to each other, but the land
in-between can be used for other things.
Although wind power only accounts for about 2.5% of total worldwide electricity
production, the capacity is growing at an incredible rate of 25% per year
(2010). This does not only contribute in the fight against global warming, but also helps
lowering costs:
Prices have decreased over 80% since 1980. Thanks to technological advancements
and increased demand, prices are expected keep decreasing in the foreseeable future.
It is generally true that operational costs tend to be low once the turbines first have
been manufactured and erected. However, not every wind turbine is created equal –
some are more susceptible to maintenance than others.
Disadvantages






Aesthetic impact: Many people are concerned with the visual effects that wind turbines
have on the beautiful scenery of nature. They believe that giant wind turbines distract
viewers from the beautiful surroundings.
The initial cost is high.
Wildlife: Wind turbines may be dangerous to flying animals. Many birds and bats have
been killed by flying into the rotors. Experts are now conducting research to learn more
about the effects that wind turbines have on marine habitats.
Remoteness of location: Although this may be an advantage (placing wind turbines in
desolate areas, far away from people), it may also be a disadvantage. The cost of travel
and maintenance on the turbines increases and is time consuming. Offshore wind turbines
require boats and can be dangerous to manage.
Noise: Some wind turbines tend to generate a lot of noise which can be unpleasant
Safety at Sea: In the darkness/at night it may be difficult for incoming boats to see wind
turbines thus leading to collisions.
Source:
https://www.google.com/search?q=wind+power+station&rlz=1C1GGRV_enKE867KE867&oq=
wind+power+station&aqs=chrome..69i57.9264j0j7&sourceid=chrome&ie=UTF-8
https://energyinformative.org/wind-energy-pros-and-cons/
Transmission and distribution systems
Fundamentally electric power transmitted by overhead systems or underground system. In both
cases, there are two types of process for electrical power transmission either by A.C or by D.C.
The process of A.C and D.C transmission has its own advantage and disadvantages also over one
another.
i.
D.C. Transmission:
This type of power transmission can be possible by several ways.
a) D.C two wires,
b) D.C two wires with midpoint earth and
c) D.C three wires.
Advantages of D.C transmission:
a)
b)
c)
d)
e)
f)
g)
It requires only two conductors.
There is no problem of inductance and capacitance.
There is no surge problem.
Voltage drop is less than A.C transmission, so better voltage regulation.
There is no skin effect. So we can utilize the entire cross-section of the conductor.
Less corona loss which reduces the interference with communication circuit.
Less insulating material than A.C is required.
Disadvantages are:
a) Due to commutation problem, electric power cannot be generated at D.C voltage.
b) D.C voltage cannot be stepped up or down because transformer uses only A.C. voltage
and current.
c) D.C. switch, circuit breaker has its own limitations.
ii.
A.C. Transmission:
The most common way of transmitting electrical power is A.C transmission.
The various process of A.C. transmission are
-
-
Single phase A.C. system: These are also subdivided by
 Single phase two wire.
 Single phase two wire with point earthed.
 Single phase three wire.
 Two phase A.C system:
 Two phase four wires.
 Two phase three wires.
Three phase A.C. system:
 Three phase four wires.
 Three phase three wires.
Advantages of A.C transmission:
a) A.C voltage can easily be stepped up and stepped down at desired level by transformer.
b) Maintenance of A.C. substation is easy and cheaper.
c) In electric power system, from generation to distribution, throughout the network, a.c.
voltage is handled. So no extra care is needed like D.C. voltage.
d) Power can be generated at high voltages
Disadvantages of A.C transmission:
a)
b)
c)
d)
A.C. lines require more copper as it requires more conductor than D.C.
A.C. transmission line construction is more complicated than D.C.
As skin effect occurs only in A.C, effective resistance of the line increased.
For line capacitance, continuous loss of power occur due to charging current.
e) The voltage drop is greater than D.C.
Distribution system
This is the part of power system which distributes electric power for local use.
It is the electrical system between the sub-station fed by the transmission system and the
consumers’ meter. It consists of feeders, distributors and the service mains.
-
-
-
Feeders – a conductor which connects the sub-station (or localized generating station) to
the area where power is to be distributed. No tappings are taken from the feeder so that
current remains the same throughout.
Distributor – a conductor from which tappings are taken for supply to the consumers.
The current throughout a distributor is not constant because tappings are taken at various
places along its length.
Service mains – a small cable which connects the distributor to the consumers’
terminals.
Ac distribution
Nowadays electrical energy is generated, transmitted and distributed in the form of alternating
current because it can be conveniently changed in magnitude by means of a transformer.
This system is the electrical system between the step down substation fed by the transmission
system and the consumers’ meters. The AC distribution system is classified into;
- Primary distribution system
- Secondary distribution system
i.
Primary distribution system
This operates at voltages higher than general utilization and handles large blocks of electrical
energy than the average low-voltage consumer uses. The voltage used depends on the amount of
power to be conveyed and the distance of the substation required to be fed. The most commonly
used primary voltages are 11kV, 6.6kV and 3.3kV. Due to economic considerations, primary
distribution is carried out by 3-phase, 3-wire system. Power is supplied to various substations for
distribution or to big consumers at this voltage.
Typical schematic layout
ii.
Secondary distribution
This includes range of voltages at which the ultimate consumer utilizes the electrical energy
delivered to him. It employs 400/230 V, 3-phase, 4-wire system. The substations are situated
near the consumers’ localities and contain step-down transformers. The voltage is stepped down
to 400V and power is delivered by 3-phase, 4-wire AC system. The voltage between phases is
400V and between any phase and neutral is 230V. The single phase domestic loads are
connected between one phase and the neutral, whereas 3-phase 400V motor loads are connected
across 3-phase lines directly.
DC distribution
The DC supply from the substation may be obtained in the form of;
- 2-wire distribution
- 3-wire distribution
i.
2-wire distribution
It consists of two wires, one is the outgoing/positive wire and the other is the return/negative
wire. The loads i.e. lamps, motors etc. are connected in parallel between the two wires. This
system is never used for transmission purposes due to low efficiency but may be employed for
distribution of DC power.
ii.
3-wire DC system
It consists of two outers and a middle/neutral wire which is earthed at the substation. The voltage
between the outers is twice the voltage between outer and neutral wire. The advantage of this
system is that it makes available two voltages at the consumer terminals between any outer and
the neutral and 2V between the outers. Loads requiring high voltages (e.g. motors) are connected
across the outers, whereas lamps and heating circuits requiring less voltages are connected
between either outer and the neutral.
CABLES
A cable is a length of insulated single conductor (solid or stranded), or of two or more such
conductors, each provided with its own insulation, which may be laid up together.
A cable consists of two basic parts;
 The conductor
 The insulator
 A means of protection
Types and sizes of cable conductors
The conductor is the type of metal which allows the electrical current to flow through it. The
electrical conductor is generally made up of metals like copper, aluminium and their alloys.
The electrical conductor which is used for power transmission is usually stranded. Stranded
conductors have great flexibility and mechanical strength as compared to a single wire of the
same cross-section area. In stranded conductor usually, the centre wire is surrounded by the
successive layers of wires containing 6, 12, 18, 24, wires.
a. Hard drawn copper conductor
Such type of conductors gives high tensile strength. It has high electrical conductivity, long life,
and high scrap value. It is most suitable for distribution work where spans and tapping are more.
b. Cadmium copper conductor
The tensile strength of the copper is increased by approximately 50 percent by adding about 0.7
to 1.0 percent cadmium to it, but their conductivity is reduced by about 15 to 17 percent. The
property of higher tensile strength enables the conductor to be erected on longer spans with the
same sag.
c. Steel-cored copper conductor (SCC)
In steel cored copper conductor one or two layers of copper strands surround a steel cored copper
conductors. The steel core adds the tensile strength to the conductor.
d. Copper welded conductor
In such type of conductors, the uniform layers of copper are welded onto a steel wire. The
conductivity of the copper welded conductor varies from 30 to 60 percent to that a solid copper
conductor with the same diameter. Such types of conductors may be used for longer span such as
a river crossing.
e. Hard-Drawn Aluminium Conductor or All-Aluminum Conductor
The cost of the copper conductor is very high, and hence it is replaced by the aluminium
conductor. The handling, transportation and erection of the aluminium wires become very
economical. It is used in distribution lines in the urban area and short transmission line with the
lower voltages.
f. Aluminium conductor steel reinforced
All aluminium conductors are not sufficiently strong mechanically for the construction of long
span lines. This deficiency in strength can be compensated by adding a steel core to the
conductor. Such a conductor is called steel-cored aluminium conductor (SCA) or aluminium
conductor steel reinforced (ACSR).
The ACSR conductor has seven steel strands forming a central core around which there are two
layers of 30 aluminium strands. The conductor stranding is specified as a 30 Al/7 St. The ACSR
conductors have high tensile strength and light weight and hence it is used for small sag.
g. Smooth body ACSR conductor
Such type of conductor is also called Compacted ACSR. The conventional ACSR conductor is
pressed through dies to flatten the aluminium strands into segmental shape. The interstrand space
is filled, and the diameter of the conductor reduces without affecting its electrical and
mechanical properties. This conductor can be made with different ratios of aluminium to steel.
The figure shows below the conductor having ratio 6 Al/1 St.
h. Expanded ACSR Conductor
For reducing the corona loss and radio interference at a high voltage a fibrous or plastic material
is filled between the strands. The diameter of the conductor expands due to the filling material
and hence, it is called an expanded conductor. These conductors consist paper material which
separates the inner aluminium strands from the outer steel strands.
i.
All Aluminum Alloy Conductor
Such type of conductor is mostly used in urban areas. This conductor has a good combination of
conductivity and tensile strength. One of the alloys which are used for making such conductor is
Silmalec. This alloy contains 0.5% silicon, 0.5% magnesium and the remainder aluminium.
These alloys are very costly as they are heat treated.
j. ACAR Conductor
Aluminium Conductor Alloy Reinforced has a central core of alloys of aluminium surrounded by
the layers of conductor aluminium. Such conductor gives a better conductance with the strengthweight ratio equal to ACSR construction of the same diameter.
k. Alumoweld Conductor
Aluminium powder is welded onto a high strength steel wire. About 75% of the area of the
conductor is covered by aluminium. This is more costly than core silicon conductor. It has been
used an earth wire for making cores of SCA conductors.
l.
Phosphor Bronze Conductor
Phosphor bronze is used as a conductor material for very long spans such as river crossing. It is
stronger than copper conductor but has got a low conductivity. This conductor is superior to the
aluminium bronze conductor for atmospheres containing harmful gases such as ammonia.
m. Galvanized Steel Conductor
The galvanised steel conductor has high tensile strength. They are used in a very long span and
in a rural area where the load is small. In such cases, the steel conductors may be replaced by a
steel core conductor to deal with the extra future load. This conductor has a large resistance,
inductance and voltage drop. But it has a small life as compared to other conductors.
Classification of cables
Cables can be classified according to their construction
a) Polyvinylchloride cable (PVC) – it is termed as a ‘thermo-plastic’ cable as the insulation is
formed from a synthetic resin which softens when heated.
b) Multi-core cable – it is made up of two or more insulated conductors and is sheathed in a
protective covering i.e. tough rubber and PVC.
c) Tough rubber sheathed (TRS) cable – this is made of specially toughened rubber which is
resistant to acids and alkalis.
d) Polychloroprene (PCP) cable – an insulation somewhat similar to that of TRS but capable of
withstanding most weather conditions and particularly direct sunlight.
e) Heat resisting, Oil-resisting and Flame-retardant (HOFR) cables – they are used in
conditions damaging to PVC cables i.e. high temperature and oil.
f) Flexible cables and flexible cords
i.
Flexible cable – it is a cable consisting of one or more cores, each containing a
group of wires, the diameters of the wires and construction of the cable being such
as to afford flexibility.
ii.
Flexible cord – it is a flexible cable in which the cross-sectional area of each
conductor doesn’t exceed 4mm2.
g) Twisted twin flex – it is made up of a multi-strand tinned-copper conductor with rubber
insulation. Used in lighting.
h) Circular flex – the rubber insulated cores are formed into a circular section with cotton
worming and contained in a cotton braiding. Used in connections to household appliances
(irons, kettles)
i) Circular flex, rubber sheathed – it is packed with jute or cotton to form a circular crosssectional but an outer sheath of rubber replaces the cotton braiding. Used in vacuum cleaners
and portable drill leads (3-core).
j) Workshop (or industrial) flex – it is similar in construction to the above but has the addition
of a compound braiding. Used in connections to industrial lighting.
k) HSOS (House Service Overhead System) cable – it is used in house-to-house overhead
supplies. It is constructed as follows;
 Hard-drawn copper conductor
 Rubber insulation
 Varnished tape
 Outer coating of compounding braiding
Armoured cables
This cable is used where there is a likelihood of mechanical damage to conductors or insulation,
i.e. underground cable runs.
Types
a) Lead-covered paper-insulated steel wire, or steel tape, armoured cables
(P.I.L.C.S.W.A/P.I.L.C.S.T.A). It consists of the following parts:
o An inner ‘heart’ of jute used to keep the cable circular.
o Copper or aluminium conductors insulated with mineral oil-impregnated paper
o A lead sheath which contains the insulation and is also used as an earth continuity
conductor.
o Jute bedding tape impregnated with bitumen, used to protect the lead against
armouring.
o Galvanized steel wire (one layer) or steel tape (two layers).
o Bitumen-impregnated jute serving.
b) PVC armoured cable – it is made up of PVC insulated cores packed PVC to give a circular
cross-sectional area. An outer sheath covers the galvanized steel wire.
Factors Governing the Current Rating
The current rating of a cable or wire indicates the current capacity that the wire or cable can
safely carry continuously. If this limit, or current rating, is exceeded for a length of time, the heat
generated may burn the insulation.
The current rating of a wire is used to determine what size is needed for a given load, or current
drain. The factors that determine the current rating of a wire are the conductor size, the location
of the wire in a circuit, the type of insulation, and the safe current rating.
a) Conductor Size
An increase in the diameter, or cross section, of a wire conductor decreases its resistance and
increases its capacity to carry current. An increase in the specific resistance of a conductor
increases its resistance and decreases its capacity to carry current.
b) Wire Location
The location of a wire in a circuit determines the temperature under which it operates. A wire
may be located in a conduit or laced with other wires in a cable. Because it is confined, the wire
operates at a higher temperature than if it were open to the free air. The higher the temperature
under which a wire is operating, the greater will be its resistance. Its capacity to carry current is
also lowered. Note that, in each case, the resistance of a wire determines its current-carrying
capacity. The greater the resistance, the more power it dissipates in the form of heat energy.
Conductors may also be installed in locations where the ambient (surrounding) temperature is
relatively high.
c) Insulation
The insulation of a wire does not affect the resistance of the wire. Resistance does, however,
determine how much heat is needed to burn the insulation. As current flows through an insulated
conductor, the limit of current that the conductor can withstand depends on how hot the
conductor can get before it burns the insulation. Different types of insulation will burn at
different temperatures. For