Module 3

Domestic Installation (10 hours)

General aspects as per NEC and IS 732 related to the design of domestic dwellings availing
single phase supply (LV) and three phase supply (MV) for a connected load less than
15kW.
Load Survey- common power ratings of domestic gadgets- connected load-diversity factor
selection of number of sub circuits (lighting and power)-selection of MCB distribution
boards to provide over load, short circuit and earth leakage protection.
Principle of operation of MCB, MCB Isolator, ELCB/RCCB and RCBO. Selection of CBs
for protection and grading between major and minor sections.
Selection of wiring cables, conduits as per NEC and IS 732
Design of electrical schematic and physical layout drawings for low and medium class
domestic installation. Preparation of schedule of works and bill of quantities (cost
estimation excluded).
Pre-commissioning tests- Insulation resistance measurement, continuity test, polarity test,
and earth resistance measurement as applicable to domestic installations.
•Domestic dwellings
Residential or domestic dwellings are those buildings intended for
normal residential purpose.
The primary considerations in planning the electrical layout in domestic
dwellings are economy and safety. Besides these, other considerations
such as efficiency and reliability, convenience and provisions for future
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expansion are also valid.
These buildings are classified into
• One or two family dwellings or Stand alone building (occupied by
members of single family not more than 20 persons)
• Apartment buildings/flats (3 or more families)
Electrical system design of domestic buildings
Objective
• Safety and economy
• Energy efficiency
• Reliability
• Convenience
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• Provision for expansion
• Estimation of load requirements
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Calculation of total connected load
•When all the light, fan and socket points are decided, total connected
load is calculated.

•Total kVA is the sum of kVA of all individual loads. Maximum demand

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kVA is not equal to installed kVA.

•To find the maximum kVA demand of the building two factors are
useful:

• Factor of maximum utilisation ku

• Factor of simultaneity ks.
 Utilisation factor (Ku)
Normally power consumed by load is less than the normal rating of
the load. So utilisation factor Ku is used to calculate the estimate of
load.
Std values of Ku are

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• For fluorescent lighting Ku=1
• For water heaters Ku=1
• For industrial loads(motor) Ku=0.75
• For socket outlets Ku=depends on type of apparatus.
 Factor of simultaneity (ks)
•All loads will not be operated simultaneously. So this factor is taken
into account for the purpose of estimating. Simultaneity factor is the
reciprocal of diversity factor.
•For domestic installation, diversity factor is taken as 2 to 2.5.
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Relation between connected load and type of supply

Supply Voltage Max. connected Max. contract

load kW/kVA demand kVA

240V,1 phase 5kW

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415,3 phase 100kVA
11kV 3000
22/33 kV 8000
66kV 8000
Assumptions made in estimation

Height of batten 2.5m above floor level

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Height of switch board

Height of distribution board

1.3m above floor level

1.5m above floor level

Height of brackets for light 2.5m above floor level

points
General requirements of electrical installation
• All electrical installation should be carried out according to India
Electricity Act and regulations of concerned electric supply authority.
• Following steps should be adopted for estimation of electrical
installation.
1) Layout wiring
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• Power and heating subcircuit should be separate and distinct from
lighting and fan subcircuit.
Circuit and Subcircuit
• Electrical apparatus are connected to supply main and to the associated protective and
controlling devices. This arrangement of cables is known as circuit.
• A circuit that feeds apparatus directly is known as a subcircuit. From the distribution board
the supply can be directly taken to the loads as shown in the fig.

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• In the above connection if there is a short circuit in any part of the wiring the fuse will
disconnect the supply to the whole installation. Also If the number of load points is large
,the voltage available at the farthest end will be low due to the voltage drop in the line. So
avoid this situation the wiring is divided into smaller circuits called subcircuit.
• Subcircuits are divided into two groups
• Light and fan subcircuit:- This subcircuit includes fan ,light and 5A
socket outlet points. Maximum points should not exceed ten. The load
on each subcircuit shall not exceed 800 watts.
• Power sub-circuit:-Maximum point should not exceed two. The load

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is restricted to 3000 watts.
2) Conductors
• All conductors should be of copper or aluminium.

• The conductors for final subcircuit wiring should have a nominal cross
sectional area not less than 1 mm2 for copper and 1.5 mm2 if aluminium is
used .

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• For power wiring minimum cross sectional area is 1.5 mm2 for copper
2.5mm2 for aluminium .
3) Rating of fan ,lamp and socket outlet points.
4) Type of connection
Select either looping-in or joint box type connection. Normally looping –in
system connection is preferred.

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5) Single phase supply
• All loads are connected in parallel and supplied at same potential.

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6) Three phase supply

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Diversity factor

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Protective devices used in domestic installations
Device Full Form Purpose
MCB Miniature Circuit Breaker Over current/Short circuit
protection
MCCB Molded Case Circuit Breaker Over current/Short circuit
protection
ELCB

RCCB
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Earth Leakage Circuit Breaker

Residual Current Circuit Breaker

Earth leakage protection

Earth leakage protection

RCD Residual Current Device Earth leakage protection

RCBO Residual-Current circuit Breaker Earth leakage protection +

with Overcurrent protection Over current protection +
Short circuit
Miniature Circuit Breaker (MCB)

A miniature circuit breaker (MCB) automatically switches off electrical circuit during an abnormal
condition of the network such as overload and faulty condition.

Nowadays MCB are used in low voltage electrical network instead of a fuse.

Comparison of fuse and MCB

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• The fuse may not sense it but the miniature circuit breaker does it in a more reliable way.

• MCB is much more sensitive to overcurrent than fuse.

• Handling an MCB is electrically safer than a fuse.

• Quick restoration of supply is possible in case of a fuse as because fuses must be re-wirable or
replaced for restoring the supply. Restoration is easily possible by just switching it ON.
MCB working

• Under normal working conditions, MCB operates as a switch (manual one) to make the circuit
ON or OFF.

• Under overload or short circuit condition, it automatically operates or trips so that current
interruption takes place in the load circuit.

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• The visual indication of this trip can be observed by automatic movement of the operating
knob to OFF position.

• This automatic operation of MCB can be obtained in two ways; they are magnetic tripping
and thermal tripping.

• Under overload condition, the current through the bimetal causes to raise the temperature of
it. This causes the deflection of bimetal
• This deflection releases the trip latch and hence contacts get separated by thermal tripping.

• Under short circuit or heavy overload conditions, magnetic tripping occurs.

• Under normal working condition, the moving contact is held in a position by light spring
because magnetic field generated by the coil is not sufficient to attract the latch.

• When a fault current flows, the magnetic field generated by the coil is sufficient to overcome
the spring force.

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• So the moving contact moves and then actuate the tripping mechanism.

• In both magnetic and thermal tripping operations, an arc is formed when the contacts start
separating. This arc is then forced into arc splitter plates via arc runner.

• These arc splitter plates are also called arc chutes where arc is formed into a series of arcs and
at the same time energy extracted and cools it. Hence this arrangement achieves the arc
extinction.
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• https://youtu.be/oYLIpN4RwEg

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Miniature Circuit Breakers have the following Specifications
• Current rating Amperes (A)
• Short Circuit Rating Kilo Amperes (kA)
• Operating Characteristics B, C , D Curves etc.
Miniature Circuit Breakers are usually available in the range of 0.5 A to 100 A.

An MCB's Short circuit rating is given in Kiloamps (3-10 kA and this indicates the
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level of its ability to work).

For example a domestic MCB would normally have a 6 kA fault level, whereas one
used in an industrial application may need a unit with a 10 kA fault capability.
MCBs are classified according to tripping over range of fault current as
follows:

1.Type B MCB

2.Type C MCB

3.Type D MCB

4. Type K MCB
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5. Type Z MCB
• Type B MCB

This type of MCB trips between 3 and 5 times full load current. Type B
devices are mainly used in residential applications or light commercial
applications. The surge current levels in such cases are relatively low.

• Type C MCB
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This type of MCB trips between 5 and 10 times full load current. This is used
in commercial or industrial type of applications .The connected loads are
mainly inductive in nature (e.g. induction motors).
• Type D MCB

This type of MCB trips between 10 and 20 times full load current. These
MCBs are use in specialty industrial / commercial uses where current inrush
can be very high. Examples include transformers or X-ray machines, large
winding motors etc.

• Type K MCB
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MCB with class K trip characteristics trips instantaneously when the current
flowing through it reaches between 8 to 12 times the rated current. Suitable
for inductive and motor loads with high inrush currents.
• Type Z MCB

MCB with class Z trip characteristics trips instantaneously when the

current flowing through it reaches between 2 to 3 times the rated
current. These type of MCBs are highly sensitive to short circuit and

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are used for the protection of highly
semiconductor devices.
sensitive devices such as
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Type Tripping Current Operating Time

Type B 3 To 5 time full load current 0.04 To 13 Sec

Type C 5 To 10 times full 0.04 To 5 Sec

Type D
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load current
10 To 20 times full 0.04 To 3 Sec
load current
Type K 8 To 12 times full <0.1 Sec
load current
Type Z 2 To 3 times full load <0.1 Sec
current
•Types of MCBs based on poles

No. of Pole for MCB depends on Single Phase & Three

Phase Power

•Single Pole (SP) MCB

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•A single pole MCB provides switching and protection for one single
phase of a circuit. Used for Single Phase circuit
• Double Pole (DP) MCB

A two Pole MCB provides switching and protection both for a phase and the
neutral. Used for Single Phase circuit

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• Triple Pole (TP) MCB

A triple/three phase MCB provides switching and protection only to three

phases of the circuit and not to the neutral. Used for Three Phase circuit.
• 3 Pole with Neutral (TPN (3P+N) MCB)

A TPN MCB, has switching and protection to all three phases of circuit
additionally Neutral is also part of the MCB as a separate pole. However , Neutral
pole is without any protection and can only be switched. Used for Three Phase
circuit with Neutral.

• 4 Pole (4P) MCB

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A 4 pole MCB is similar to TPN but additionally it also has protective release for
the neutral pole. This MCB should be used in cases where there is possibility of
high neutral current flow through the circuit as in cases of an unbalanced circuit.

• Used for Three Phase circuit with Neutral.

MCCB-Molded Case Circuit Breaker

• The main distinction between molded case and miniature circuit breaker are that the MCCB can
have current ratings of upto 2,500 amperes, and its trip settings are normally adjustable.

• The MCCB provides protection against overload, short circuit faults and is also used for switching
the circuits.

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• The wide current ratings and high breaking capacity in MCCB find their use in industrial
applications. MCCB can be used for protection of capacitor bank, generator protection and main
electric feeder distribution.

• The Frame, also known as the molded case, provides an insulated housing to mount all of the
circuit breaker components. This will often be made of a glass-polyester material or thermoset
composite resin that combines ruggedness and high dielectric strength in a compact design.
• Main difference between MCB & MCCB
Sl.
MCB MCCB
No
It stands for Molded Case Circuit
1 It stands for Miniature Circuit Breaker.
Breaker.
2 Rated current not more than 125 Ampere. Rated Current up to 1600A

3
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Its interrupting current rating is under Their interrupting current ranges from
around 10KA -85KA

MCCB is mainly used for both low and

MCB is mainly used for low Breaking
4 high Breaking capacity requirements
capacity requirement mainly domestic.
mainly industrial.
Its trip characteristics are normally not Its trip current may be fixed as well as
5 adjustable since they basically cater to low adjustable for overload and magnetic
circuits. setting.
•ELCB -Earth Leakage Circuit Breaker
•ELCB, an acronym for Earth Leakage Circuit Breaker detects directly
the current leaking to the earth from a load device or installation.
•The main purpose of the device is to prevent damages and injuries due

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to electric shocks. 2 types :

•Voltage operated ELCB

•Current operated ELCB (RCCB / RCD)
Voltage operated ELCB

• Voltage Operate ELCB contains a relay coil or ELCB coil. One

end of the ELCB coil is given connection to the body of the
metal part and the other end to the earth wire. When the
voltage of the load rises, there will be a difference in voltage
between the load and the earth wire resulting in electric shock.
This potential or voltage difference causes a current to flow

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from the load to the ground through the relay coil loop. When
the voltage difference becomes greater than 50 volt, current
through the loop moves the relay and hence disconnect the
supply.

• Voltage ELCBs are not recommended over current ELCB

(RCCB) and no longer available because of old technology.
Current operated ELCB (RCCB /
RCD)
• The circuit consists a phase coil, neutral
coil, search coil wound around the core of
a transformer.

• In normal conditions, same electric current

flows through the phase and neutral. the

such that an
produced.
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neutral and phase coils are wound in a way
opposing magnetic flux is

• Since the current passing is same through

the phase and neutral at normal conditions,
their net magnetic effect and flux cancels
out each other.
• When there is an earth fault, the current flowing through the phase and
neutral will be different. In this case, magnetic flux in the core is not
balanced, i.e., the sum of magnetic flux of phase and neutral coil is not zero.

• Net remaining flux is called as Residual Flux. The residual flux changing
periodically within the core of the transformer crosses the path of the search

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coil and produces an alternating voltage. This voltage will produce the
current essential to trip the circuit breaker.
RCBO( Residual Circuit Breaker with Overload)

• RCDs or RCCB detect an imbalance in the live and neutral currents. A current
overload, however large, cannot be detected. RCDs don’t offer protection
against current overloads.

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• It is now possible to get an MCB and RCD in a single unit, called
RCBO.RCBOs are commonly used in applications where there is the need to
an

combine protection against overcurrent (overload and short-circuit) and

protection against earth leakage currents. They can generally be fitted into a
Consumer Unit in place of an MCB. They tend to be quite expensive.
Cross Section of Conductors

The cross section of conductors shall be determined according to:

• their admissible maximum temperature
• the admissible voltage drop

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• the electromechanical stresses likely to occur due to short circuits
• the maximum impedance with respect to the functioning of the
short-circuit protection.
Type of Wiring and methods of Installation
The choice of the type of wiring and the methods of installation depend on:

a) the nature of the locations

b) the nature of the walls or other parts of the building supporting the wiring

c) accessibility of wiring to persons and live; stock

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e) the electromechanical stresses likely to occur due to short circuits, and

f) other stresses to which the wiring may be exposed during the erection of the electrical
installation or in service.
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• A typical range of single phase RCBO – 6A, 10A, 16A, 20A, 25A, 32A,
40A, 63A.
• A typical range of Three phase RCBO – 16A, 25A, 32A, 40A, 63A.

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A three occupant building has to be electrified independently from a
common energy meter. Design the distribution boards with accessories
for each resident having 10nos of light circuits, 6 no.s of power circuits.

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• Load for each residents
• Total circuits = 10 nos = 10*800watts/circuit = 8000 watts
• Total no. of power circuits = 6 nos. = 6*3000 watts/circuit = 18000 watts
• Total load estimated as per given number of circuits = 26kWatts.
• Select 3 phase supply

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• Accessories required per resident
Incoming:
Outgoing:
 Fig. shows the plan of a small flat. The position of light and fan points
and switch board have been shown.
a) Decide the no.of subcircuits and show these in the installation plan.
b) Calculate the size and length of the wire required for wiring.
c) Estimate the quantity of material.

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Design an electrical schematic for the residential building with
following details. Locate the positions of meter board, Main Switch
board, DB, switch boards.

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A single storied residential building with two bedrooms with attached toilets, one
dining room, one living room, kitchen, and sit out. Decide the required number of light
points, fan points, 5A socket outlet, 15A socket outlet. Decide the number of sub
circuits required. Determine the connected load, type of supply required, sub circuits
required, maximum demand.

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• The plan layout of a two bed room domestic building is shown in
figure. Locate the light, fan, socket points etc., required for the
electrification of the building as per NEC requirements. Calculate (a)
Connected load of the building (b) Maximum demand in kW (c) Type
of supply required (d) Number of light and power circuits (e) Details
of the distribution board

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A rest house has four air-conditioned bed rooms with attached toilets,
dining hall and kitchen. Prepare the room wise list of electrical
materials for the installation. Draw the schematic diagram showing the
ratings of MCBs and sub circuits. Design is based on the NEC guide
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lines. Assume all required data.
Precommissioning Test
• It is a necessary prerequisites under statutory provisions. This is done in
accordance with IS-732.
It ensures
• that the installations satisfies all the safety regulations.
• Good material and workmanship have gone into the work
• Installation is good in working condition

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• Faults if any in the installation are located and remedial measures are suggested
before energising.
Precommissioning test includes:-
• Visual Inspection:-This is necessary to verify the installation is
In compliance with Indian standard specifications
Correctly erected in accordance with the relevant code
No visual damage is seen outside.
Testing :- testing is done in the following sequence

1. Continuity of the final circuit conductors. :- It is carried out in all final

circuits to make sure that there is no discontinuity in the circuit. The
most common and basic way of performing a continuity test is with the

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help of a multimeter. Connect one lead to phase wire and other to
neutral wire. The device will give a beep when there is continuity.

2. Continuity of protective conductors

All protective conductors should be tested separately to make sure that
they are electrically sound and are correctly connected.
3. Insulation resistance

It is measured between the live conductors and earth and between live
conductors. For larger installation this is done by dividing the
installation into smaller units, each containing not less than 50 outlets.
The test voltage applied for insulation measurement is

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For voltages upto 250V 500 V

250 to 500 V 1000V

Above 500 V 2500V

The following steps are involved in the
measurement of insulation between live
conductors and earth.
• Main switch is in OFF position.
• All fuses, MCBs are ON
• All lamps and other accessories are in
place.
• Phase and neutral conductor are
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connected together near the main switch.
• The insulation resistance between
conductor and earth is measured using a
megohm meter of appropriate voltage
class.
• Measured value shall not be less than 1
Megaohm.
The following steps are involved in the
measurement of insulation resistance
between conductors
Steps involved are
1) The main switch is placed in the OFF
position
2) The phase and neutral conductors are
not connected together.
3) All switches are placed in the ON
position.
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4) All lamps and accessories are
removed.
5) All fuses/MCBs beyond the main
switch on the consumer side are ON.
6) The insulation resistance between any
two phases are recorded using a
megaohm meter.
Measured value shall not be less than 1
Megaohm.
3. Polarity Test
This test will verify that all the switches
installed in the system are connected in
current carrying conductor and not in
neutral.
Steps involved in Polarity Test are
• All equipment are disconnected.
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• All switch covers are open.
• Use a test lamp with long leads.
• Touch one end of the lead to the neutral
point.
• Touch the other lead to the input
terminal of the switch. If the lamp burns
,the polarity is correct.
4. Earth electrode resistance test
• Earth electrode–It is the conductive part which may be embedded in the
soil.
• Earth electrode resistance–It is the resistance of an earth electrode to
Earth.
• The purpose of this test is to establish that the resistance of the soil
surrounding an earth electrode is suitable and that the electrode makes
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contact with the soil.
• Two auxiliary earth electrodes, besides the test electrode, are placed at
suitable distance from the test earth electrode (see Fig). A measured
current is passed between the earth electrode to be tested and an
auxiliary current electrode ‘C’, and the potential difference between the
electrode earth electrodec and auxiliary electrode ‘P’ is measured.
Measured voltage and current values are used to calculate the electrode
resistance.
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