Some of the frequently asked viva questions on electrical engineering are as follows:
Q. 1. What are the laws of resistance?
Ans. The resistance of a conductor must obey the following laws:
(i) The resistance of a conductor is directly proportional to its length, when its cross-section remains constant.
ADVERTISEMENTS:
If a conductor has a resistance R, length l and cross-section A, then,
R ∝
l, when A remains constant.
(ii) The resistance of a conductor is inversely proportional to its cross-sectional area, when its length remains constant. Thus,
R ∝ l/A, when l remains constant.
ADVERTISEMENTS:
A From laws (i) & (ii) above R ∝ l/A, when both l and A vary.
... R = ρ l/A
where ρ is a constant, called Specific Resistance or Resistivity of the material of the conductor, ρ depends on material and temperature of the conductor.
If R is expressed in ohm, l in metre and A in square metre, ρ is expressed in ohm-metre.
ADVERTISEMENTS:
Again, when l = 1 metre and A = 1 square metre.
ρ = R
Thus, the specific resistance or resistivity of a material may also be defined as the resistance of a piece one metre long and one square metre in cross- section, i.e. it is the resistance between opposite faces of a metre cube (figure 3) of the material. Sometimes centimetre cube is used in place of metre cube.
In that case ρ is expressed in ohm-centimetre (Ω-cm) or ohm per cubic centimetre or micro ohm-centimetre (μΩ- cm) or micro ohm per cubic centimetre. Table 4 shows the values of specific resistance of different materials in μΩ-cm (1 micro ohm = 10-6 ohm).
Ex:
A Copper wire of length 200 metres has cross-sectional area of 0.5 cm2. If the specific resistance of copper is 1.72 μΩ-cm, what is the resistance of the wire?
Solution:
ADVERTISEMENTS:
Here, l = 200 metres = 2×104 cm,
A = 0.5 cm2, and
P = 1.72 μΩ – cm = 1.72 x 10-6 x (2 x 104/0.5) = 0.06888 ohm
Q. 2. What is meant by resistance-temperature coefficient?
Ans. The resistance of all metals increases with the rise in temperature and decreases when the temperature falls. If a resistance has a value of R0 at 0°C, at 1°C it will increase by a small amount x. The ratio x/R0 is called the Resistance-Temperature Coefficient of the metal. It is usually denoted by a (alpha, a Greek alphabet).
α = x/R0 or x = R0α
Thus, α may be defined as the fraction of the resistance at 0oC by which the resistance increases for 1oC rise in temperature, α depends on material and temperature of a conductor. For copper it is equal to 0.00428 per oc at 0oc.
Now, increase in resistance for 1oc rise in temperature = x ohm. Therefore increase in resistance for toc rise in temperature = xt ohm.
Again, resistance at:
toc = resistance at 0°c + increase in resistance
or Rt = R0 + xt = R0 + R0αt (... x = R0α)
= R0 (1 + αt) ohm.
α varies with temperature. If the resistance-temperature coefficient of a metal is α0 at 0°c and αt at t0c, then,
αt = α0/1 + α0t ohm.
The relation between resistances at temperatures t1°c and t2°c may be given by-
Rt2 = Rt1 {1 + αt1 (t2 – t1)}
In case of carbon, insulators and electrolytes which have negative temperature coefficient-
Rt = R0 (1 – α0t)
Q. 3. What is energy?
Ans. Energy is the capacity for doing work. In other words, the amount of work done within a particular time is the energy.
Energy = power x time.
Obviously, the unit of energy depends upon the units of power and time.
There are various forms of energy, such as mechanical energy, electrical energy, heat energy, chemical energy, etc. According to law of conservation of energy, energy can never be created nor destroyed; only it can be transformed from one form into the other.
The unit of electrical energy is watt-second or joule which means a power of one watt is consumed in the circuit for one second. If a p.d. of V-volt is applied across a circuit having a resistance of R-ohm and carrying a current of I-ampere continuously for t- second.
Energy expended in the circuit = VIt joule
= I2Rt joule
A larger unit of energy is the watt-hour. A watt-hour is the amount of work done or the amount of energy transformed between two points in a circuit when the potential difference between them is 1 volt and 1 ampere of current flows continuously between them for 1 hour.
A still larger unit is kilowatt-hour (kwh), also called the Board of Trade Unit. The Board of Trade Unit or B.O.T. Unit is one Kilowatt-hour and is the amount of electrical energy when it is consumed in one hour at a power of one kilowatt. It may also be regarded as the amount of energy consumed in half-an-hour at a power of 2 KW or in two hours at a power of 500 watts, and so on, as long as the product of the number of hours of consumption and the number of kilowatts of power is unity.
The electric supply concerns usually sell their electrical energy to consumers at a certain rate per kilowatt-hour, and hence it is called Board of Trade Unit.
Q. 4. Explain joule’s law of electric heating?
Ans. Joule found that there exists a definite relation between the electrical energy expended and the amount of heat produced. This relation is known as ‘Joule’s Law’ of electric heating.
Joule’s law states that the amount of heat produced 1s directly proportional to the electrical energy expended, i.e.
H ∝ I2Rt,
where H is the amount of heat generated for I amperes flowing continuously for t seconds through a resistance of R ohms.
where J is a constant, known as ‘Joule’s Constant’. By experiment it has been found that-
J = 4.18 joules per calorie
Q. 5. When does power loss in the conductor takes place?
Ans. The resistance of a conductor opposes the flow of current through it. To overcome this opposition certain amount of supply pressure is dropped which is known as ‘voltage drop’. In addition to voltage drop the resistance also causes loss of power in the form of heating the conductor. This loss of power is directly proportional to square of the current flowing through the conductor and the resistance of the conductor.
Thus, if a current of I ampere flows through a conductor having a resistance of R ohm, then (according to Joule’s Law),
power loss = I2R watt.
If the current flows continuously for T hours,
loss of energy = I2RT watt-hour
Q. 6. What is the meaning of alternating current?
Ans. An alternating quantity, either current or e.m.f., is one which periodically passes through a definite cycle of changes. Each of these cycles consists of two half-cycles. During one half-cycle the quantity acts in one direction around the circuit and during the other in the opposite direction.
Thus, an alternating current is that current which flows first in one direction in a circuit, called the positive direction, then in the reverse or negative direction, repeating this cycle of changes continuously, and has an average value zero over a period.
When a conductor moves in a magnetic field and thereby cuts the flux, an e.m.f. is induced in the conductor. The magnitude of this induced e.m.f. is given by-
e = Blv sin θ volt,
where B = flux density of the magnetic field in weber per square metre,
l = effective length (length linking with flux) of the conductor in metre,
v = velocity of the conductor in metre per second, and
θ = angle between direction of the magnetic field and the direction of motion of the conductor.
Thus, the alternating e.m.f. and hence the current can be represented by a sine curve. Such an e.m.f. is called sinusoidal e.m.f. or simple harmonic e.m.f. The e.m.f. waves of some alternators (specially the older ones) may deviate from a sine wave considerably, but the designers of commercial alternators always endeavour to obtain a wave form as closely as possible to that of a sine wave. Hence, alternating current theory and analysis are based on sine waves of voltage, current and power. The sine wave has the advantage that it greatly simplifies the theory and calculations of a.c. circuits.
Q. 7. Explain cycle, time period and frequency.
Ans. When an alternating quantity passes through one complete set of positive values and one complete set of negative values, it is said to have completed one cycle.
The time taken by an alternating quantity to complete one cycle is called its time period or periodic time. It is generally denoted by T and is expressed in second.
Frequency is the number of cycles completed in one second time. It is usually denoted by f and is expressed in hertz (i.e. cycles per second).
In our country electricity is supplied usually at a frequency of 50 hertz, i.e., the current flowing through the supply line completes 50 cycles in one second time.
Q. 8. Explain root mean square (R.M.S.) value or effective value or virtual value of alternating quantity.
Ans. An alternating current changes in magnitude from instant to instant. At first it might seem the value in ampere of such a current should be based on the average value; but taken over a complete cycle the average or mean value is zero, since there is just as much positive value as the negative value.
The logical way of measuring alternating current is by the amount of work it will do or the amount of heat it will produce. An alternating current ampere is that current which, when flowing through a given ohmic resistance for a given time, produces the same amount of heat as direct current ampere.
Heat produced by alternating current flowing through a resistance of R ohm in time t second = mean value of i2 x Rt joule,
where i indicates the instantaneous values of current. If the average or mean value of squares of all the instantaneous values of current over a complete cycle is determined, it will be the mean value of i2.
Let I be the d.c. ampere which, flowing through the same resistance, produces the same amount of heat during the same time.
Then,
This is called the Root Mean Square Value or Effective Value or Virtual Value of alternating current.
Similarly, R.M.S. value of alternating e.m.f.-
These are the values used in practice and registered by a.c. ammeters and a.c. voltmeters.
Q. 9. Explain average or mean value of alternating current and alternating E.M.F.
Ans. Taken over a complete cycle the mean or average value of sinusoidal current or e.m.f. is zero. Hence, for determining the average value, only one half cycle is taken into consideration. The average height of this one half cycle gives the mean value of current. It has been found that average height of one half cycle is equal to 0.636 times the maximum height of the current wave. Therefore, average or mean value of sinusoidal current.
Iav = 2/π Im = 2/3.14 Im
= 0.636 Im
and average or mean value of alternating e.m.f.-
Eav = 0.636 Em.
Q. 10. Explain practical importance of mean or average value of alternating current.
Ans. Alternating current flows as much in one direction during one half cycles as flows in the opposite direction during the next half cycle. For this reason alternating current cannot be used for electro-chemical work, such as battery charging, electro-plating etc. But if this current is rectified, a fluctuating but unidirectional current is obtained, the effective value of which is numerically equal to average height of the rectified wave.
It is in this connection that the average or mean value of a.c. has any practical importance.
Q. 11. What is form factor?
Ans. Form factor is the ratio R.M.S. value/Average value. For an alternating current of sine wave form,
form factor = (0.707 x maximum value)/(0.636 x maximum value) = 0.707/0.636 = 1.11
Q. 12. What is peak factor or crest factor?
Ans. Peak factor or crest factor is the ratio Maximum value/Average Value. For an alternating current of sine wave form,
peak factor or crest factor = maximum value/(0.707 x maximum value) = 1/0.707
= 1.414.
Q. 13. What is phase angle or angle of phase difference?
Ans. The angle between the directions of two vectors is called phase angle or angle of phase difference between those two vectors.
In general, however, angle of phase difference of an a.c. circuit is the angular distance between the applied voltage and the current flowing through the circuit. The angular distance is measured in degree or in radian. If the current lags behind the applied voltage, the phase angle is called a lagging angle; when the current leads the applied voltage, the phase angle is called a leading angle.
Q. 14. What are artificial magnets?
Ans. Natural magnets are rugged and clumsy and are comparatively weak. These are, therefore, unsuitable for scientific and technological work. To execute useful work, magnets are made artificially.
These artificial magnets may be classified into two types:
a. Permanent magnets and
b. Electro-magnets.
An iron piece may be magnetised by repeatedly rubbing a magnet over its entire length in the same direction. If the piece is made of soft iron, it cannot retain its magnetic properties for a long time. But a steel piece, once magnetised, retains its magnetism permanently.
Hence, permanent magnets are usually made from steel which is in general much harder than soft iron. Permanent magnets are made in three different forms—magnetic needle, bar magnet and horse-shoe magnet.
If an insulated wire is wound on a soft iron piece and electric current is passed through the wire, the iron piece becomes a temporary magnet. It is because the iron piece will retain magnetism so long as the current is flowing. Such a magnet is called an Electro-magnet.
If the current stops flowing, the iron piece will lose its magnetism. But if a bar of steel is wound with an insulated wire and current is passed through that wire, the bar becomes a permanent magnet. It will not lose its magnetism totally even when the flow of current is stopped.
Initially the magnetic strength of a piece of iron or steel increases very rapidly with the increase of current flowing through the coil, then increases slowly and finally the strength remains virtually constant, although the current is increased as before. At this stage the magnet is said to be saturated.
Q. 15. What is meant by magnetic materials?
Ans. The materials which are appreciably attracted by a magnet are called magnetic materials. Iron, steel and some of their alloys are the best among all magnetic materials. Cobalt, nickel and some of their alloys have certain magnetic properties, but these are far inferior to iron and steel.
Other materials like glass, paper, wood, water, oil etc. are not attracted by a magnet. Again, metals like copper, aluminium, manganese etc. have no individual magnetic property, but when they are mixed together in certain proportion, the alloy possesses magnetic properties.
The power of attraction or repulsion of a magnet can be felt even through a non-magnetic substance, i.e. the lines of force produced by a magnet can pass through any medium. For example, if a magnet is placed on one side of a paper and an iron piece on the other side, the magnet will attract the iron piece through the paper which is a non-magnetic substance.
Q. 16. What are magnetic poles?
Ans. The strongest parts of a magnet lie near its two ends. These are called magnetic poles—one North Pole and the other South Pole.
When a magnet is freely suspended, one end always points towards Earth’s north and the other end towards south. The end which points towards north is called North Pole and the end pointing towards south is called South Pole.
The polarities of a magnet have the following properties:
(i) Both the poles of a magnet have equal strength.
(ii) Similar poles repel each other and dissimilar poles attract each other.
(iii) The force of attraction or repulsion between two magnetic poles is directly proportional to the product of their strengths and inversely proportional to the square of the distance between them.
Q. 17. How to determine polarities of a magnet?
Ans. When a bar magnet is suspended horizontally at its centre of gravity from a wooden stand, it always sets itself approximately along north-south direction. The end of the magnet pointing towards north is called North-Seeking Pole or North Pole, and the other end pointing towards south is called South-Seeking Pole or South Pole. The magnet may be rotated in different directions, but when it comes to rest, its north pole always points towards north and South Pole towards south.
Again, suspend a bar magnet horizontally at its centre of gravity. Bring the north pole of another bar magnet slowly near a particular end of the suspended magnet. If the suspended magnet moves away indicating a repulsion, that particular end is a north pole; but if the suspended magnet turns towards the other magnet indicating an attraction, the particular end is south pole.
In the same way the polarities of a bar magnet can be determined with the help of a magnetic needle or compass needle. A particular end of bar magnet is brought slowly from a distance near the north pole of the needle. If the needle moves away indicating a repulsion, the particular end of the bar magnet is north pole. But if the needle turns towards the bar magnet, thereby indicating attraction, the particular end is South Pole.
Caution must be taken when the experiment is conducted with a magnetic needle. The end of a bar magnet (specially when the magnet is powerful) should always be brought slowly from a distance near the end of the needle. Otherwise the bar magnet may induce opposite polarity at the near end of the needle suppressing its original polarity. As a result opposite phenomenon will take place when the end of one is brought near the end of the other.
Q. 18. What is meant by magnetic induction?
Ans. If a piece of soft iron is dipped into iron filings, it will not attract any filing. But if one end of a bar magnet is brought near or in contact with the soft iron piece, at once the iron filings will cling to it. This shows that when a magnetic substance is placed in the neighbourhood of or in contact with a magnet, it acquires magnetism. This phenomenon is known as magnetic induction.
It is a temporary phenomenon, since when the bar magnet is removed, the iron filings fall off from the soft iron. In this case the magnetic substance (i.e. the soft iron) is said to be magnetised by induction, the magnetisim developed in it is called induced magnetism and the magnet responsible for the induced magnetism (i.e. the bar magnet) is called the inducing magnet.
Magnetic induction can take place unopposed through non-magnetic substances but not through magnetic substances. This can be easily proved by placing a sheet of fibre or glass between the bar magnet and the piece of soft iron. The iron filings still cling to the soft iron. But if a sheet of iron or steel is interposed, the iron filings will fall off.
Q. 19. Explain terrestrial magnetism or the earth’s magnetism.
Ans. Wherever may be placed, a magnetic needle or a compass needle always sets itself approximately along north-south direction. This leads to the conclusion that it must be under the influence of a magnetic field surrounding the earth. Other evidence is that iron beams lying for a long time in a north-south direction become, slightly magnetised, the end pointing towards north acquiring north polarity and the other end acquiring south polarity.
If a closed coil of any conductor is rotated in a magnetic field, electric current is generated in the coil. It has been found that, if the coil is rotated at any place on the earth, a flow of current through it is produced. This also proves that there exist a magnetic field surrounding the earth.
Q. 20. Explain absolute permeability and relative permeability.
Ans. The ability of a material to conduct magnetic flux through it is known as magnetic permeability of that material. It is generally denoted by Greek alphabet μ (mu). Iron and steel are very good conductors of magnetic flux. If a piece of soft iron is placed in a magnetic field, most of the lines of force will pass through it. This proves that soft iron has. high permeability. But non-magnetic materials like air, glass, paper etc. are poor conductors of magnetic flux.
They all have low value of permeability:
i. Relative Permeability:
The relative permeability of a material is the ratio of the flux existing in the material to the flux which would exist in the same space if the material were replaced by vacuum or air, the m.m.f. acting on the space remaining unchanged. It is generally denoted by μr. Thus,
μr for a material = (no. of lines of force passing through the material / no. of lines of force passing through air or vacuum for the same space and for the same m.m.f. acting in the magnetic circuit).
The relative permeability is a pure number and therefore has no unit. For air and other non-magnetic materials, its value is unity.
It is, therefore, evident that material having higher relative permeability is a better magnetic substance.
ii. Absolute Permeability:
The absolute permeability (μa or μ) of a material is the product of permeability of free space (μ0) and the relative permeability (μr) of the material. Thus,
μa or μ = μ0 x μr
In S.I. unit the value of no is taken to be 4π x 10-7 henry per metre.
Q. 21. What is retentivity?
Ans. If a piece of soft iron is magnetised by induction and later the inducing magnet is removed, it will lose its induced magnetism almost immediately. But in case of hard steel magnetism will continue for some time. This property of hard steel is known as retentivity.
The magnetism left in a magnetic substance after the inducing magnet is removed is known as Residual Magnetism.
Q. 22. What is coercivity?
Ans. The power to retain residual magnetism in a magnetic substance is known as coercivity of that substance.
The residual magnetism in soft iron can be destroyed easily, but that in hard steel cannot be destroyed so easily. Therefore hard steel has more coercivity than soft iron. The material selected for permanent magnet must have high retentivity as well as high coercivity.
Q. 23. What is magnetic circuit?
Ans. Magnetic Circuit is the path followed by magnetic lines of force. Every line of force follows a complete loop or a closed circuit, coming back to its starting point. In any magnet the lines of force leave its north pole, passing through air or other surrounding media, enter the magnet at its south pole, and finally reach the point from where they start through the magnet itself. Thus, the magnetic flux forms a closed circuit exactly as electric current does in an electric circuit.
Q. 24. What is meant by magneto motive force (M.M.F.)?
Ans. Magneto motive force or M.M.F. is the force which drives or tends to drive magnetic flux through a magnetic circuit. It corresponds to electromotive force or E.M.F. causing or tending to cause electric current to flow through an electric circuit.
M.M.F. is produced when electric current flows through a coil of number of turns. It is measured in ampere-turns which is the product of number of turns of the coil and the current in ampere flowing through the coil.
If I be the current in ampere flowing through a coil of N turns, then,
M.M.F. = NI ampere-turns.
Q. 25. Explain the reason of leakage of current into earth, its effects and remedy.
Ans. The current leaking through Insulation into earth of an electric appliance operated at a voltage higher than extra low voltage is called Earth Leakage Current or in brief Leakage Current. Sometimes this leakage current may be so high that it becomes a source of danger.
In order to prevent such high leakage of current, one of the four methods mentioned below is to be adopted:
(i) The outer insulating covering of wires and other electrical appliances must be strong and continuous.
[Permanently installed appliances should be placed within a box or within some other cover in such a way that nobody can touch it or it does not come in contact with bare metal. Such appliances are deemed to be ‘totally insulated’]
(ii) Double insulation should be applied on the appliances. When a layer of insulation is applied between conductor and its cover and another layer of insulation between cover and earth, it is known as ‘double insulation’. Again, in case of portable appliances if the outer insulation applied on all metallic parts is protected by some other arrangement, it is also deemed to be double insulation.
(iii) The bare metallic parts should be properly earthed.
(iv) The metallic parts should be separated from current-carrying parts or earthed conductors in such a way that they cannot come in contact with each other.
Q. 26. What is earthing?
Ans. To connect any wire, outer covering of wire, outer covering of electrical machines and other appliances to the general mass of earth by wire of negligible resistance is known as Earthing. Metal conduit, outer cover of lead-covered wiring system, table lamp fitted with metallic tube, electric iron, electric heater and other appliances, metallic frame of generator, motor and other electric machines must be properly connected to earth.
Earthing does not mean that the earth lead is simply touched or somehow connected to earth. It means earthing of an installation in such a way that the earth lead is in constant touch with wet soil and the leakage current is always discharged to earth without causing danger.
According to Indian Electricity Rules the earthed or connected with earth means connected with the general mass of the earth in such manner as to ensure at all times an immediate discharge of energy without danger. Earth lead may be connected with a water pipe or an iron beam grouted deep into the earth if it is available nearby.
But usually the house wiring and specially the outer frames of machines and other electrical appliances are earthed with earth lead connected to separate earth electrode. Again, if the supply pressure exceeds 125 volts, according to Indian Electricity Rule No. 61(1) the outer frame of each machine and apparatus should be connected to earth by two separate and distinct earth connections.
Where supply pressure is only 230-volt, the earth lead of the wiring or installation may be connected with the earth terminal provided by the supplier. In case no earth terminal is provided by the supplier, a separate earthing arrangement is to be made by the consumer. But in case of 3-phase a.c. supply, one earth lead of the wiring is to be connected with the earth terminal provided by the supplier and another earth lead of the same wiring is to be connected with the separate earthing arrangement made by the consumer himself.
Q. 27. Why an electric installation is earthed?
Ans. If the metal parts of an electric machine, appliance or equipment of an electric installation come in direct contact with bare conductors or current-carrying wires, these metal parts become electrically charged and an inadvertent contact with these parts will give shock to the operating personnel or to persons using it.
It is therefore essential from safety point of view to provide arrangement so that the charge of these metal parts may be discharged direct to earth without causing any danger. The safety is ensured only when the metallic frames of machines and apparatus are efficiently connected to earth by means of proper size of earthing conductor.
If the resistance of the earth lead and the earth electrode is negligible, a large amount of current flows towards the earth as soon as the metal parts come in contact with current-carrying wires. When this current exceeds pre-determined value, the fuse-wire of the circuit melts or the circuit breaker trips, and the electric installation is immediately disconnected from the supply line. This is the reason for which efficient earthing of an electric installation is considered to be the best arrangement for safety afforded to consumers.
Q. 28. What is main earthing lead?
Ans. The wire used to connect consumer’s earth terminal or the coil of the earth leakage circuit-breaker or the outer frame of electrical machines and appliances with earth plate or earth electrode is called Earth Lead. In Indian Electricity Rules only copper wire has been mentioned as earth wire, although in almost all places galvanized iron wire is being extensively used as earth wire.
The significance of Indian Electricity Rules is to be noted here. According to these rules use of galvanised iron wire as earth wire is not prohibited, but only the copper wire has been mentioned as earth lead.
In many earthing system often wrong size of earth wire is selected. One must be cautious about this. In many cases almost arbitrarily No. 8, 6, or 4 S.W.G. galvanized iron wire is selected as earth lead. This should not be the practice. The practice is to follow the method mentioned below for selecting proper size of earth wire.
Galvanized iron wire is used as earth wire with galvanized iron earth plate or galvanized iron earth electrode, while copper wire is used with copper earth plate or copper earth electrode. The size of copper earth wire should not be less than 0.02 square inch (0.129 square centimetre) or greater than 0.1 square inch (0.645 square centimetre).
In general size of a copper earth wire must not be less than one half of that of the largest size of wire used in house wiring. If galvanized iron earth wire is used, its size should be such that its current-carrying capacity is the same as that of copper earth wire.
If copper wire is used as earth lead, its size should be one half of that of the largest size of copper wire used for house wiring. If galvanized iron wire is used as earth lead, its size should be the same as that of largest wire used in the wiring system. When aluminium wire is used for wiring system, the size of earth wire should be selected with respect to copper wire equivalent to largest aluminium wire.
Metallic conduit pipe is not normally used as earth lead. If at all such a pipe is to be used, the workmanship must be perfect. Where one pipe ends and another pipe begins or where the end of a pipe enters into a machine or a switch or a starter or a junction box, the end of pipe must be properly threaded and the two pipes or the pipe and the outer frame of machine or other appliance are so joined together with lock nuts or jam nuts that the current-carrying capacity of such joint is not less than that of pipe itself.
Q. 29. Explain the arrangement for protection of earth lead.
Ans. In order to protect an earth lead from external injury, it may run through a channel cut in the wall or in the floor. The portion of this wire placed in the soil must remain at least 60 cm below the ground level and must be drawn through a galvanized iron pipe of diameter 12.7 mm up to earth electrode. For the protection from Injury sometimes wire armoring is used over the earth lead. Such a lead with armoring has been shown in fig. 174.
An earth lead should be connected with an earth electrode or an earth plate at a point which lies within easy reach, and the joint is made either by soldering or a by a clamp made of material other than iron. If a cable sheath or a cable armoring or both are to be connected with an earth wire by means of a clamp, care should be taken to see that the clamp does not damage the cable in any way and the connection is also perfect.
Wherever-a circular wire is used for earth connection, a suitable socket is to be soldered at its end, the size of the socket being such that all the strands of a stranded earth wire can be inserted into the socket.
Q. 30. State the points to be specially observed in a house wiring for the sake of safety?
Ans. According to rules laid down in “Indian Standard Code of Practice”:
1. Everywhere power circuits for motors, heaters etc. should be kept separate from lighting circuits of lights and fans.
2. At the point of entry of low voltage supply line in a house, a double-pole switch must be provided with only one fuse in the live line. Throughout the house wiring single-pole switches or fuses should not be so connected as to disrupt the continuity of neutral wire. Moreover, the neutral wire must bear a clear and self-evident mark.
3. Main switch, fuses etc. should be within easy reach and should be as near the supply mains as possible.
4. In the main switchboard there must be an earthed wire in case of two-wire supply. If, however, the supply is three-wire d.c. or four-wire a.c., one of the wires must be an earthed neutral or it should remain connected with earthed neutral. Besides, this wise should bear such a permanent mark that one can easily recognize it to be an earthed neutral.
Q. 31. Explain the points to be kept in mind while installing a switchboard?
Ans. 1. Main switch may be either metal clad (iron clad) or insulated and of enclosed pattern. It should be installed near the point of entry of the supply line in a house.
2. A bare or partly covered switchboard may be installed only in dry and well-ventilated place where no fume arises due to acid or other chemicals or where no storage battery has been installed nearby. If, however, the place is damp or full of combustible particles or where there is continuous presence of gas or vapour, the switchboard to be installed must be totally enclosed type and made of flame-proof material.
3. In case the front side of the switchboard cannot be totally closed by a hinged cover or if it is not installed in a place where entry is restricted only to authorised persons, the bottom of the switchboard must remain at a height of at least 1.25 metres (4 feet) above-the floor level.
4. A switchboard must not be installed in a kitchen, lavatory or bathroom or within 2.5 metres (8 feet) from a gas stove, washing basin, sink or laundering machine (in a laundry).
5. If a switchboard is required to be installed in an open space where water drips or where space is very much humid, its outer cover must not be affected by the surrounding environment. Moreover, this type of outer cover must be fitted with screwed conduit, gland or bush.
Q. 32. How a supply line enters into premises?
Ans. The service main of the suppliers, after entering the premises, first goes to the Suppliers cut-out and from there to the supply meter after which the suppliers leave the lead wires for the consumer to start his wiring. The system of conductors from the distribution lines on the street up to consumer’s terminals is called “service cable” or “service main”.
The consumer’s wiring starts after the meter. The two, three or four cable leads left by the supplier after his energy meter first go to consumers linked S.P. & N or T.P. or T.P. & N. iron clad switch and from there the lines go to consumer’s main fuse or fuses. These fuses can be inserted in the live wires or phase wires only and never in the earthed neutral.
From the main fuse the cable comes to main distribution board. But at the very beginning of Wiling, the cable leads left by the supplier are not connected to the consumer’s main switch; — this connection is made at the last stage. Starting from farthest point of the load and after completing the wiring of different load points, finally the wiring is finished at the main distribution board.
The supply co.’s personnel point out a place near the meter box where the main distribution board is to be installed. When the entire wiring is finished, distribution board is installed. At last, after satisfactory tests of insulation resistances of the wiring, supply company’s personnel themselves connect the supply line to the main switch of the consumer.
Q. 33. What is the use of distribution board?
Ans. In old wiring installation it is found that a double-pole switch and two fuse cut-outs are fixed on a wooden board. Above the cut-outs main distribution board has been placed with the required number of grip fuses fixed inside a wooden box with glass cover. Behind the fuses there are two heavy copper bars (or strips), called bus-bars. With one of the bus-bars is connected the live wire, while with the other bar neutral wire is connected.
Fig. 234 shows that consumer’s area begins from a double-pole switch after which only a single main fuse has been provided in the live line. Immediately after this there is main distribution fuse board. The wires from main fuse and link are connected to two bars. These are the two bus-bars.
A number of fuses remain fixed with the live bus-bar. Two wires, one from live bus-bar through a fuse and the corresponding wire from the neutral bus-bar, make up a circuit (e.g. 1-1; 2-2; 3-3; etc.). Current flows to a load circuit from live bus-bar through a suitable fuse and returns to neutral bus-bar through neutral wire. In this manner different circuits are supplied with current from the distribution board.
If the house is a small one, a single distribution board is sufficient. Each circuit of this board supplies current to lamps, fans etc. at one portion of the house. But in case of a large building, one distribution board (D.B.) will not do. In that case there must be one main distribution board. From this main D.B. one (or more if number of load points is large) circuit is drawn up to a separate board at different portion or floor of the house.
The separate board is known as sub-distribution board or fuse board. Use of sub-distribution board has been shown in Fig. 235. In this figure it will be seen that current at first comes from the consumer’s main fuse to main D.B. and thence to no. 1 sub-distribution board through no. 1 fuse, no. 2 sub-distribution board through no. 2 fuse and so on.
Now, if the house is a two-storeyed one, apart from main distribution board, ground floor must have one sub-distribution board also. In this way each floor must have a separate sub-distribution board. For a very large building where a single floor has large number load points, more than one sub-distribution board may be provided for that floor. Main distribution board is usually installed at a place in the ground floor, and the sub-distribution board of that floor is installed near the main distribution board. Number of circuit or ‘way’ provided in a main D.B. is usually one more than the number of sub-distribution board installed in a house.
In an earthed system each fuse is considered to be one way. One extra way is provided in main D.B. for provision of future expansion when it may be necessary to install one more sub-distribution board. Reasonable number of way to be provided in a sub-distribution board will be discussed later.
Where more than one sub-distribution boards are used, wiring may be completed without a main distribution board, if desired. In that case each sub-distribution board works as a main distribution board. An example of such connections is given in Fig. 236.
Fig. 236 shows that the supply line, coming through meter, switch, fuse etc. goes at first to the distribution board at the ground floor. There it makes a loop connection with a link and a cut-out and thence go to first floor cut-out. Later on, it goes to cut-out of the board at second floor.
In this way we can do away with a main D.B. no doubt, but in this arrangement main cables of comparatively large cross-sectional area are to be drawn up to each distribution board. Hence, if the cost of this system of wiring is less than the main D.B. system, then only we can afford to adopt loop connection method.
The system becomes more convenient if the D.B. of each floor can be installed on the same place at each floor (e.g. just besides the staircase). The main cables may then be drawn through a hole in the ceiling vertically upwards to the next D.B.
For a big house if it becomes necessary to install more than one distribution board at each floor, arrangements as shown in Fig. 237 may be adopted.
Q. 34. Explain what is D.C. three-wire or A.C. three-phase supply.
Ans. Suppose Fig. 237 is the plan of the ground floor of a house. The service main enters into the house through right-hand corner. Since the house is a big one, it is natural to give supply with three-wire d.c.
The service main, on entering the premises, goes to the meter box and thence at first goes to the main distribution board at, the ground floor. In case of a large two- or three-storeyed building, a main distribution board for each floor is to be installed at the ground floor. Space must be spared for main distribution board as near the entry of service main as possible. Three supply cables (positive, neutral and negative), after entering the premises, come up to main distribution board.
Now, the neutral wire is split up into two wires. Positive cable and one neutral cable go to one main distribution board of the ground floor through a meter, while the negative cable and the other neutral cable go to another main distribution board of the upper floor through a separate meter. These connections have been well-explained in the description of Fig. 238.
In case of a multi-storeyed building, the total number of load points have to be so divided that the positive side and negative side of supply are loaded equally as far as practicable. When the supply is 3-phase, 4-wire a.c., total loads are to be divided into three equal parts, each part being supplied by a separate phase.
Positive and negative wires of three-wire d.c. supply and three phase wires of 3-phase, 4 wire a.c. supply are the live wires. Unless these live wires are metal-sheathed with the sheath earthed, one should be kept at least 2 metres (6 feet) away from the other. But in that case a clear indication on the sheath must remind the possibility of danger arising out of voltage across the wires.
It is evident from the plan shown in Fig. 237 that each floor of this building may conveniently be divided into three portions, — left portion, central portion and right portion. If there be a fuse board to control current in each portion, it would be sufficient for the main board at the ground floor to have four ways.
Similarly but separately the number of ways necessary for the main D.B. of the first floor should be considered and determined. If, however, the plan of the upper floor is the same as that of the ground floor as is usually the case, four ways in the main D.B. of the upper floor may also be deemed to be sufficient.
Now, a detailed description of these connections is given below:
When three-wire supply, after entering a house, first comes to the supplier’s main fuse, only two fuses should be provided — one in the positive line and the other in the negative line. Fuse wire must not be inserted in the neutral line. A detachable metal link may, however be used in the neutral line so that It may be taken off.
If necessary, at the time of testing. Now, the positive wire is connected to one meter board and the negative wire is connected to another meter board, while neutral is split up and two cables are drawn from it. One neutral cable is connected to one terminal of D.P. switch of a meter board and the other neutral cable is connected to one terminal of D.P. switch of the other meter board. Thereafter, one cable from each meter is connected to the remaining terminal of the corresponding D.P. switch.
The meter board connected to positive and neutral is known as the positive side of supply, and the board connected to negative and neutral is the negative side of supply. Therefore, two-wire supply is available from each meter board. As usual, the two-wire connection is made with the main distribution board. But no fuse can be inserted In any wire connected to neutral. If desired, a double-pole switch may be provided in each way or in case of an unearthed pole a single-pole switch may be used.
Size of Fuse Board:
15-ampere fuse board is the smallest size of fuse boards. So, whenever a fuse board is required, at least a 15-ampere board has to be installed. Only the sizes of fuse wires are different for different sub-circuit currents in that case.
In this case double-pole switch, fuse etc. may be installed side by side. But the switch etc. of each pole should be in an iron-clad switch-fuse box which must be duly earthed. Besides, the arrangement should be such that when the cover of switch etc. of one pole is opened up, the switch etc. of the other pole having a potential difference of more than 250 volts between poles should not be uncovered.
Further, if all these switch boxes are installed at the same place, the existence of high or medium pressure between them should be indicated by a danger board hung there or this notice may be written on the surface of the box in a permanent way or all the boxes may be Installed in a room which should be kept under lock and key so that only the authorised person or persons ran enter the room.
Q. 35. Which line can include fuse cut-out and which line cannot?
Ans. Special attention is drawn to ‘Rules for Main Switch and Fuse’ in Table 24. Moreover, Fig. 239 and Fig. 240 explain clearly where fuses should not be used in d.c. and a.c. circuits, respectively,
Note:
Fuse should not be inserted at any point of the middle wire. If switch is to be provided in all the three wires, a triple pole linked switch should be used. Alternatively, a double-pole switch may be used for positive and negative wires, the neutral wire at the middle having no switch at all.
Everything said here refers to earthed neutral system. If the neutral is earthed (neutral wire is earthed at a generating station or at a sub-station; almost everywhere the supply system has the earthed neutral), no fuse can be inserted in wires (whether main wire or small branch wire) wherever these are connected to neutral wire. Nor any single-pole switch can be used in a neutral wire.
A switch may be used in the neutral wire provided it is a multi-pole linked switch and the neutral is switched on or oil together with live wire or wires. If it is deemed to be unnecessary, a double-pole linked switch may be used with positive and negative wires, the neutral wire having no switch at all.
In case of 3-phase, 4-wire a.c. supply a four-pole linked switch inclusive of neutral pole may be used; but fuses can be inserted in three phase wires only and not In the neutral. Fixing a copper link plate in the neutral, alternatively a triple-pole linked switch may be used for three phase wires only.
Note:
Nowhere fuse should be inserted in the neutral line. It should be used only in phase wires. If switch is to be provided in the neutral wire, a four-pole linked switch should be used. Otherwise a triple-pole linked switch may be used for three phase wires, the neutral wire having no switch.
Q. 36. State the approximate estimates of allowable voltage drop in different parts of wiring system of a large building.
Ans. There is no hard and fast rule in this respect. Ordinarily, however, in a lighting circuit containing lights and fans, the total voltage drop is kept within three per cent of the declared voltage.
The maximum allowable voltage drop is 1 volt from main fuse to main distribution board, 4.5 volts from main D.B. to each sub-distribution board and 1.5 volts in each final sub-circuit.
The voltage drop in the connection line of a pump motor in a house may go up to 7.5 per cent of the declared pressure, but as is the case with a lighting circuit, it is better to keep this drop within 3 per cent, if possible.
Q. 37. What is meant by main cables?
Ans. In most cases of large buildings—where the distance between main distribution board and sub-distribution board or fuse board is rather long, the use of paper-insulated lead-covered and armored cable or P.V.C. insulated with or without armor cable (e.g. tropodur cable) as main cable is economical as well as long lasting.
It makes no difference whether the wiring of the house is done by means of cables drawn through conduit or by means of lead-covered cables. However; in the latter case, the cables must not be single-core,—twin-core or three-core or three-core with earth wire cables are to be used.
As paper is very much nygroscopic, both ends of a paper-insulated cable, whenever used, should be fitted with end-box or sealing chamber. In one type of sealing chamber, there is an iron box from one side of which a brass tube is projected outside. Lead-covered paper-insulated main cable is inserted into this tube and the cover is soldered to the box.
On the other side of the chamber there is another tube with screw threads for connection with conduit pipes. Through this tube connection is made with main board by V.I.R. or P.V.C. wires. The conductors of V.I. R. or P.V.C. wires and those of paper-insulated or tropodur cables are soldered together by means of brass connectors.
These connectors are available from the manufacturers of the cable. If necessary, arrangement; for keeping the conductors away from one another may –be made by means of suitable pieces of wooden separators.
There are two plugs or holes on the top cover of the box. Sealing compound suitable for junction boxes is melted and poured into the box through one of them. The air inside is then expelled through the other.
Power consumptions of electrical equipment’s usually used in a house are as follows:
Q. 38. How to make the correct estimation of sizes of cables?
Ans. If the size of cable is determined on the basis of total load connected in the circuit, i.e. on the basis of sum of wattages of all lamps, fans, wall-plugs etc., the size will be so large as to be never required in practice. The reason is lamps, fans, wall-plugs etc. are not used at a time, and oftener than not, the points are not loaded to their full capacity.
For these reasons it is considered to be sufficiently accurate if an estimate is prepared according to the following rules:
Usually it is assumed that:
(a) If the correct wattage of load to be connected to an ordinary socket outlet is not known, it is assumed to be 60 watts (generally a 60-watt lamp or a table fan is used with such an outlet). If, however, it is known to be more, the actual wattage of the load should be taken into account.
(b) Every point of power socket outlet is to be assumed 1,000 watts.
(c) Unless specifically mentioned, wattage of every incandescent lamp is assumed to be 60 watts and that of every fluorescent lamp 40 watts.
(d) In case the exact wattage of every ceiling fan and table fan is not known, it should be deemed to consume 60 watts when in use.
Now, the total wattage of every final sub-circuit is obtained by adding up the wattage of individual loads connected to that circuit. Two-thirds of this total wattage should be taken into consideration for determining the size of cable to be used for this sub-circuit. But the current corresponding to this wattage must not be less than the current drawn by the single maximum wattage point.
If, however, a sub-circuit has only one point, cable suitable for full load current of that point is to be used. Again, if a sub-circuit has only three 5-ampere plug-socket, it will not be erroneous to determine the size of the cable on the basis of two-thirds of 180 watts (i.e. 120 watts).
The above rules are applicable to ordinary dwelling-house, but not to all buildings. Three- fourths of the total wattage is to be considered for hotels, boarding houses etc. and nine-tenths for office etc. For the auditorium of cinema, theatres etc., cables suitable for full connected load are to be used.
If in a house there is electric cooker or electric oven, full load up to 10 amperes and one half of any extra load (in excess of 10 amperes) should be taken into account. The load of every sub-circuit thus calculated, the current drawn by a sub-distribution board is determined.
The load of wall-plug connected to a sub-distribution board in a dwelling house where there are wall-plugs of various sizes, will be the full-load of the plug drawing maximum current plus four-tenths of all the remaining plugs. In hotels etc., three-fourths of the total loads of all the remaining plugs have to be added to the full-load of the plug drawing maximum current.
(i) At first currents for the sub-circuits are to be determined, one by one.
(ii) According to currents suitable sizes of fuse which can continuously carry the respective currents should be determined.
(iii) The size of cable for each sub-circuit is determined according to the current drawn by that sub-circuit.
(iv) Finally, the sizes of flexible cord and wall-socket for the respective sub-circuit have to be fixed.
Q. 39. What is splitter unit?
Ans. A kind of distribution board is very much in use nowadays. This board can be installed anywhere. It is known as ‘splitter unit’ or ‘splitter box’. The unit is prepared by setting a pair of main switches as well as a pair of main fuses or a single fuse inside a cast iron box.
An external handle is attached to the body of the box. It is so arranged that the cover of the box cannot be opened when the switch is on or the switch cannot be on when the cover is open, i.e. the cover cannot by any means be opened unless the switch is off. It is for this arrangement that the unit is quite good from the point of view of safety. The arrangement is known as ‘foolproof arrangement, i.e. the switch cannot be misused even foolishly. The box is also known as Iron-clad Switch-Fuse Box.
The switch-fuse box is installed at a point where from consumer’s zone starts. Cables are drawn from the switch and connected to the bus-bars of a fuse board. This is the main distribution board. Nowadays iron-clad fuse-box is very much in use. A screw is attached to the body of this box. The risk of electric shock is avoided by connecting .earth wires to that screw. The box is to be earthed by two separate and distinct earth connections.