SSC JE Electrical Previous Year Question Paper 2018-SET2|MES Electrical

Ques.71. Name the generating station where electrical energy is generated through the steam and coal.

  1. Thermal power station
  2. Diesel Power station
  3. Hydro Power station
  4. Nuclear Power station

The steam power plant is also called as the thermal power plant. It is the important source to produce the electricity.

In thermal power plants, steam is generated by heating the water and is used to rotate the turbines which are coupled with the synchronous generators (also known as alternators) to produce electricity. Steam can be generated from coal, gas or nuclear as the main fuel. Coal, which is the main fuel used in thermal power plants, is fired in the boiler to generate heat for producing steam. The thermal efficiency of a steam power plant mainly depends on the choice of the steam cycle and varies from 28% to 35%. The principal equipment of steam power plants are the boiler, super-heater, feed water pump, steam reheater, condenser, turbine, and generator. The major components of the steam generating plants are shown in Figure. In large thermal power plant unit, several stages of turbines such as high pressure, intermediate pressure and low pressure are used to extract more power from the steam and thus, to increase the efficiency of the machine with minimum cost.

For generating the electricity using the steam turbine, high-speed synchronous generators al used because the efficiency of steam turbines is high at high speed. Since the speed of turbo alternator is high, the diameters of the machine are kept minimum so that the centrifugal fort acting on the rotor is minimized. To keep the same electrical loading (which is proportional area x length), the length of the turbo-alternators is increased.

Note:-

In the hydro-electric power station, the potential energy of the water head is used to generate electrical energy.

A nuclear power plant is almost similar to the coal-fired power plant except for the steam generation part where nuclear energy is used for the steam production.

 

Ques.72. In a 3-phase 4-wire cable, the cross-sectional area of the neutral conductor is

  1. Equal to the phase conductor
  2. More than the phase conductor
  3. Half the phase conductor
  4. None of these

The 3-Phase four wire system is used for the distribution system. A three-phase four-wire supply gives a consumer the choice of a 400 V three-phase supply and a 230 V single-phase supply. Many industrial loads such as motors require a three-phase 400 V supply, while the lighting load in a factory, as in a house, will be 230 V. Industrial loads usually demand more power than a domestic load, and more power can be supplied by a 400 V three-phase supply than is possible with a 230 V single-phase supply for a given size of cable since power = VI cosφ (watts).

This form of connection is shown in Fig. and is known as a star or Y connection. With the resulting 4-wire system, the three loads also are connected in star configuration The three outer wires are called the lines, and the common wire in the center is called the neutral. One reason for the connection of the neutral conductor is to provide a path for currents if the system became unbalanced. Another is that it enables single-phase loads to be connected to a three-phase system. 

 

The addition of the conductor between the star points converts the system into what is known as a ‘three-phase four-wire system‘. We can see that the currents supplied by the generator flow along the lines, through the load and return via the neutral conductor.

However, in a balanced three-phase system are equal, and add up to the neutral current, which is zero:

IBr + IBk + IGr = IN = 0

If the three loads were identical in every way (same impedance and phase angle), then the currents flowing in the three lines would be identical. The three currents meet at the star point of the load. The resultant current returning down the neutral wire would, therefore, be zero. The load, in this case, is known as a balanced load, and the neutral is not strictly necessary. However, it is difficult, in practice, to ensure that each of the three loads is exactly balanced. For this reason, the neutral is left in place. Also, since it has to carry only the relatively small ‘out-of-balance’ current, it is made half the cross-sectional area of the lines.

In some cases, the same-sized cable is normally used for all four wires both to allow for inevitable imbalances and to also ensure that the neutral cable has equal tensile strength as the phase conductors.

 

Ques.73. Fuse is always made up of alloys and metals having

  1. High resistance and high melting points
  2. High resistance and low melting points
  3. Low resistance and low melting points
  4. Low resistance and high melting points

Fuse is the current interrupting device which breaks or open the circuit (in which it is inserted) by fusing the elements when the current in the circuit exceeds a certain value.

Fuse is a simplest and cheapest device used for interrupting an electrical circuit under the condition of short-circuiting, or excessive overload, current magnitudes.

A fuse is a safety device having a short length of a thin, tin-plated copper wire having the low melting point, which melts and breaks the circuit if the current exceeds a safe value. The thickness and length of the fuse wire depend on the maximum current allowed through the circuit. An electric fuse works on the heating effect of current. The fuse for protecting our domestic wiring is fitted just above our main switch on the switchboard. A fuse wire is connected in series in the electric circuits.

The main fuse in domestic wiring consists of a porcelain fuse holder H having two brass terminals T1 and T2 in it. This is connected in the live wire. The other part of the fuse is a removable fuse grip G which is also made of porcelain. The fuse grip has a fuse wire fixed in it. When fuse grip is inserted in the fuse holder as shown in Figure, then the circuit of our domestic wiring is completed. So, under normal circumstances when the current is within limit, then the fuse wire is intact and electric current is available in our wiring.

When a short circuit takes place, or when overloading takes place, the current becomes large and her the fuse wire too much. Since the melting point of fuse wire is much lower than copper wires, the fuse wire melts and breaks the circuit as shown in Figure. When the fuse wire breaks, electricity supply automatically switched off before any damage can be done to the rest of the wiring (or the electric appliances being used).

We will now give some important points about the fuse wire to be used in electrical circuits. First of all, we should know why we use a thin wire as a fuse wire and not a thick wire. We use a thin wire in a fuse because it has a much greater resistance than the rest of connecting wires. Due to its high resistance, the heating effect of current will be much more in the fuse wire than anywhere else in the circuit. This will melt the fuse wire whereas other wirings will remain safe. We should not use a thick wire as a fuse wire because it will have a low resistance and hence it will not get heated to its melting point easily. The fuse wire usually made from tin-plated copper wire having the low melting point so that it may melt easily. A pure copper wire cannot be used as a fuse wire because it has a high melting point due to which it will not me easily when a short circuit takes place.

Fuse wire is made with an alloy of lead and tin having the low melting point and low resistance (although the resistance of fuse wire is higher than that of electrical appliances). If due to any malfunction or fault, excessive current begins to flow through the circuit, the fuse wire immediately melts due to the heat generated by the flowing current. The circuit is broken and the excess current, which may damage equipment is prevented from flowing. 

It is used for Overload and for short-circuit protection in high voltage (upto 66 kV) and for low Voltage (upto 120 V – 240 V) installations/circuits.

Characteristics of a fuse are:-

  1. It should have the low melting point.
  2. It should have low ohmic losses.
  3. It should have high conductivity. ( or low resistivity)
  4. It should be economical.
  5. It should be free from detraction.

 

Ques.74. In case of staircase wiring which type of switch is used?

  1. 2 one way switch
  2. 1 one way switch
  3. 2 two way switch
  4. 1 two way switch

Staircase wiring is commonly used for the staircase and corridor lighting. Normally, only a single light lamp is provided in a staircase which is controlled by either of the two switches – one installed at the top and other at the bottom. Similar is the situation in a bathroom common to two rooms The light lamp in the bathroom can be controlled by the switches installed in the two rooms. 

Two-way switches have a central terminal. Central terminal of the first switch is connected to the lower position of the second switch, whereas the central terminal of the second is connected to the upper terminal of the first. We can connect terminal in reverse fashion also i.e Central terminal of the first switch is connected to the upper position of the second switch, whereas the central terminal of second is connected to the lower terminal of the first. The figure shows a diagram of two-way switching, that is control of light from two different switches (staircase lighting). When the phase is completed the Lamp start to glow.

 Suppose you are at the top of the staircase and the light is in switched off condition, you can switch the light on as you go down, then switch it off as you leave. 

 

Ques. 75.  Two incandescent lamps of wattage 40W, 60 W are connected in series with the voltage of 230 V. Which out of the two lamps will glow brighter?

  1. 40 W
  2. 60 W
  3. Both brightly
  4. Both dim

Comparing the 60W bulb to a 40W bulb means that the 60W bulb consumes 20W more power than the 40W bulb. Now note the question its a 40W and 60W bulbs we talking about, not resistors! For the bulbs, wattage is brightness. The 40W and 60W bulbs will be rated for a particular (I mean same) voltage, not different. So considering power P=V2/R power is inversely proportional to resistance (bulbs rated for same voltage). Which means that 40W bulb has more resistance across it compared to the 60W bulb. ( Compare in terms of brightness, a 0W bulb has to glow so dim just because it has high resistance while a 100W bulb glows very much bright because its resistance is low)

Connecting the 2 bulbs in series implies that the current is the same, hence the power consumption totally depends on how voltage gets divided between the bulbs. Naturally, the 40W bulb will have more voltage across it since it has more resistance. Hence, the 40W bulb will glow brighter.

 

Ques.76. Rating of fuse wire is expressed in terms of _____

  1. Ohms
  2. Mhos
  3. Amperes
  4. Watts

An electric fuse is a device used in the household electric wiring that protects appliances from high current. Fuse wire is connected between the mains and leads of the primary circuit of an electric appliance. When the excess amount of electricity passes through the wire, the fuse wire gets heated and the heat produced fuses or melts the wire breaking the power supply to the circuit of the appliance. Thus, the electric appliance is protected from the passage of a high electric current which destroys the appliance.

A fuse rating is a current need to blow the fuse. Fuses are rated in amps and will be placed as close to a power distribution point as possible to minimize runs of unprotected cable.

Generally, the lowest rated fuse is selected consistent with reliable operation, but for emergency equipment, the highest rating is used consistent with cable protection.

 

Ques.77. If a live wire comes in contacts with the metal casing excess current moves to _____

  1. Power House
  2. Dynamics
  3. Earth
  4. Transformer

Sometimes due to short circuit or due to any other reason large amount of current flows in the circuit. This current produces excessive heat (Q = I2Rt) in the circuit and may cause the fire as well. To prevent the damage of electrical appliances against this heating and fire a local earthing is provided just below the electric meter in the house. For this purpose, a thick copper wire is connected from the earth wiring. The other end of this copper wire goes 3 to 4 metros deep inside the earth and is connected to a thick copper plate (50 cm x 50 cm) buried with the mixture of water, salt, and charcoal. To keep the copper plate conducting the ground above the plate is made wet time to time. In case of excessive current in the live wire, it comes to earth wire and passes through the earth. 

In order to work an electrical appliance like an electric iron, electric kettle or a room cooler, we need two wires of the supply line, the live wire, and the neutral wire. Sometimes, due to wear and tear or due to excessive heating, the plastic covering (or insulation) of the connecting wires gets removed or gets burnt and the live wire (which is at a high potential of 220 volts) becomes naked. This naked live wire may touch the metal case (or metal body) of the electrical appliance due to which the case becomes live and comes to the high voltage of 220 volts. If we happen to touch any part of this live appliance, a very high current flows through our body into the earth Due to this high current flowing through our body we get an electric shock.

To avoid the risk of electric shocks, the metal body of an electrical appliance is “earthed”. Earthing means to connect the metal case of the electrical appliance to the earth (at zero potential) by means of a metal wire called “earth wire”. in household circuits, we have three wires, the live wire, the neutral wire and the earth wire. One end of the earth wire is buried in the earth. We connect the earth wire to the metal case of the electrical appliance by using a three-pin plug. The metal casing of the appliance will now always remain at the zero potential of the earth. We say that the appliance has been earthed or grounded. Let us make it more clear with the help of a diagram. The figure shows the earthing of an electric iron or press.

The live wire and the neutral wire are connected to the two ends of the heating element whereas the earth wire is connected to the metal body of the electric iron. These three wires are connected to a three-pin plug P. The plug P is connected to a three-pin socket S. Let us see how the earth connection actually works.

If by chance, the live wire touches the metal case of the electric iron (or any other appliance), which has been earthed, then the current passes directly to the earth through the earth wire. It does not need our body to pass the current and, therefore, we do not get an electric shock. Actually, a very heavy current flows through the earth wire and the fuse of household wiring blows out or melts. And it cuts off the power supply. In this way, earthing also saves the electrical appliance from damage due to excessive current.

From the above discussion, we conclude that we earth the metallic body of an electrical appliance to save ourselves from electric shocks. Thus, the earthing of electrical appliances is used as a safety measure. It should be noted that we give earth connections to only those electrical appliances which have the metallic body, which draw heavy current, and which we are liable to touch. For example, electric iron, electric heater, room cooler, and refrigerator, are all provided with earth connections. We, however, do not do earthing of an electric bulb or a tube-light because we hardly touch them when they are on. 

Hence in case of proper earthing no sooner the live wire touches the metal body then a large amount of current flows in the body of nearly zero resistance and the fuse connected to the appliance blows off to disconnect the circuit. Clearly, the fuse has to be connected to the live wire.

 

Ques.78. A 3-Phase, 4 wire system is commonly used on___

  1. Primary transmission
  2. Secondary transmission
  3. Primary distribution
  4. Secondary distribution

3-Phase, 4 wire system is commonly used on the secondary distribution system.

The secondary distribution system receives power from the secondary side of distribution transformers at low Volta and supplies power to various connected loads via service lines. The secondary distribution system is the final sub-system of the power system.

At the local distribution centers, there is step down distribution transformers. The voltage level of 6.6 kV, 11 kV is further reduced to 400 V using distribution transformers. Sometimes it may be reduced to 230 V. The power is then transmitted using distributors and service mains to the consumers. This secondary distribution also called low voltage distribution. This uses 3 phase 4 wire system.

The advantages of 

  • For Distribution, the load is not constant as it depends upon consumer and loads continuously keep varying. The balanced condition is tried to obtain by distributing the load equally on all 3 phase and the under the faulty condition, the short-circuit current flows through neutral.

The voltage between any two lines is 400 V while the voltage between any of the three lines and a neutral is 230 V. The single-phase lighting loads are supplied using a line and neutral while loads like motors are supplied using three phase lines.

 

Ques.79  Insulator is used in low voltage distribution lines are known as____

  1. Shackle
  2. Strain
  3. Pin
  4. Suspension

Shackle Insulators: This insulator is also known as spool insulator. Basically, this insulator is used for low voltages, i.e., below 11 kV. It can be used in both the direction either vertical or horizontal. This insulator is mounted in a C type clamp fitted to the cross arms of the pole with the help of bolts. The conductor is passed from the groove and fixed with the binding wire. The schematic diagram of this insulator is shown in Fig. This insulator is generally used at the dead end of the line where mechanical stress is more.

Strain Insulator: Large tension is experienced by the line when there is a sharp curve in the line or it crosses a river or at the dead end. In such cases, the pin insulators are useful. For low voltage line, shackle insulators can be used but for high voltage line, strain insulators are used.

Pin-Type Insulator: This is very simple, economic and efficient method to support the line conductor and bus bars for voltage up to 33 kV. The pin-type insulator is generally made of glass or porcelain material. This insulator is used only for low voltages because for high voltages it becomes uneconomical and bulky.

Suspension-Type Insulator: As discussed above, for high voltage the pin-type insulator is not economical. Hence, for high voltages more than 33 kV, the suspension-type insulator is used. In the suspension-type insulator, large numbers of porcelain discs are used. The various discs are connected in series with the metal link forming a string.

 

Ques.80. Which of the following type of lamp gives more illumination from low wattage?

  1. Incandescent lamp
  2. Fluorescent lamp
  3. Compact fluorescent lamp
  4. LED lamp

Light-emitting diode (LED)

Light-emitting diode (LED) lamps arc fairly new to the general illumination scene. As technology continues to improve, they will become a great option.

  1. LED lamps are available in a wide spectrum of colors (2700 being the closest to incandescent.
  2. LED lamps are cool to the touch.
  3. LED lamps are high efficiency.
  4. LED can emit 300 lumens/watt
  5. LED lamps have the longest life expectancy (50,000 hours).
  6. LED lamps are expensive.

Incandescent lamps

Incandescent lamps have been around for a long time and are still commonly used, but they don’t perform as well as some of the other lamps on the market.

  1. Incandescent lamps produce a warm light that is pleasing in homes.
  2. Incandescent lamps are sensitive to vibration.
  3. Incandescent lamps are hot to the touch.
  4. The incandescent lamp produces 15 – 20 lumens/watt
  5. Incandescent lamps are low efficiency. e. Incandescent lamps have a short life expectancy (1200 hours).
  6. Incandescent lamps are inexpensive.

 

Compact fluorescent (CFL) 

Compact fluorescent (CFL) lamps are becoming more popular.

  1. CFLs are available in a wide spectrum of colors (2700 K is the closest to incandescent
  2. CFLs take a few minutes to warm up and produce full intensity.
  3. CFLs are cool to the touch.
  4. CFL’s can emit 70 – 90 lumens watts.
  5. CFLs are high efficiency.
  6. CFLs have a long life expectancy (8000 hours).
  7. CFLs have a moderate cost.

4 thoughts on “SSC JE Electrical Previous Year Question Paper 2018-SET2|MES Electrical”

  1. sir ek request hai kripya aap answer ko hide kar dijiye taki as a practise ise solve iske baad click karne pe answer dikhaye. ye achha huga practise ke liye.

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