ELECTRICAL ENGG. INTERVIEW QUESTION PART 5 TO 9

HI, ENGINEERS

All Electrical engineering last year's students will give the interview.
we help you to crack the interview. 
we are posting interview questions and their relevant answer in part wise refer to all parts for your interview preparation.

also sharing interview books that help you a lot. it also helps you to make a perfect resume.

the link is given at the bottom of the page. (must refer)

HAVE YOU SEEN PART 1 TO 4

TAP TO SEE👉 PART 1 TO 4

Electrical Engineering Interview Questions and Answers - Part 05

Question No. 01: What is the difference between Isolator and Circuit Breaker? What is bus-bar?

Answer: Isolator is an off load device which is used for isolating the downstream circuits from upstream circuits for the reason of any maintenance on downstream circuits. It is manually operated and does not contain any solenoid unlike circuit breaker. It should not be operated while it is having load. First the load on it must be made zero and then it can safely operated. Its specification only rated current is given. But circuit breaker is on load automatic device used for breaking the circuit in case of abnormal conditions like short-circuit, overload etc., it is having three specifications:

1. 1st is rated current and
2. 2nd is short circuit breaking capacity and
3. 3rd is instantaneous tripping current.

Bus bar is nothing but a junction where the power is getting distributed for independent loads.

Question No. 02:  What is stepper motor. What are its uses?

Answer: Stepper motor is the electrical machine which acts upon input pulse applied to it. It is one type of synchronous motor which runs in steps in either direction instead of running in complete cycle. So, in automation parts it is used.

Question No. 03: What is the difference between synchronous generator & asynchronous generator?

Answer: In simple, synchronous generator supply’s both active and reactive power but asynchronous generator (induction generator) supply’s only active power and observes reactive power for magnetizing. This type of generators is used in windmills.

Question No. 04: Which motor has high Starting Torque and Staring current DC motor, Induction motor or Synchronous motor?

Answer: DC Series motor has high starting torque. We cannot start the Induction motor and Synchronous motors on load, but cannot start the DC series motor without load.

Question No. 05: Why the capacitors work on ac only?

Answer: Generally capacitor gives infinite resistance to dc components (i.e., block the dc components). It allows the ac components to pass through.

Question No. 06: Which power plant has high load factor?
Answer: All base load power plants have a high load factor. If we use high efficiency power plants to supply the base load, we can reduce the cost of generation. Hydel power plants (hydroelectric power plant) have a higher efficiency than thermal & nuclear power plants.

Question No. 07: What is meant by knee point voltage?

Answer: Knee point voltage is calculated for electrical Current transformers and is very important factor to choose a CT. It is the voltage at which a CT gets saturated.(CT-current transformer).

Question No. 08: Use of switch mode power converter in real-time basis?
Answer: Switch mode power converter can be used in the following 5 different ways

1. Step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS,
2. Step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost Converter or Step-Up SMPS,
3. Step up or step down an unregulated dc input voltage to produce a regulated dc output voltage,
4. Invert the input dc voltage using usually a circuit such as the Cuk converter, and
5. Produce multiple dc outputs using a circuit such as the fly-back converter. 

Question No. 09: Why we use ac system in India why not dc? 

Answer: Firstly, the output of power stations comes from a rotary turbine, which by its nature is AC and therefore requires no power electronics to convert to DC. Secondly it is much easier to change the voltage of AC electricity for transmission and distribution. Thirdly the cost of plant associated with AC transmission (Circuit breakers, transformers etc) is much lower than the equivalent of DC transmission AC transmission provides a number of technical advantages. When a fault on the network occurs, a large fault current occurs. In an AC system this becomes much easier to interrupt, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Question No. 10: What is difference between fuse and breaker?
Answer: Fuses are burned at the time of over current flows in the circuit but breakers are just open (not burn) at the time of over current flow. Fuses are used in only one time but breakers are used by multiple numbers of times.

Question No. 11: State the difference between generator and alternator

Answer: Generator and alternator are two devices, which converts mechanical energy into electrical energy. Both have the same principle of electromagnetic induction, the only difference is that their construction. Generator persists stationary magnetic field and rotating conductor which rolls on the armature with slip rings and brushes riding against each other, hence it converts the induced emf into dc current for external load whereas an alternator has a stationary armature and rotating magnetic field for high voltages but for low voltage output rotating armature and stationary magnetic field is used.

Question No. 12: What are the different methods for the starting of a synchronous motor?

Answer: Starting methods: Synchronous motor can be started by the following two methods:

1. By means of an auxiliary motor: The rotor of a synchronous motor is rotated by auxiliary motor. Then rotor poles are excited due to which the rotor field is locked with the stator-revolving field and continuous rotation is obtained.
2. By providing damper winding: Here, bar conductors are embedded in the outer periphery of the rotor poles and are short-circuited with the short-circuiting rings at both sides. The machine is started as a squirrel cage induction motor first. When it picks up speed, excitation is given to the rotor and the rotor starts rotating continuously as the rotor field is locked with stator revolving field.

Question No. 13: Mention the disadvantages of low power factor? How can it be improved?

Answer: Disadvantages of low power factor:

• Line losses are 1.57 times unity power factor.
• Larger generators and transformers are required.
• Low lagging power factor causes a large voltage drop, hence extra regulation equipment is required to keep voltage drop within prescribed limits.
• Greater conductor size: To transmit or distribute a fixed amount of power at fixed voltage, the conductors will have to carry more current at low power factor. This requires a large conductor size.

Question No. 14: Which type of A.C motor is used in the fan (ceiling fan, exhaust fan, pedestal fan, bracket fan etc) which are find in the houses?

Answer: Its Single Phase induction motor which mostly squirrel cage rotor and are capacitor start capacitor run.

Question No. 15: Why Human body feel Electric shock? And in an Electric train during running, we didn't feel any Shock? Why?

Answer: Unfortunately our body is a pretty good conductor of electricity, The golden rule is Current takes the lowest resistant path if you have insulation to our feet as the circuit is not complete (wearing rubber footwear which doing some repairs is advisable as our footwear is a high resistance path not much current flows through our body).The electric train is well insulated from its electrical system.

Top Important Electrical Interview Questions - Part 06

Question No. 01: Why link is provided in neutral of an ac circuit and fuse in phase of ac circuit?

Answer: Link is provided at a Neutral common point in the circuit from which various connections are taken for the individual control circuit and so it is given in a link form to withstand high Amps. But in the case of Fuse in the Phase of AC circuit it is designed such that the fuse rating is calculated for the particular circuit (i.e. load) only. So if any malfunction happens the fuse connected in the particular control circuit alone will blow off.

Question No. 02: Mention the methods for starting an induction motor? 
Answer: The different methods of starting an induction motor

1. DOL: direct online starter
2. Star delta starter
3. Auto transformer starter
4. Resistance starter
5. Series reactor starter

Question No. 03: Which type of oil is used as transformer oil?

Answer: Transformer oil, or insulating oil, is usually a highly-refined mineral oil that is stable at high temperatures and has excellent electrical insulating properties. It is used in oil filled transformers, some types of high voltage capacitors, fluorescent lamp ballasts, and some types of high voltage switches and circuit breakers. Its functions are to insulate, suppress corona and arcing, and to serve as a coolant.

Well into the 1970s, polychlorinated biphenyls (PCB)s were often used as a dielectric fluid since they are not flammable. They are toxic, and under incomplete combustion, can form highly toxic products such as furan. Starting in the early 1970s, concerns about the toxicity of PCBs have led to their banning in many countries.

Today, non-toxic, stable silicon-based or fluoridated hydrocarbons are used, where the added expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Combustion-resistant vegetable oil-based dielectric coolants and synthetic pentaerythritol tetra fatty acid (C7, C8) esters are also becoming increasingly common as alternatives to naphthenic mineral oil. Esters are non-toxic to aquatic life, readily biodegradable, and have a lower volatility and higher flash points than mineral oil.

Question No. 04: What is different between resistance grounding system and resistance earthing system?
Answer: Resistance grounding system means connecting the neutral point of the load to the ground to carry the residual current in case of unbalanced conditions through the neutral to the ground whereas resistance earthing system is done in electric equipment in order to protect he equipment in occurrence of fault in the system.

Question No. 05: What is inrush current?
Answer: Inrush current is the current drawn by a piece of electrically operated equipment when power is first applied. It can occur with AC or DC powered equipment, and can happen even with low supply voltages.

Question No. 06: In the motors starting purpose why the armature rheostats at maximum position while the field rheostat is kept in minimum position?
Answer: In the time of starting a motor the field rheostat is kept in minimum state for the high starting torque.
In the time of starting a motor the armature rheostats is used to control or reduce the high starting current. This high starting current can damage the motors starting systems.

Question No. 07: Comment on the working principle of operation of a single-phase transformer.

Answer: Working principle of operation of a single-phase transformer can be explained as
An AC supply passes through the primary winding, a current will start flowing in the primary winding. As a result, the flux is set. This flux is linked with primary and secondary windings. Hence, voltage is induced in both the windings. Now, when the load is connected to the secondary side, the current will start flowing in the load in the secondary winding, resulting in the flow of additional current in the secondary winding. Hence, according to Faraday’s laws of electromagnetic induction, emf will be induced in both the windings. The voltage induced in the primary winding is due to its self inductance and known as self induced emf and according to Lenze’s law it will oppose the cause i.e. supply voltage hence called as back emf. The voltage induced in secondary coil is known as mutually induced voltage. Hence, transformer works on the principle of electromagnetic induction.

Question No. 08: Compare JFET’s and MOSFET’s.

Answer: Comparison of JFET’s and MOSFET’s:

• JFET’s can only be operated in the depletion mode whereas MOSFET’s can be operated in either depletion or in enhancement mode. In a JFET, if the gate is forward-biased, excess-carrier injunction occurs and the gate-current is substantial.
• MOSFET’s have input impedance much higher than that of JFET’s. Thus is due to negligible small leakage current.
• JFET’s have characteristic curves more flat than that of MOSFET is indicating a higher drain resistance.
• When JFET is operated with a reverse-bias on the junction, the gate-current IG is larger than it would be in a comparable MOSFET.

Question No. 09: Why Delta Star Transformers are used for Lighting Loads?

Answer: For lighting loads, neutral conductor is must and hence the secondary must be star winding. And this lighting load is always unbalanced in all three-phases. To minimize the current unbalance in the primary, we use delta winding in the primary. So delta / star transformer is used for lighting loads.

Question No. 10: What happens if I connect a capacitor to a generator load?

Answer: Connecting a capacitor across a generator always improves power factor, but it will help depends up on the engine capacity of the alternator, otherwise the alternator will be over loaded due to the extra watts consumed due to the improvement on pf. Secondly, don't connect a capacitor across an alternator while it is picking up or without any other load.

Question No. 11: What is the one main difference between UPS & inverter?

Answer: Uninterrupted power supply is mainly use for short time means according to UPS VA it gives backup.

UPS is also two types: on line and off-line. Online UPS having high volt and ampere for long time backup with high dc voltage, UPS start with 12 V dc with 7 ampere. But inverter is start with 12 V -24v - 36 V dc and 120-180 ampere battery with long time backup.

Question No. 12: What is the count of hvdc transmission lines in India?
Answer: At present there are three hvdc transmission lines in India 

1. Chandrapur to Padghe (Mumbai)--(1500 MW at ± 500 kV DC)
2. Rehand to Delhi (1500 MW at ± 500 kV DC)
3. Talchal to Kolar (2500 MW) 

Question No. 13: What is use of lockout relay in ht voltage?
Answer: A lock-out relay is generally placed in line before or after the e-stop switch so the power can be shut off at one central location. This relay is powered by the same electrical source as the control power and is operated by a key lock switch. The relay itself may have up to 24 contact points within the unit itself. This allows the control power for multiple machines to be locked out by the turn of a single key switch.

Question No. 14: What are the points to be considered for MCB (miniature circuit breaker selection?
Answer: I(L) × 1.25=I(MAX) maximum current. MCB specifications are done on maximum current flow in circuit.

Question No. 15: Battery banks are in connected in series or parallel and why?

Answer: Battery banks are always connected in series in order to get a multiplied voltage where the AH or current capacity remaining same. Ex: 24 nos. 2 V, 200 Ah batteries connected in series will give 48 V, 200 Ah output (Ah = Ampere hours)

Placement Interview Questions Electrical Engineering - Part 07

Question No. 01: What are the advantages of VSCF wind electrical system?

Answer: Advantages of VSCF wind electrical system are:

• No complex pitch changing mechanism is needed.
• Aero turbine always keeps going at maximum efficiency point.
• Extra energy in the high wind speed region of the speed - duration curve can be extracted
• Significant reduction in aerodynamic stresses, which are associated with constant - speed operation.

Question No. 02: What are the various kind of cables used for transmission?

Answer: Cables, which are used for transmitting power, can be categorized in three forms:

• Low-tension cables, which can transmit voltage upto 1000 volts.
• High-tension cables can transmit voltage upto 23000 volts.
• Super tension cables can transmit voltage 66 kV to 132 kV.

Question No. 03: Why in a three pin plug the earth pin is thicker and longer than the other pins?

Answer: It depends upon R = rho l/a where area (a) is inversely proportional to resistance (R), so if (a) increases, R decreases & if R is less the leakage current will take low resistance path so the earth pin should be thicker. It is longer because the First to make the connection and last to disconnect should be earth Pin. This assures Safety for the person who uses the electrical instrument.

Question No. 04: What are the transformer losses?

Answer: TRANSFORMER LOSSES - Transformer losses have two sources-copper loss and magnetic loss. Copper losses are caused by the resistance of the wire (I2R). Magnetic losses are caused by eddy currents and hysteresis in the core. Copper loss is a constant after the coil has been wound and therefore a measurable loss. Hysteresis loss is constant for a particular voltage and current. Eddy-current loss, however, is different for each frequency passed through the transformer

Question No. 05: What is an exciter and how does it work?

Answer: There are two types of exciters, static exciter and rotary exciter. Purpose of exciter is to supply the excitation dc voltage to the fixed poles of generator. Rotary exciter is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotor poles through slip-ring and brushes( conventional alternator). if it is an ac exciter, output of ac exciter is rectified by rotating diodes and supply dc to main fixed poles.ac exciter is the ac generator whose field winding are stationary and armature rotates. Initial voltage is built up by residual magnetism. It gives the starting torque to the generator.

Question No. 06: Why synchronous generators are used for the production of electricity?

Answer: Synchronous machines have capability to work on different power factor (or say different imaginary power varying the field emf. Hence synchronous generators r used for the production of electricity.

Question No. 07: Why, when birds sit on transmission lines or current wires doesn't get shock?

Answer: It’s true that if birds touch the single one line (phase or neutral) they don't get electrical shock. If birds touch 2 lines than the circuit is closed and they get electrical shock. So if a human touch single one line (phase) then he doesn't get shock if he is in the air (not touching - standing on the ground if he is standing on the ground then touching the line (phase) he will get a shock because the ground on what we standing is like line (ground bed - like neutral) and in the most of electric lines the neutral is grounded, so that means that human who touch the line closes the circuit between phase and neutral.

Question No. 08: Operation carried out in Thermal power station?

Answer: The water is obtained in the boiler and the coal is burnt so that steam is obtained this steam is allowed to hit the turbine; the turbine which is coupled with the generator generates the electricity

Question No. 09: Why most of analog o/p devices having o/p range 4 to 20 mA and not 0 to 20 mA?

Answer: 4-20 mA is a standard range used to indicate measured values for any process. The reason that 4 mA is chosen instead of 0 mA is for fail safe operation .For example- a pressure instrument gives output 4 mA to indicate 0 psi, up to 20 mA to indicate 100 psi, or full scale. Due to any problem in instrument i.e. broken wire, its output reduces to 0 mA. So if range is 0-20 mA then we can differentiate whether it is due to broken wire or due to 0 psi.

Question No. 10: What is the full form of KVAR? 

Answer: We know there are three types of power in Electrical as Active, apparent & reactive. So KVAR is stand for Kilo Volt Amps with Reactive component.

Question No. 11: What is the difference between delta-delta, delta-star transformer?
Answer: Delta-delta transformer is used at generating station or a receiving station for Change of Voltage (i.e.) generally it is used where the Voltage is high & Current is low. Delta-star is a distribution kind of transformer where from secondary star neutral is taken as a return path and this configuration is used for Step down voltage phenomena. 

Question No. 12: Explain the application of storage batteries.

Answer: Storage batteries are used for various purposes, some of the applications are mentioned below:

• For the operation of protective devices and for emergency lighting at generating stations and substations.
• For starting, ignition and lighting of automobiles, aircrafts etc.
• For lighting on steam and diesel railways trains.
• As a supply power source in telephone exchange, laboratories and broad casting stations.
• For emergency lighting at hospitals, banks, rural areas where electricity supplies are not possible.

Question No. 13: Explain thin film resistors and wire-wound resistors

Answer:

• Thin film resistors: It is constructed as a thin film of resistive material is deposited on an insulating substrate. Desired results are obtained by either trimming the layer thickness or by cutting helical grooves of suitable pitch along its length. During this process, the value of the resistance is monitored closely and cutting of grooves is stopped as soon as the desired value of resistance is obtained.
• Wire wound resistors: Length of wire wound around an insulating cylindrical core are known as wire wound resistors. These wires are made of materials such as Constantan and Manganin because of their high resistivity, and low temperature coefficients. The complete wire wound resistor is coated with an insulating material such as baked enamel

Question No. 14: Where should the lighting arrestor be placed in distribution lines?

Answer: Near distribution transformers and outgoing feeders of 11 kV and incoming feeder of 33 kV and near power transformers in sub-stations.

Question No. 15: Why series motor cannot be started on no-load?

Answer: Series motor cannot be started without load because of high starting torque. Series motors are used in Trains, Crane etc.

Electrical Job Interview Questions and Answers - Part 08

Question No. 01: Tell me in detail about C.T. and P.T.?

Answer: The term C.T means current transformer and the term P.T means potential transformer. In ckt where measurements of high voltage and high current is involved they are used there.

Particularly when a measuring device like voltmeter or ammeter is not able to measure such high value of quantity because of large value of torque due to such high value it can damage the measuring device. So, CT and PT are introduced in the ckts. They work on the same principle of transformer, which is based on linkage of electromagnetic flux produced by primary with secondary. They work on the ratio to they are designed. E.g. if CT is of ratio 5000\5 A and it has to measure secondary current of 8000 A, then ANS = (8000 ×5)/5000 = 8 A and this result will be given to ammeter   and after measuring 8 A we can calculate the primary current. Same is the operation of PT but measuring voltage.

Question No. 02: What is the difference between surge arrestor and lightning arrestor?

Answer: LA is installed outside and the effect of lightning is grounded, where as surge arrestor installed inside panels comprising of resistors which consumes the energy and nullify the effect of surge.

Question No. 03: What is vacuum circuit breaker? Define with cause and where be use this Device?

Answer: A breaker is normally used to break a circuit. While breaking the circuit, the contact terminals will be separated. At the time of separation an air gap is formed in between the terminals. Due to existing current flow the air in the gap is ionised and results in the arc. Various mediums are used to quench this arc in respective CB's. But in VCB the medium is vacuum gas. Since the air in the CB is having vacuum pressure, the arc formation is interrupted. VCB's can be used upto 11 kV.

Question No. 04: Why temperature rise is conducted in bus bars and isolators? 

Answer: Bus bars and isolators are rated for continuous power flow that means they carry heavy currents which rises their temperature. So it is necessary to test these devices for temperature rise.

Question No. 05: Explain the operation of variable frequency transformer?
Answer: A variable frequency transformer is used to transmit electricity between two asynchronous alternating current domains. A variable frequency transformer is a doubly-fed electric machine resembling a vertical shaft hydroelectric generator with a three-phase wound rotor, connected by slip rings to one external ac power circuit. A direct-current torque motor is mounted on the same shaft. Changing the direction of torque applied to the shaft changes the direction of power flow; with no applied torque, the shaft rotates due to the difference in frequency between the networks connected to the rotor and stator. The variable frequency transformer behaves as a continuously adjustable phase-shifting transformer. It allows control of the power flow between two networks.

Question No. 06: Why should be the frequency 50 Hz 60 Hz only why not others like 45, 95 56 or anything, why should we maintain the frequency constant if so why it is only 50 Hz 60 Hz? 
Answer: We can have the frequency at any frequency you like, but then you must also make your own motors, high voltage transformers or any other equipment you want to use. We maintain the frequency at 50 Hz or 60 Hz because the world maintains a standard at 50 /60 Hz and the equipments are made to operate at these frequency.

Question No. 07: What is slip in an induction motor?

Answer: Slip can be defined as the distinction between the flux speed (Ns) and the rotor speed (N). Speed of the rotor of an induction motor is always less than its synchronous speed. It is usually expressed as a percentage of synchronous speed (Ns) and represented by the symbol ‘S’.

Question No. 08: Name power loses in rotating electrical machines.
Answer: Power loses in rotating electrical machines are Copper losses, core losses, mechanical losses and stray losses.

Question No. 09: Explain the terms real power, apparent power and reactive power for ac circuits and also the units used.

Answer:

• Real Power: It is the product of voltage, current and power factor i.e. P = V I cos j and basic unit of real power is watt. I.e. Expressed as W or kW.
• Apparent power: It is the product of voltage and current. Apparent power = V I and basic unit of apparent power is volt- ampere. Expressed as VA or KVA.
• Reactive Power: It is the product of voltage, current and sine of angle between the voltage and current i.e. Reactive power = voltage × current × sin j or Reactive power = V I sin j and has no other unit but expressed in VAR or KVAR.

Question No. 10: What is a differential amplifier? Also, explain CMRR.

Answer:

• Differential Amplifier: The amplifier, which is used to amplify the voltage difference between two input-lines neither of which is grounded, is called differential amplifier. This reduces the amount of noise injected into the amplifier, because any noise appearing simultaneously on both the input-terminals as the amplifying circuitry rejects it being a common mode signal.
• CMRR: It can be defined as the ratio of differential voltage-gain to common made voltage gain. If a differential amplifier is perfect, CMRR would be infinite because in that case common mode voltage gain would be zero.

Question No. 11: Difference between a four point starter and three point starters?

Answer: The shunt connection in four point starter is provided separately from the line where as in three point states it is connected with line which is the drawback in three point starters

Question No. 12: How do you select a cable size (Cu & Al) for a particular load?
Answer: At first calculate the electrical current of the load, after that derate the electrical current considering derating factor(depending on site condition and laying of cable) after choose the cable size from cable catalogue considering derating electrical current. After that measure the length of cable required from supply point of load to load point. Calculate the voltage drop which will max 3% (resistance and reactance of cable found from cable catalogue of selecting cable) if voltage drop>3%
then choose next higher size of cable.

Question No. 13: Why computer humming sound occurred in HT transmission line?
Answer: This computer humming sound is coming due to ionization (breakdown of air into charged particles) of air around transmission conductor. This effect is called as Corona effect, and it is considered as power loss.

Question No. 14: Capacitor is load free component but why ampere meter shows current when capacitor bank breaker closes?
Answer: As we know that Electrical is having two type of load, Active and Reactive. Capacitor is a reactive load which is not considering as a load, & its factor is Isin@ .Meter is design based on Current RMS value because of it meter is showing the current RMS value.

Question No. 15: Explain the process of commutation in a dc machine. Explain what are inter-poles and why they are required in a dc machine.

Answer: Commutation: It is phenomenon when an armature coil moves under the influence of one pole- pair; it carries constant current in one direction. As the coil moves into the influence of the next pole- pair, the current in it must reverse. This reversal of current in a coil is called commutation. Several coils undergo commutation simultaneously. The reversal of current is opposed by the static coil emf and therefore must be aided in some fashion for smooth current reversal, which otherwise would result in sparking at the brushes. The aiding emf is dynamically induced into the coils undergoing commutation by means of Compoles or Interpoles, which are series excited by the armature current. These are located in the interpolar region of the main poles and therefore influence the armature coils only when these undergo commutation.

Question No. 16: There are a Transformer and an induction machine. Those two have the same supply. For which device the load current will be maximum? And why?

Answer: The motor has max load current compare to that of transformer, because the motor consumes real power and the transformer is only producing the working flux and it is not consuming. Hence the load current in the transformer is because of core loss so it is minimum.

Question No. 17: Why ELCB can't work if N input of ELCB do not connect to ground?

Answer: ELCB is used to detect earth leakage fault. Once the phase and neutral are connected in an ELCB, the current will flow through phase and that much current will have to return neutral so resultant current is zero. Once there is aground fault in the load side, current from phase will directly pass through earth and it will not return through neutral through ELCB. That means once side current is going and not returning and hence because of this difference in current ELCB will trip and it will safe guard the other circuits from faulty loads. If the neutral is not grounded, fault current will definitely high and that full fault current will come back through ELCB, and there will be no difference in current.

Question No. 18: Explain the working principal of the circuit breaker?

Answer: Circuit Breaker is one which makes or breaks the circuit. It has two contacts namely fixed contact & moving contact. Under normal condition the moving contact comes in contact with fixed contact thereby forming the closed contact for the flow of current. During abnormal & faulty conditions (when current exceeds the rated value) an arc is produced between the fixed & moving contacts & thereby it forms the open ckt. Arc is extinguished by the Arc quenching media like air, oil, vacuum etc.

Question No. 19: What is the function of anti-pumping in circuit breaker?

Answer: when breaker is close at one time by close push button he anti pumping contractor prevent re close the breaker by close push button after if it already close.

Question No. 20: What is Buchholz relay and the significance of it into the transformer?

Answer:  Buchholz relay is a device which is used for the protection of transformer from its internal faults; it is a gas based relay. whenever any internal fault occurs in a transformer, the Buchholz relay at once gives a horn for sometime, if the transformer is isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own tripping mechanism.

Electrical Job Interview Questions and Answers - Part 09

Question No. 01: What is the principle of motor?

Answer: Whenever a current carrying conductor is placed in a magnetic field it produce turning or twisting movement is called as torque.

Question No. 02: What is the Polarization index value? Give simple definition of polarization index?

Answer: Its ratio between insulation resistance (IR) i.e. megger value for 10 min to insulation resistance for 1 min. It ranges from 5-7 for new motors & normally for motor to be in good condition it should be Greater than 2.5

Question No. 03: What will happen when power factor is leading in distribution of power?

Answer: If there is high power factor, i.e. if the power factor is close to one:

1. Losses in form of heat will be reduced,
2. Cable becomes less bulky and easy to carry, and very cheap to afford, and
3. It also reduces over heating of transformers.

Question No. 04: What is encoder, how it function?

Answer: An encoder is a device used to change a signal (such as a bit stream) or data into a code. The code may serve any of a number of purposes such as compressing information for transmission or storage, encrypting or adding redundancies to the input code, or translating from one code to another. This is usually done by means of a programmed algorithm, especially if any part is digital, while most analog encoding is done with analog circuitry.

Question No. 05: What is electric traction?

Answer: Traction means using the electric power for traction system i.e. for railways, trams, trolleys etc. electric traction means use of the electricity for all these. Now a days, magnetic traction is also used for bullet trains. Basically dc motors aroused for electric traction systems.

Question No. 06: When voltage increases then current also increases then what is the need of over voltage relay and over current relay? Can we measure over voltage and over current by measuring current only? 
Answer: No, we can't sense the over voltage by just measuring the current only because the current increases not only for over voltages but also for under voltage (As most of the loads are non-linear in nature). So, the over voltage protection & over current protection are completely different. Over voltage relay meant for sensing over voltages & protect the system from insulation break down and firing. Over current relay meant for sensing any internal short circuit, over load condition, earth fault thereby reducing the system failure & risk of fire. So, for a better protection of the system, it should have both over voltage & over current relay.

Question No. 07: What will happen if DC supply is given on the primary of a transformer?
Answer: Mainly transformer has high inductance and low resistance. In case of DC supply there is no inductance, only resistance will act in the electrical circuit. So, high electrical current will flow through primary side of the transformer. So for this reason coil and insulation will burn out.

Question No. 08: What are the advantages of freewheeling diode in a Full Wave rectifier?
Answer: It reduces the harmonics and it also reduces sparking and arching across the mechanical switch so that it reduces the voltage spike seen in an inductive load

Question No. 09: What is the function of interposing current transformer?
Answer: The main function of an interposing current transformer is to balance the currents supplied to the relay where there would otherwise be an imbalance due to the ratios of the main current transformers.  Interposing current transformer is equipped with a wide range of taps that can be selected by the user to achieve the balance required.

Question No. 10: What is excitation?
Answer: Excitation is applying an external voltage to DC shunt coil in DC motors.

Question No. 11: In three pin plug 6 Amp. 220 V AC rating. Why earth pin diameter is higher than other two pin? What its purpose? 

Answer: Because Current flow in the conductor is inversely proportional to the conductor diameter. So, if any short circuits occur in the system, first high currents bypassed in the Earthling terminal. (R=Pl/a area of the conductor increases resistance value decreases) 

Question No. 12: What is electrical diversity factor in electrical installations? 
Answer: Electrical diversity factor is the ratio of the sum of the individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system, or part of the system, under consideration. Electrical diversity factor is usually more than one.

Question No. 13: What is rated speed?
Answer: At the time of motor taking normal current (rated current) the speed of the motor is called rated speed. It is a speed at which any system take small current and give maximum efficiency.

Question No. 14: Which type of motor is used in trains, what is the rating of supply used explain Working principal?

Answer: Dc series is in the trains to get high starting torque while starting of the trains and operating voltage is 1500v dc.

Question No. 15: In a Tap changing transformer where is the tap connected, is it connected in the primary side or secondary side?

Answer: Tapings are connected to high voltage winding side, because of low current. If we connect tapings to low voltage side, sparks will produce while tap changing operation due to high current.

Question No. 16: State Maximum power transfer theorem

Answer: The Maximum power transfer theorem explains about the load that a resistance will extract from the network. This includes the maximum power from the network and in this case the load resistance is being is equal to the resistance of the network and it also allows the resistance to be equal to the resistance of the network. This resistance can be viewed by the output terminals and the energy sources can be removed by leaving the internal resistance behind.

Question No. 17: Define the following terms:-

• Reliability,
• Maximum demand,
• Reserve-generating capacity, 
• Availability (operational).

Answer:

• Reliability: It is the capacity of the power system to serve all power demands without failure over long periods.
• Maximum Demand: It is maximum load demand required in a power station during a given period.
• Reserve generating capacity: Extra generation capacity installed to meet the need of scheduled downtimes for preventive maintenance is called reserve-generating capacity.
• Availability: As the percentage of the time a unit is available to produce power whether needed by the system or not.

Question No. 18: State the factors, for the choice of electrical system for an aero turbine.

Answer: The choice of electrical system for an aero turbine is guided by three factors:

• Type of electrical output: dc, variable- frequency ac, and constant- frequency ac.
• Aero turbine rotational speed: constant speed with variable blade pitch, nearly constant speed with simpler pitch- changing mechanism or variable speed with fixed pitch blades.
• Utilization of electrical energy output: in conjunction with battery or other form of storage, or interconnection with power grid.

Question No. 19: Define the following: Average demand, Maximum demand, Demand factor, Load factor.

Answer:

• Average Demand: the average power requirement during some specified period of time of considerable duration is called the average demand of installation.
• Maximum Demand: The maximum demand of an installation is defined as the greatest of all the demand, which have occurred during a given period. It is measured accordingly to specifications, over a prescribed time interval during a certain period.
• Demand Factor: It is defined as the ratio of actual maximum demand made by the load to the rating of the connected load.
• Load Factor: It is defined as the ratio of the average power to the maximum demand.

Question No. 20: How does Zener phenomenon differ from Avalanche breakdown?

Answer: The phenomenon when the depletion region expands and the potential barrier increases leading to a very high electric field across the junction, due to which suddenly the reverse current increases under a very high reverse voltage is called Zener effect. Zener-breakdown or Avalanche breakdown may occur independently or both of these may occur simultaneously. Diode junctions that breakdown below 5 V are caused by Zener Effect. Junctions that experience breakdown above 5 V are caused by avalanche-effect. The Zener-breakdown occurs in heavily doped junctions, which produce narrow depletion layers. The avalanche breakdown occurs in lightly doped junctions, which produce wide depletion layers.

 Link for the interview books:interview book

hope this helps you to crack the interview.
Share with your friends     

                                                                                                           --Engineersvision