ELECTRICAL ENGG. INTERVIEW QUESTION PART 1 to 4

 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)

PART 0:Question that ask at the beginning of the engineering interview

1. Tell me about the most challenging engineering project that you have been involved with during the past year.

This question is apt because it immediately pulls the candidate into a specific mindset of describing what type of project details they consider to be ‘challenging,’ and why. Assuming the interviewer probes beyond the initial question, the response will reveal not only what particular project stretched them, but also how they navigated through the challenges of that project and what the outcome was.

2. Describe the most challenging written technical report or presentation that you’ve had to complete.

Again, the question focuses on a ‘most challenging’ initiative, but this time it further focuses in on writing and/or communication skills. For a candidate to successfully navigate this question, you might expect details of the report or presentation, building a case for complexity and/or difficulty, but also evidence of their communications prowess (or, lack thereof).

3. What checks and balances do you use to make sure that you don’t make mistakes?

This is a great thinking question. For those candidates who may never have thought about a checks-and-balances system in this way, the question may spur their creative juices.

More than likely, a strong engineering candidate will have some sort of system in place, even if they aren’t consciously aware of it. It may be as simple as running ideas and initiatives by a colleague . Or, maybe they have notes or a digital system they use to assess tasks or project steps.  The bottom line, though, is that you are seeking a candidate who can elucidate an ability to reduce the margin of error, thus improve quality, productivity and ultimately, corporate profit margins.

In addition to Doyle’s suggestions, I consider the following questions to be illuminating:

4. In what instances have you demonstrated leadership skills, and how would you describe your style?

Most employees, at one point or another and regardless of level, have been tasked to guide another person or team of people. This may be for a very specific task or could involve a longer, more drawn-out initiative. By prompting the candidate with this question, you gain a sense of how they define leadership as well as how they exude leadership in their day to day. In today’s more collaborative culture, it is important to have employees who know how to step up and steer the way, when needed.

5. What processes have you helped develop or singularly created that enhanced engineering performance capabilities? What was the impact of this process on your team’s/department’s/division’s performance and/or how did this impact something bigger company-wide?

This multilayered question is important. Because of the process-oriented nature of engineering roles, it is likely that the candidate can provide a solid story that describes a process they helped build. The key will be their ability to translate this initiative further into a bottom-line impact.

6. Have you been involved in cost reductions? Have you shaved expenses or been asked to work within a slashed budget?

In most economic climates, cost reduction factors into a company’s success at some point. Even if the company itself is doing well, it may be that a particular project has exceeded the allocated budget, for example. Or, perhaps a new competitor has challenged profit margins, creating a battening down of financial hatches. Whatever the case, a candidate’s ability to prove they can help cut costs or work within a suddenly decreased budget framework is important.

Doyle’s article inspired the final four interview questions helpful in shaping specialty-specific engineering questions:

7. Why did you select civil(any branch) engineering as your field of major?

Of course, you can substitute civil engineering with a mechanical, chemical, electrical engineer, etc. Answers to this question may unearth a plethora of insights about the candidate. Some candidates may answer from the gut and with passion when describing how they decided to major in this area of specialty. Others may offer a more pragmatic or mechanical response. Moreover, a candidate’s ability to be concise or to ramble on about their career-paving story could enhance, or detract, from their interview.

8. How much oil is necessary to pollute the ocean?

This is an intriguing question and one that most likely would be posed to an environmental engineer. A candidate’s ability to apply logical reasoning, prior experience or studies on the topic will reveal itself in the answer.

9. Describe the differences between Corsim and Vissim models.

Corsim and Vissim models relate to traffic flow; as such, this question would apply well to civil engineers. By demonstrating an understanding of the differences between the models, a candidate may prove they have the depth of knowledge to make the appropriate decision when called to employ either — or both — of these traffic analysis tools.

10.  Describe the process you use for writing a piece of code, from requirements to delivery.

Designed for a software engineer, this question is helpful to extract a candidate’s ability to communicate a process. It will also help the interviewer determine strengths and potential weaknesses in the candidate’s chosen processes.

Electrical Engineering Important Interview Questions and Answers - Part 01

Question No. 01: What is Ferranti effect?

Answer: Output voltage is greater than the input voltage or receiving end voltage is greater than the sending end voltage.

Question No. 02: 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. 03: Why use the VCB at High Transmission System? Why can't use ACB?

Answer: Actually the thing is vacuum has high arc quenching property compare to air because in VCB, the die electric strength is equal to 8 times of air. That always vacuum used as in HT breaker and air used as in LT.

Question No. 04: Enlist types of dc generator?

Answer: D.C. Generators are classified into two types

1) Separately excited D.C. generator

2) Self excited D.C. generator,

Which is further classified into:

• Series
• Shunt and
• Compound

(Which is further classified into cumulative and differential). 

Question No. 05: What happen if we give 220 volts dc supply to a bulb or tube light?

Answer: Bulbs (devices) for AC are designed to operate such that it offers high impedance to AC supply. Normally they have low resistance. When DC supply is applied, due to low resistance, the current through lamp would be so high that it may damage the bulb element.

Question No. 06: What is "pu" in electrical engineering?

Answer: Pu stands for per unit and this will be used in power system single line diagram there it is like a huge electrical circuit with no of components (generators, transformers, loads) with different ratings (in MVA and KV). To bring all the ratings into common platform we use pu concept in which, in general largest MVA and KV ratings of the component is considered as base values, then all other component ratings will get back into this basis. Those values are called as pu values. (p.u=actual value/base value).

Question No. 07: How electrical power is generated by an A.C Generator? 

Answer: For the generation of elect power we need a prime mover which supplies mechanical power input to the alternator, can be steam turbines, or hydro turbines .When poles of the rotor moves under the armature conductors which are placed on the stator, field flux cut the armature conductor, therefore voltage is generated and is of sinusoidal in nature, due to polarity change of rotor poles (i.e.) N-S-N-S.

Question No. 08: What is the difference between earth resistance and earth electrode resistance?
Answer: Only one of the terminals is evident in the earth resistance. In order to find the second terminal we should recourse to its definition: Earth Resistance is the resistance existing between the electrically accessible part of a buried electrode and another point of the earth, which is far away.
The resistance of the electrode has the following components:

1. The resistance of the metal and that of the connection to it.
2. The contact resistance of the surrounding earth to the electrode.

Question No. 09: Two bulbs of 100 W and 40 W respectively connected in series across a 230 V supply which bulb will glow bright and why?

Answer: Since two bulbs are in series they will get equal amount of electrical current but as the supply voltage is constant across the bulb (P=V2/R).So the resistance of 40 W bulb is greater and voltage across 40 W is more (V=IR) so 40 W bulb will glow brighter.

Question No. 10: If we give 2334 A, 540 V on Primary side of 1.125 MVA step up transformer, then what will be the Secondary Current, If Secondary Voltage = 11 KV? 
Answer: As we know the Voltage & current relation for transformer- V1/V2 = I2/I1
We Know, VI= 540 V; V2=11 KV or 11000 V; I1= 2334 Amps.
By putting these value on Relation-
540/11000= I2/2334
So, I2 = 114.5 Amps

Question No. 11: Why field rheostat is kept in minimum position while armature rheostat at maximum position?
Answer: In motors at the time of starting the armature resistance is introduced to reduce the high starting current and the field resistance is kept minimum to have high starting
torque.

Question No. 12: Why transformer ratings are in kVA? 

Answer: Since the power factor of transformer is dependent on load we only define VA rating and does not include power factor .In case of motors, power factor depend on construction and hence rating of motors is in KW (Kilo-Watts) and include power factor.

Question No. 13: In the magnetic fluxes, what is the role of armature reaction?
Answer: The armature flux has an important role for the running condition. This armature flux can oppose the main flux or it may support the main flux for better running condition. This effect of supporting and opposing of main flux to armature flux is called armature reaction.

Question No. 14: Why star delta starter is preferred with induction motor?

Answer: Star delta starter is preferred with induction motor due to following reasons:

• Starting current is reduced 3-4 times of the direct current due to which voltage drops and hence it causes less losses.
• Star delta starter circuit comes in circuit first during starting of motor, which reduces voltage 3 times, that is why current also reduces up to 3 times and hence less motor burning is caused.
• In addition, starting torque is increased and it prevents the damage of motor winding.

Question No. 15: Explain advantages of storage batteries

Answer: Few advantages of storage batteries are mentioned below:

• Most efficient form of storing energy portably.
• Stored energy is available immediately because there is no lag of time for delivering the stored energy.
• Reliable source for supply of energy.
• The energy can be drawn at a fairly constant rate.

Question No. 16: State Thevenin’s Theorem:

Answer: According to Thevenin’s theorem, the current flowing through a load resistance
Connected across any two terminals of a linear active bilateral network is the ratio open circuit voltage (i.e. the voltage across the two terminals when RL is removed) and sum of load resistance and internal resistance of the network. It is given by Voc / (Ri + RL).

Question No. 17: Explain different types of D.C motors? Give their applications

Answer: Different type of DC motors and their applications are as follows:-

• Shunt motors: It has a constant speed though its starting torque is not very high. Therefore, it is suitable for constant speed drive, where high starting torque is not required such as pumps, blowers, fan, lathe machines, tools, belt or chain conveyor etc.
• Service motors: It has high starting torque & its speed is inversely proportional to the loading conditions i.e. when lightly loaded, the speed is high and when heavily loaded, it is low. Therefore, motor is used in lifts, cranes, traction work, coal loader and coal cutter in coal mines etc.
• Compound motors: It also has high starting torque and variable speed. Its advantage is, it can run at NIL loads without any danger. This motor will therefore find its application in loads having high inertia load or requiring high intermittent torque such as elevators, conveyor, rolling mill, planes, presses, shears and punches, coal cutter and winding machines etc.

Question No. 18: What is 2 phase motor?

Answer: A two phase motor is a motor with the starting winding and the running winding have a phase split. E.g. AC servo motor, where the auxiliary winding and the control winding have a phase split of 90 degree.

Question No. 19: 1 ton is equal to how many watts?

Answer: 1 ton = 12000 BTU/hr and to convert BTU/hr to horsepower, 12,000 × 0.0003929 = 4.715 hp therefore 1 ton = 4.715 × 0.746 = 3.5 KW.

Question No. 20: What is ACSR cable and where we use it?

Answer: ACSR means Aluminum conductor steel reinforced, this conductor is used in transmission & distribution.

Question No. 21: What does quality factor depend on in resonance?

Answer: Quality factor q depends on frequency and bandwidth.

Question No. 22: What does the KVAR means?
Answer: The KVAR indicates the electrical power. KVAR means “Kilo Volt Amperes with Reactive components”

Question No. 23: What's the other name for short circuit?
Answer:  Fault.

Question No. 24: How can you relate power engineering with electrical engineering?

Answer: Power engineering is a sub-division of electrical engineering. It deals with the generation, transmission, and distribution of energy in electrical form. The design of all power equipment also comes under power engineering. Power engineers may work on the design and maintenance of the power grid i.e. called on-grid systems and they might work on off-grid systems that are not connected to the system.

Question No. 25: Why back emf used for a dc motor? Highlight its significance.

Answer: The induced emf developed when the rotating conductors of the armature between the poles of the magnet, in a DC motor, cut the magnetic flux, opposes the current flowing through the conductor, when the armature rotates, is called back emf. Its value depends upon the speed of rotation of the armature conductors. In starting, the value of back emf is zero.

Electrical Interview Questions for Freshers and Experienced - Part 02

Question No. 01: What is the difference between electronic regulator and ordinary rheostat regulator for fans?

Answer: The difference between the electronic and ordinary regulator is that in electronic regulator power losses are less i.e. for as we decrease the speed, the electronic regulator gives the power needed for that particular speed but in case of ordinary rheostat type regulator, the power wastage is same for every speed and no power is saved. In electronic regulator TRIAC is employed for speed control by varying the firing angle speed is controlled but in rheostat control resistance is decreased by steps to achieve speed control.

Question No. 02: If one lamp connects between two phases it will glow or not?
Answer: If the voltage between the two phases is equal to the lamp voltage then the lamp will glow. When the voltage difference is big it will damage the lamp and when the difference is smaller the lamp will glow depending on the type of lamp.

Question No. 03: How to determine capacitor tolerance codes?
Answer: In electronic circuits, the capacitor tolerance can be determined by a code that appears on the casing. The code is a letter that often follows a three-digit number (such as 130Z).The first two are the 1st and 2nd significant digits and the third is a multiplier code. Most of the time the last digit tells you how many zeros to write after the first two digits and these are read as Pico-Farads. 

Question No. 04: What is the different between digital phase converter and ordinary phase converter? 
Answer: Digital phase converter is a recent development in phase converter technology that utilizes proprietary software in a powerful microprocessor to control solid state power switching components. This microprocessor, called a digital signal processor (DSP), monitors the phase conversion process, continually adjusting the input and output modules of the converter to maintain perfectly balanced three-phase power under all load conditions.

Question No. 05: What is meant by regenerative braking?

Answer: When the supply is cut off for a running motor, it still continue running due to inertia. In order to stop it quickly we place a load (resistor) across the armature winding and the motor should have maintained continuous field supply, so that back e.m.f voltage is made to apply across the resistor and due to load the motor stops quickly. This type of breaking is called as "Regenerative Breaking".

Question No. 06: What is the difference between MCB & MCCB, Where it can be used?

Answer: MCB is miniature circuit breaker which is thermal operated and use for short circuit protection in small current rating circuit. MCCB moulded case circuit breaker and is thermal operated for over load current and magnetic operation for instant trip in short circuit condition. Under voltage and under frequency may be inbuilt. Normally it is used where normal current is more than 100A.

Question No. 07: Why ac solenoid valves attract the plunger even though we interchange the terminal? Will the poles changes?
Answer: Yes because the pole changes for every half-cycle of ac voltage so the polarity of AC voltage is continuously changing for every half cycle. So, interchanging of terminals in ac system does not show any difference. That's why the ac solenoid attracts the plunger even though its terminals are interchanged. 

Question No. 08: What is the main use of rotary phase converter? 
Answer: Rotary phase converter will be converting single phase power into true balanced 3 phase power, so it is often called as single phase to three phase converter .Often the advantages of 3 phase motors, and other 3 phase equipment, make it worthwhile to convert single phase to 3 phase so that small and large consumers need not want to pay for the extra cost of a 3 phase service but may still wish to use 3 phase equipment.

Question No. 09: Why electricity in India is in the multiples of 11 like 11kv, 22kv, 33kv?

Answer: Transformer Induced voltage equation contains 4.44 factors. 

E = 4.44 × f × T × π 

E - Induced emf per phase 

T - Number of turns 

f - Frequency 

π - Maximum flux per pole 

From the equation we see that E is proportional to 4.4 and it is intern multiple of 11. So always transmission voltage is multiple of 11 

Question No. 10: What are the types of power in electrical power?

Answer: There are normally three types of power are counted in electrical power. They are,

• Apparent power
• Active power
• Reactive power

Question No. 11: Name the types of motors used in vacuum cleaners, phonographic appliances, vending machines, refrigerators, rolling mills, lathes, power factor improvement and cranes.

Answer: Following motors are used: -

• Vacuum cleaners- Universal motor.
• Phonographic appliances – Hysteresis motor.
• Vending machines – Shaded pole motor.
• Refrigerators – Capacitor split phase motors.
• Rolling mills – Cumulative motors.
• Lathes – DC shunt motors.
• Power factor improvement – Synchronous motors.

Question No. 12: Explain different losses in a transformer.

Answer: There are two types of losses occurring in transformer:

Constant losses or Iron losses: The losses that occur in the core are known as core losses or iron losses. Two types of iron losses are:

• eddy current loss
• Hysteresis loss. 

These losses depend upon the supply voltage, frequency, core material and its construction. As long as supply voltage and frequency is constant, these losses remain the same whether the transformer is loaded or not. These are also known as constant losses.

Variable losses or copper losses: when the transformer is loaded, current flows in primary and secondary windings, there is loss of electrical energy due to the resistance of the primary winding, and secondary winding and they are called variable losses. These losses depend upon the loading conditions of the transformers. Therefore, these losses are also called as variable losses.

Question No. 13: Explain forward resistance, static resistance and dynamic resistance of a PN junction diode.

Answer:

• Forward Resistance: Resistance offered in a diode circuit, when it is forward biased, is called forward-resistance.
• DC or Static Resistance: DC resistance can be explained as the ratio of the dc-voltage across the diode to the direct current flowing through it.
• AC or Dynamic Resistance: It can be defined as the reciprocal of the slope of the forward characteristic of the diode. It is the resistance offered by a diode to the changing forward current.

Question No. 14: Why is the starting current high in a DC motor?

Answer: In DC motors, Voltage equation is V = Eb - IaRa

Where, (V = Terminal voltage, Eb= Back emf in Motor, Ia = Armature current, Ra = Armature resistance).

At starting, Eb is zero.

Therefore, V = IaRa, Ia = V/Ra, where Ra is very less like 0.01ohm.i.e, Ia will become enormously increased.

Question No. 15: Define IDMT relay?

Answer: It is an inverse definite minimum time relay. In IDMT relay its operating is inversely proportional and also a characteristic of minimum time after which this relay operates. It is inverse in the sense; the tripping time will decrease as the magnitude of fault current increases.

Interview Questions for Electrical Engineers - Part 03

Question No. 01: what is meant by insulation voltage in cables? Explain it?

Answer: It is the property of a cable by virtue of it can withstand the applied voltage without rupturing it is known as insulation level of the cable.

Question No. 02:  What is power factor? Whether it should be high or low? Why?

Answer: Power factor should be high in order to get smooth operation of the system. Low power factor means losses will be more. It is the ratio of true power to apparent power. It has to be ideally 1. If it is too low then cable over heating &equipment overloading will occur. If it is greater than 1 then load will act as capacitor and starts feeding the source and will cause tripping.(if pf is poor ex: 0.17 to meet actual power load has to draw more current (V constant),result in more losses if pf is good ex: 0.95 to meet actual power load has to draw less current (V constant),result in less losses).

Question No. 03: What is Automatic Voltage regulator (AVR)?

Answer: AVR is an abbreviation for Automatic Voltage Regulator. It is important part in Synchronous Generators; it controls the output voltage of the generator by controlling its excitation current. Thus it can control the output Reactive Power of the Generator.

Question No. 04: Give two basic speed control scheme of D.C. shunt motor?

Answer:

1. By using flux control method: in this method a rheostat is connected across the field winding to control the field current. So by changing the current the flux produced by the field winding can be changed, and since speed is inversely proportional to flux speed can be controlled.
2. Armature control method: in this method a rheostat is connected across armature winding by varying the resistance the value of resistive drop (IaRa) can be varied, and since speed is directly proportional to Eb-IaRa the speed can be controlled.

Question No. 05: Advantages of (Variable Voltage Variable Frequency (VVVF) drives over non-vvvf drives for EOT cranes?

Answer:

1. Smooth start and stop.
2. No jerking of load.
3. Exact positioning
4. Better protection for motor.
5. High/low speed selection.
6. Reliability of break shoe.
7. Programmable break control.
8. Easy circuitry.
9. Reduction in controls
10. Increases motor life

Question No. 06: What is MARX CIRCUIT?

Answer: It is used with generators for charging a number of capacitor in parallel and discharging them in series. It is used when voltage required for testing is higher than the available.

Question No. 07: How can you start-up the 40 W tube light with 230 V AC/DC without using any choke/Coil?

Answer: It's possible by means of Electronic choke. Otherwise it's not possible to ionise the particles in tube light, with normal voltage.

Question No. 08: What is derating? Why it is necessary, it is same for all means for drives, motors, and cables. 

Answer: The current currying of cables will change depending upon the site temperature (location of site), type of run (it will run through duct, trench, buried etc.), number of tray, depth of trench, and distance between cables. Considering this condition actual current currying capacity of cable reduce than current currying capacity (which given to cable Catalog) this is called derating.

Question No. 09: What is the power factor of an alternator at no load?
Answer: At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor.

Question No. 10: What is power quality meter?
Answer: Power Quality meters are common in many industrial environments. Small units are now available for home use as well. They give operators the ability to monitor the both perturbations on the power supply, as well as power used within a building, or by a single machine or appliance. In some situations, equipment function and operation is monitored and controlled from a remote location where communication is via modem or high-speed communication lines. So we can understand the importance of power measurement through power quality meters.

Question No. 11: When we connect the large capacitor bank in series? 
Answer: we connect large capacitor bank in series to improve the voltage power supply at the load end in balanced transmission line when there is considerable voltage drop along the balanced transmission line due to high impedance of the line. So in order to bring the voltage at the load terminals within its limits i.e. (+ or - %6) of the rated high terminal voltage) the large capacitor bank is used in series.

Question No. 12: How to determine alternating current frequency?
Answer: Zero crossings of the sine wave to trigger a monostable (pulse generator) is a way to determine alternating current frequency. A fixed width pulse is generated for each cycle. Thus there are "n" pulses per second, each with a constant energy. The more pulses there are per second, the more the energy. The pulses are integrated (filtered or averaged) to get a steady DC voltage which is proportional to frequency. This voltage can then be displayed on an analogue or digital voltmeter, indicating frequency. This method is more suitable than a direct counter, as it can get good accuracy in a second or so.

Question No. 13: What type’s domain of Laplace transforms? What behavior can Laplace transform predict how the system work?
Answer: Types domain of Laplace transforms is s-domain, Laplace transforms provide a method to find position, and acceleration or voltage the system will have.

Question No. 14: What is the voltage gain or transfer function of amplifier?
Answer: Vout/Vin

Question No. 15: Why AC systems are preferred over DC systems?

Answer: Due to following reasons, AC systems are preferred over DC systems:

1. It is easy to maintain and change the voltage of AC electricity for transmission and distribution.
2. Plant cost for AC transmission (circuit breakers, transformers etc) is much lower than the equivalent DC transmission
3. From power stations, AC is produced so it is better to use AC then DC instead of converting it.
4. When a large fault occurs in a network, it is easier to interrupt in an AC system, as the sine wave current will naturally tend to zero at some point making the current easier to interrupt.

Interview Questions and Answers on Electrical Engineering - Part 04

Question No. 01: State Norton’s Theorem

Answer: The Norton’s theorem explains the fact that there are two terminals and they are as follows:

• One is terminal active network containing voltage sources 
• Another is the resistance that is viewed from the output terminals. The output terminals are equivalent to the constant source of current and it allows giving the parallel resistance.

The Norton’s theorem also explains about the constant current that is equal to the current of the short circuit placed across the terminals. The parallel resistance of the network can be viewed from the open circuit terminals when all the voltage and current sources are removed and replaced by the internal resistance.

Question No. 02: State the methods of improving power factor?

Answer: Methods of improving power factor:

• By connecting static capacitors in parallel with the load operating at lagging power factor. 
• A synchronous motor takes a leading current when over excited and therefore behaves like a capacitor.
• By using phase advancers to improve the power factor of induction motors. It provides exciting ampere turns to the rotor circuit of the motor. By providing more ampere-turns than required, the induction motor can be made to operate on leading power factor like an overexcited synchronous motor.

Question No. 03: What is Difference between Induction Motor & Servo Motor?

Answer:

1. An induction motor has high inertia and servo motor has a very low inertia. Hence servo motors are used in applications where instant and accurate positioning of load is required.
2. As in induction motor stator is formed by coils while in servo motor stator is formed with permanent magnet. So in induction motor very high inertia is there while in servo inertia is low. Due to that servo is very easy to stop at instant. So it is used in position control while in  application where at instant position stop is not required induction motor is used
3. Servo motor is actual normal ac induction motor with added fetcher as

• Low inertia rotor
• High torque break and
• Inbuilt encoder for speed and position feedback. All this components combine work with servo drive.

Question No. 04: What are the advantages of star-delta starter with induction motor?

Answer: The advantages of star-delta starter with induction motor are:

1. The main advantage of using the star delta starter is reduction of current during the starting of the motor. Starting current is reduced to 3-4 times of current of Direct online starting.
2. Hence the starting current is reduced; the voltage drops during the starting of motor in systems are reduced.

Question No. 05: Why we do 2 types of earthing on transformer, i.e.: body earthing & neutral earthing, what is function. I am going to install a 500 kVA transformer & 380 kVA DG set what should the earthing value?

Answer: The two types of earthing are Familiar as Equipment earthing and System earthing.

In Equipment earthing: body (non conducting part) of the equipment should be earthed to safeguard the human beings.

System Earthing: In this neutral of the supply source (Transformer or Generator) should begrounded. With this, in case of unbalanced loading neutral will not be shifted, so that unbalanced voltages will not arise. We can protect the equipment also. With size of the equipment (transformer or alternator) and selection of relying system earthing will be further classified into directly earthed, Impedance earthing, resistive (NGRs) earthing.

Question No. 06: What is SF6 Circuit Breaker?

Answer: SF6 is Sulphur hexafluoride gas. If this gas is used as arc quenching medium in a Circuit breaker means SF6 CB.

Question No. 07: How many types of cooling method of transformers are there?

Answer: Cooling methods of transformers are:

1. ONAN (oil natural, air natural)
2. ONAF (oil natural, air forced)
3. OFAF (oil forced, air forced)
4. ODWF (oil direct, water forced)
5. OFAN (oil forced, air forced)

Question No. 08: What is meant by armature reaction?

Answer: The effect of armature flu to main flux is called armature reaction. The armature flux may support main flux or opposes main flux.

Question No. 09: What is the significance of vector grouping in Power Transformers?

Answer: Every power transformer has a vector group listed by its manufacturer. Fundamentally it tells you the information about how the windings are connected (delta or wye) and the phase difference between the current and voltage. E.g. DYN11 means Delta primary, Wye Secondary and the current is at 11 o’clock referred to the voltage.

Question No. 10: What are the advantages of speed control using thyristor?

Answer: Advantages:

1. Fast Switching characteristics than Mosfet, BJT, IGBT
2. Low cost
3. Higher Accuracy

Question No. 11: How tube light circuit is connected and how it works?

Answer: A choke is connected in one end of the tube light and a starter is in series with the circuit. When supply is provided, the starter will interrupt the supply cycle of AC. Due to the sudden change of supply the chock will generate around 1000 volts. This volt will capable of to break the electrons inside the tube to make electron flow. Once the current passes through the tube the starter circuit will be out of part. Now there is no change of supply causes choke voltage normalized and act as minimize the current.

Question No. 12: What is reverse power relay?

Answer: Reverse Power flow relay are used in generating stations’ protection. A generating stations is supposed to fed power to the grid and in case generating units are off, there is no generation in the plant then plant may take power from grid. To stop the flow of power from grid to generator we use reverse power relay.

Question No. 13: What are Motor Generator Sets and explain the different ways the motor generator set can be used? 

Answer: Motor Generator Sets are a combination of an electrical generator and an engine mounted together to form a single piece of equipment. Motor generator set is also referred to as a genset, or more commonly, a generator. The motor generator set can be used in the following different ways: 

1. Alternating current (AC) to direct current (DC)
2. DC to AC
3. DC at one voltage to DC at another voltage
4. AC at one frequency to AC at another harmonically-related frequency

Question No. 14: What are HRC fuses and where it is used?
Answer: HRC stand for "high rupturing capacity" fuse and it is used in distribution system for electrical transformers.

Question No. 15: Difference between megger test equipment and contact resistance meter test instruments? 
Answer: Megger test equipment used to measure cable electric resistance, conductor continuity, and phase identification where as contact resistance meter test instruments used to measure low resistance like relays, contactors.

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