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Electrical Engineering Interview Questions and Answers

Section 1: Electrical interview questions and answers about switchgear and protection

What are the functions of protective relays?

To detect the fault and initiate the operation of the circuit breaker to isolate the defective element from the rest of the system, thereby protecting the system from damages consequent to the fault.

Give the consequences of short circuit.

Whenever a short -circuit occurs, the current flowing through the coil increases to an enormous value. If protective relays are present , a heavy current also flows through the relay coil, causing it to operate by closing its contacts .The trip circuit is then closed , the circuit breaker opens and the fault is isolated from the rest of the system. Also, a low voltage may be created which may damage systems connected to the supply.

Define protected zone.

Protected zones are those which are directly protected by a protective system such as relays, fuses or switch gears .If a fault occurring in a zone can be immediately detected and or isolated by a protection scheme dedicated to that particular zone.

What are unit system and non unit system?

A unit protective system is one in which only faults occurring within its protected zone are isolated. Faults occurring elsewhere in the system have no influence on the operation of a unit system. A non unit system is a protective system which is activated even when the faults are external to its protected zone.

What is primary protection?

Is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker .It serves as the first line of defenses.

What is back up protection?

It is the second line of defense , which operates if the primary protection fails to activate within a definite time delay.

Name the different kinds of over current relays.

Define energizing quantity.

It refers to the current or voltage which is used to activate the relay into operation.

Define operating time of a relay.

It is defined as the time period extended from the occurrence of the fault through the relay detecting the fault to the operation of the relay.

Define resetting time of a relay.

It is defined as the time taken by the relay from the instant of isolating the fault to the moment when the fault is removed and the relay can be reset.

What are over and under current relays?

Over current relays are those that operate when the current in a line exceeds a predetermined value. (e .g : Induction type non-directional/directional over current relay, differential over current relay)whereas undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined value.(e g: differential over-voltage relay)

Mention any two applications of differential relay.

Protection of generator & generator transformer unit; protection of large motors and bus bars .

What is biased differential bus zone reduction?

The biased beam relay is designed to respond to the differential current in terms of   its fractional relation to the current flowing through the protected zone. It is essentially an over – current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force.

State the types of faults in Power system.

i) Symmetrical fault

ii) Unsymmetrical fault

How is ‘arcing ground’ avoided?

Arcing ground can be prevented by earthing the neutral.

What is surge absorber? How do they differ from surge diverter?

The device which reduces the steepness of the wave front of a particular surge and thus minimizes the danger due to overvoltage is surge absorber or modifier. The surge absorber absorbs the surge energy whereas surge diverter diverts the surge to earth.

Define the term ‘Insulation coordination’.

Insulation coordination is the correlation of the insulation of various equipments in a power system to the insulation of the protective devices used for the protection of those equipments against over voltages.

What is the difference between fuse and protective relay?

A relay is a switch; it is not usually meant to open under fault conditions (high current), and would likely fail if done so.

A fuse is a fault protection device. If a short circuit develops, large currents will flow, and the fuse link will melt, causing the circuit to open.

There are also protective relays, which are devices that sense fault, or other undesirable conditions, and are used to send trip commands to breakers.

Define the following terms as related to protective relaying:

(a) pick-up current,

(b) reset value

Pick-up current: The threshold value of the operating quantity above which the relay operates is known as pick-up current.

Reset value: It is the value which elapses from the moment the actuating quantity falls below its reset value to the instant when the relay comes back to its normal position.

What are the requirements of line protection?

1) Speed

2) Selectivity

3) Sensibility

4) Reliability

5) Simplicity

6) Economy

Explain the need for overlapping the zones of protection.

Adjacent protective zones must overlap each other, failing which a fault on the boundary of the zones may not lie in any of the zones, and hence no circuit breaker would trip .Thus the overlapping between the adjacent zones is unavoidable.

What is time setting multiplier and plug setting multiplier?

A relay is generally provided with control to adjust the time of operation. This adjustment is known as time setting multiplier.

Plug-setting multiplier: The ratio of actual fault current in the relay coil to the pick-up current is called plug-setting multiplier.

What is the need for power system earthing?

Earthing is done in power system by connecting the non-current carrying parts of electrical equipment or the neutral point of the supply system to the general mass of earth in such a way that an immediate discharge of electrical energy takes place without danger at all times.

Section 2: Electrical Interview Questions and Answers About Operating Principles and Relay Characteristics

What is the need of relay coordination?

The operation of a relay should be fast and selective, ie, it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system.

Mention the short comings of Merz Price scheme of protection applied to a power transformer.

In a power transformer, currents in the primary and secondary are to be compared. As these two currents are usually different, the use of identical transformers will give differential current, and operate the relay under no-load condition.

Also, there is usually a phase difference between the primary and secondary currents of three phase transformers. Even CT’s of proper turn-ratio are used, the differential current may flow through the relay under normal condition.

What are the various faults to which a turbo alternator is likely to be subjected?

Failure of steam supply; failure of speed; over current; over voltage; unbalanced loading; stator winding fault.

What is an under frequency relay?

An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value.

Define the term pilot with reference to power line protection.

Pilot wires refer to the wires that connect the CT’s placed at the ends of a power transmission line as part of its protection scheme. The resistance of the pilot wires is usually less than 500 ohms.

What are the features of directional relay?

High speed operation;

high sensitivity;

ability to operate at low voltages;

adequate Short-time thermal ratio;

burden must not be excessive.

What are the causes of over speed and how alternators are protected from it?

Sudden loss of all or major part of the load causes over-speeding in alternators.

Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over-speed occurs.

What are the main types of stator winding faults?

Fault between phase and ground; fault between phases and inter-turn fault involving turns of the same phase winding.

Give the limitations of Merz Price protection.

Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault.

What are the uses of Buchholz’s relay?

Buchholz relay is used to give an alarm in case of incipient (slow-developing) faults

in the transformer and to connect the transformer from the supply in the event of severe internal faults. It is usually used in oil immersion transformers with a rating over 750KVA.

What is meant by directional relay?

A relay which responds to the flow of power in a definite direction with reference to the location of C.T’s and P.T’s.

Why is under frequency relay required in a power system?

The frequency of induced e.m.f of synchronous generators is maintained constant at constant speed. In the event of over speeding/under speeding due to load, the system frequency tends to vary and hence incorporation of frequency relay is required.

A relay is connected to 400/5 ratio current transformer with current setting of 150%.

Calculate the plug setting multiplier when circuit carries a fault current of 4000A.

Pick up value=Rated secondary current of C.T*Current setting


Fault current in relay coil = 4000*5/400=50A

Hence P.S.M = 50/7.5=7

What is static relay?

It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device.

What are the advantages of static relay over electromagnetic relay?

Low power consumption as low as 1mW

No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts

No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc.

A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks.

This is not possible in electromagnetic relays

Static relay is compact

Superior operating characteristics and accuracy

Static relay can think, programmable operation is possible with static relay

Effect of vibration is nil, hence can be used in earthquake-prone areas

Simplified testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with transducers.

What is a programmable relay?

A static relay may have one or more programmable units such as microprocessors or microcomputers in its circuit.

What is CPMC?

It is combined protection, monitoring and control system incorporated in the static system.

Classify the different types of over current relay based on the inverse time characteristics.

Induction disc and induction cup relays are the different types of over current relays.

Section 3: Electrical Interview Questions and Answers About Apparatus Protection

What are the types of graded used in line of radial relay feeder?

Definite time relay and inverse-definite time relay.

What are the various faults that would affect an alternator?

(a) Stator faults

Phase to phase faults

Phase to earth faults

Inter turn faults

(b) Earth faults

Fault between turns

Loss of excitation due to fuel failure

(c) Over speed

Loss of drive

Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low pressure casing

(d) Fault on lines

Fault on busbars

Why neutral resistor is added between neutral and earth of an alternator?

In order to limit the flow of current through neutral and earth a resistor is introduced between them.

What is the backup protection available for an alternator?

Over current and earth fault protection is the backup protections.

What are faults associated with an alternator?

(a) External fault or through fault

(b) Internal fault

Short circuit in transformer winding and connection

Incipient or slow developing faults

What are the main safety devices available with transformer?

Oil level guage, sudden pressure delay, oil temperature indicator, winding temperature indicator .

What are the limitations of Buchholz relay?

(a) Only fault below the oil level are detected.

(b) Mercury switch setting should be very accurate, otherwise even for vibration; there can be a false operation.

(c) The relay is of slow operating type, which is unsatisfactory.

What are the problems arising in differential protection in power transformer and how are they overcome?

Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires.

Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.

Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this.

Magnetizing inrush current appears wherever a transformer is energized on its primary side producing harmonics. No current will be seen by the secondary.

CT’s as there is no load in the circuit. This difference in current will actuate the differential relay. A harmonic restraining unit is added to the relay which will block it when the transformer is energized.

What is REF relay?

It is restricted earth fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay.

What is over fluxing protection in transformer?

If the turn’s ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing.

Why busbar protection is needed?

Fault level at busbar is high

The stability of the system is affected by the faults in the bus zone.

A fault in the bus bar causes interruption of supply to a large portion of the system network.

What are the merits of carrier current protection?

Fast operation, auto re-closing possible, easy discrimination of simultaneous faults.

What are the errors in CT?

Ratio error

Percentage ratio error = [(Nominal ratio – Actual ratio)/Actual ratio] x 100

The value of transformation ratio is not equal to the turns ratio.

Phase angle error:

Phase angle _ =180/_[(ImCos _-I1Sin _)/nIs]

What is field suppression?

When a fault occurs in an alternator winding even though the generator circuit breaker is tripped, the fault continues to fed because EMF is induced in the generator itself.

Hence the field circuit breaker is opened and stored energy in the field winding is discharged through another resistor. This method is known as field suppression.

What are the causes of bus zone faults?

Failure of support insulator resulting in earth fault

Flash-over across support insulator during over voltage

Heavily polluted insulator causing flash over

Earthquake, mechanical damage etc.

What are the problems in bus zone differential protection?

Large number of circuits, different current levels for different circuits for external faults.

Saturation of CT cores due to dc component and ac component in short circuit currents.

The saturation introduces ratio error.

Sectionalizing of the bus makes circuit complicated.

Setting of relays need a change with large load changes.

What is power swing?

During switching of lines or wrong synchronization surges of real and reactive power flowing in transmission line causes severe oscillations in the voltage and current vectors. It is represented by curves originating in load regions and traveling towards relay characteristics.

What is meant by time-graded system protection?

Time setting of over current relays at different locations is graded or having different times of operation. This system is suitable for radial feeder in which power flow is only in one direction.

What are the main safety devices available with transformer?

Buchholz device

Earth fault relays

Over current relays

Differential system

Harmonic restraint and Harmonic blocking

Explain why the secondary of C.T should not open.

If the CT secondary is open circuited, the voltage across the secondary rises to a high value.

Open circuiting of secondary’s results in zero secondary current, hence reduced back emf.

The working flux increases and core gets saturated.

The primary gets overheated and core also gets overheated .Voltages is induced in secondary by electro-magnetic induction.

The peak value of secondary voltage on open circuit may be several times the r.m.s value since the core is saturated and waveform of voltage is distorted. This may cause danger to personnel working on secondary side.

Therefore when primary current is flowing, secondary should not be open.

Which type of relays is best suited for transmission lines?

Distance relays are best suited for very high voltage transmission lines.

What type of relay is suited for generator protection?

Differential relay or longitudinal differential protection is best suited for generator protection.

Electrical Engineering Interview Questions about Theory of Circuit Interruption

What is resistance switching?

It is the method of connecting a resistance in parallel with the contact space(arc).

The resistance reduces the re striking voltage frequency and it diverts part of the arc current.

It assists the circuit breaker in interrupting the magnetizing current and capacity current.

What do you mean by current chopping?

When interrupting low inductive currents such as magnetizing currents of the transformer, shunt reactor, the rapid deionization of the contact space and blast effect may cause the current to be interrupted before the natural current zero. This phenomenon of interruption of the current before its natural zero is called current chopping.

What are the methods of capacitive switching?

Opening of single capacitor bank

Closing of one capacitor bank against another

What is an arc?

Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition.

Give the two methods of arc interruption?

High resistance interruption:-the arc resistance is increased by elongating, and

splitting the arc so that the arc is fully extinguished _ Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in ac.

What is re striking voltage?

It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished.

What is meant by recovery voltage?

The power frequency r .m .s voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage.

What is RRRV?

It is the rate of rise of re striking voltage, expressed in volts per microsecond. It is closely associated with natural frequency of oscillation.

List the factors affecting transient recovery voltage.

Power factor

Armature reaction

Circuit conditions

Explain the static and dynamic arc characteristics.

A plot of instantaneous values  of voltage between the electrodes of a burning arc against the corresponding value of current give the arc characteristics .By changing the rate of change of current the characteristics shift .If the current changes rapidly with time, that characteristics is dynamic and if the rate of change of current is small the characteristics is static.

What are the problems encountered in the interruption of capacitive currents?

The amount of energy to be dissipated during the short interval of breaking is very high as compared to the conventional a.c circuit breakers.

The natural zero current does not occur as in the case of a.c circuit breakers .Resistance switching and efficient cooling by forcing the liquid or air blast are used to dissipate the high amount of energy, whereas artificial means are provided to bring the current to zero.

Section 4: Basic Electrical Engineering questions and answers about circuit breakers

What is circuit breaker?

It is a piece of equipment used to break a circuit automatically under fault conditions. It breaks a circuit either manually or by remote control under normal conditions and under fault conditions.

Write the classification of circuit breakers based on the medium used for arc extinction?

Air break circuit breaker

Oil circuit breaker

Minimum oil circuit breaker

Air blast circuit breaker

SF6 circuit breaker

Vacuum circuit breaker

What is the main problem of the circuit breaker?

When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem.

What are demerits of MOCB?

Short contact life

Frequent maintenance

Possibility of explosion

Larger arcing time for small currents

Prone to restricts

What are the advantages of oil as arc quenching medium?

It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties

It acts as an insulator and permits smaller clearance between line conductors and earthed components.

What are the hazards imposed by oil when it is used as an arc quenching medium?

There is a risk of fire since it is inflammable. It may form an explosive mixture with arc. So oil is preferred as an arc quenching medium.

What are the advantages of MOCB over a bulk oil circuit breaker?

It requires lesser quantity of oil

It requires smaller space

There is a reduced risk of fire

Maintenance problem are reduced

What are the disadvantages of MOCB over a bulk oil circuit breaker?

The degree of carbonization is increased due to smaller quantity of oil o There is difficulty of removing the gases from the contact space in time . The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization.

What are the types of air blast circuit breaker?

Arial-blast type

Cross blast


What are the advantages of air blast circuit breaker over oil circuit breaker?

The risk of fire is diminished

The arcing time is very small due to rapid buildup of dielectric strength between contacts

The arcing products are completely removed by the blast whereas oil deteriorates with successive operations

What are the demerits of using oil as an arc quenching medium?

The air has relatively inferior arc quenching properties

The air blast circuit breakers are very sensitive to variations in the rate of rise of re striking voltage

Maintenance is required for the compression plant which supplies the air blast

What is meant by electro negativity of SF6 gas?

SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro negativity of SF6 gas.

What are the characteristic of SF6 gas?

It has good dielectric strength and excellent arc quenching property. It is inert, non- toxic, noninflammable and heavy. At atmospheric pressure, its dielectric strength is2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer oil.

Write the classifications of test conducted on circuit breakers.

Type test

Routine test

Reliability test

Commissioning test

What are the indirect methods of circuit breaker testing?

Unit test

Synthetic test

Substitution testing

Compensation testing

Capacitance testing

What are the advantages of synthetic testing methods?

The breaker can be tested for desired transient recovery voltage and RRRV.

Both test current and test voltage can be independently varied. This gives flexibility to the test

The method is simple

With this method a breaker capacity (MVA) of five time of that of the capacity of the test plant can be tested.

How does the over voltage surge affect the power system?

The over voltage of the power system leads to insulation breakdown of the  equipment’s. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line.

What is pick up value?

It is the minimum current in the relay coil at which the relay starts to operate.

Define target.

It is the indicator used for showing the operation of the relay.

Define reach.

It is the distance up to which the relay will cover for protection.

Define blocking.

It means preventing the relay from tripping due to its own characteristics or due to additional relays.

Define a over current relay.

Relay which operates when the current ia a line exceeds a predetermined value.

Define an undercurrent relay?

Relays which operates whenever the current in a circuit drops below a  predetermined value.

Mention any 2 applications of differential relays.

Protection of generator and generator-transformer unit: protection of large motors and bus bars

Mention the various tests carried out in a circuit breaker at HV labs.

Short circuit tests, Synthetic tests& direct tests.

Mention the advantages of field tests.

The circuit breaker is tested under actual conditions like those that occur in the network. Special occasions like breaking of charging currents of long lines, very short line faults, interruption of small inductive currents etc… can be tested by direct testing only.

State the disadvantages of field tests.

The circuit breaker can be tested at only a given rated voltage and network capacity.

The necessity is to interrupt the normal services and to test only at light load conditions.

Extra inconvenience and expenses in installation of controlling and measuring equipment in the field.

Define composite testing of a circuit breaker.

In this method the breaker is first tested for its rated breaking capacity at a reduced voltage and afterwards for rated voltage at a low current. This method does not give a proper estimate of the breaker performance.

State the various types of earthing.

Solid earthing, resistance earthing , reactance Earthing , voltage transformer Earthing and       zig-zag transformer earthing.

What are arcing grounds?

The presence of inductive and capacitive currents in the isolated neutral system leads to formation of arcs called as arcing grounds.

What is arc suppression coil?

A method of reactance grounding used to suppress the arc due to arcing grounds.

State the significance of single line to ground fault.

In single line to ground fault all the sequence networks are connected in series. All the sequence currents are equal and the fault current magnitude is three times its sequence currents.

What are symmetrical components?

It is a mathematical tool to resolve unbalanced components into balanced components.

State the three sequence components.

Positive sequence components, negative sequence components and zero sequence components.

Define positive sequence component.

It has 3 vectors equal in magnitude and displaced from each other by an angle 120 degrees and having the phase sequence as original vectors.

Define zero sequence components.

It has 3 vectors having equal magnitudes and displaced from each other by an angle zero degrees.

State the significance of double line fault.

It has no zero sequence component and the positive and negative sequence networks are connected in parallel.

Define negative sequence component.

It has 3 vectors equal in magnitude and displaced from each other by an angle 120 degrees and has the phase sequence in opposite to its original phasors.

State the different types of faults.

Symmetrical faults, unsymmetrical faults and open conductor faults.

State the various types of unsymmetrical faults.

Line to ground, line to line and double line to ground faults

Mention the withstanding current in our human body.

9 mA

State the different types of circuit breakers.

Air, Oil, Vacuum circuit breakers.

Define per unit value.

It is defined as the ratio of actual value to its base value.

Mention the inductance value of the Peterson’s coil.


Define single line diagram.

Representation of various power system components in a single line is defined as single line diagram.

Differentiate between a fuse and a circuit breaker.

Fuse is a low current interrupting device. It is a copper or an aluminum wire. Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.

How direct tests are conducted in circuit breakers?

Using a short circuit generator as the source.

Using the power utility system or network as the source.

What is dielectric test of a circuit breaker?

It consists of overvoltage withstand test of power frequency lightning and impulse voltages. Tests are done for both internal and external insulation with switch in both open and closed conditions.

What are the basic requirements of circuit breaker?

It must make or break a circuit either manually or by remote control under normal conditions.

It must break a circuit automatically under fault conditions.

Must make a circuit either manually or by remote control under fault conditions.

Give the advantages of SF6 Circuit breaker over air blast circuit breaker.

Size of the switchgear is smaller.

Gas is inflammable,no danger of fire or explosion.

Same gas is recirculated in the circuit.Hence SF6 gas requirement is small in the long run.

The sealed construction avoids the contamination by moisture, dust, sand etc.

Its dielectric strength is 2.5 times that of air.

Suggest the type of circuit breakers for the following:3.3-33KV,132-220KV and 400-760KV.

For       3.3KV -33KV –Air blast circuit breaker, Vaccum circuit breaker

132 – 220KV –Oil circuit breaker

400 – 760KV_ SF6 circuit breaker

What is the need of multiple breakers in circuit breaking?

Breakers are provided in a substation for radial feeders separately and LV side of each power transformer and HV side also. This facilitates backup protection and also discrimination of breaking of various circuit elements. For this multiple breakers are used in circuit breaking.

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