Basic Electrical Engineering Interview Questions and Answers (Part -1)

Static electricity means electricity at rest. If we join a charged conductor to another conductor, electricity flows from one to the other. This way an electric current is produced, which lasts for a moment only. Static electricity is no use. Rubbing of two different materials having different electrons produces this.

Flow of electrons in any conductor is called electric current. Its symbol is ‘I’ and measuring unit is Ampere measured by ammeter or ampere meter.

It is the pressure that moves the electrons to flow in any conductor. It is also known as electromotive force voltage. Its symbol is ‘E’ or ‘V’ and measuring unit is volt measured by voltmeter.

It is the voltage available at the terminal of the source of supply. It’s symbol is VT.

VT = emf – P.D

Resistance is the property of a substance, which gives opposition to flow of electrons through itself. Its measuring unit is ohm and measured by ohmmeter, multi meter, wheat stone bridge, and post office box. There are two types of resistances and they are fixed resistance and variable resistance.

The international ampere is defined as that steady current which, flowing through a solution of silver nitrate, deposits silver at the rate of 0.001118 gm/sec.

The international volt is defined as 1/1.0183 of the emf of a Weston cadmium cell. It is that difference of potential which, when applied to a conductor whose resistance is 1 (one) international ohm, will cause a current of 1 (one) international ampere to flow.

The matter is defined as anything, which possesses weight and occupies space and can be in any of three forms solid, liquid or gaseous. The matter consists of three ingredients, which are protons, neutrons and electrons.

In a single phase system, there is one neutral wire and one power wire with current flowing between them.

In a 3 phase system there are three power wires. Delta and wye are the two types of circuits use to maintain equal load across a three phase system.In the delta configuration, no neutral wire is used. The wye configuration uses both a neutral and a ground wire.

(Note: In high voltage systems, the neutral wire is not usually present for a three phase system.)

If your load is IDEAL then It’s not necessary to have neutral wire in alternating current system. Because power station transmits power in 3 wires (delta connection) only, there is no neutral wire at all. Now if we connect the ideal balanced 3 Phase load at the end, that means all the power will be consumed by the load but in real life this is not possible. your load will have some unused/imbalanced current left , which needs to carry back. so system needs neutral wire for carrying imbalance current.

  • Green: It is connected to the grounding terminal in an outlet box and run from the outlet box to the ground bus bar within an electric panel
  • Blue and Yellow wire: These wires are also used to carry power but are not wiring the outlets for common plug-in electrical devices. They are used for the live wire pulled through the conduct. You will see yellow wire in the fan, structure lights, and switched outlets.
  • Black wire: This wire is used for power supply in all circuits. Any circuits with this color is considered hot or live. It is never used for a neutral or ground wire.
  • White and Gray: This color wire is used as a neutral wire. It carries the current (unbalanced load) to the ground. You can join white and gray only to other white and gray wires.
  • Red wire: This color wire is a secondary live wire in a 220 volt circuit and used in some types of inter connection. You can join the red wire to another red wire or to a black wire.

Cables are categorized into three types:

  • Low tension cables- transmits voltage upto 1000 volts
  • High tension cables- transmits voltage up to 23000 volts
  • Super tension cables- transmits voltage up to 66kv to 132kv

Miniature Circuit Breaker

MCB is Miniature Circuit Breaker used for short circuit protection and is rated for smaller short circuit current. It provides you the facility to break the contact of the circuit from supply if the supply current exceeds from the rated current (not more than 100 A).

Moulded Case Circuit Breaker

MCCB is Moulded Case Circuit Breaker which is used for the same purpose of MCB, but rated for very high circuit current (upto 1000 A).

Earth leakage circuit breaker

Earth-leakage circuit breaker (ELCB) is a safety device used in electrical installations with high Earth impedance to prevent shock. It detects small stray voltages on the metal enclosures of electrical equipment, and interrupts the circuit if a dangerous voltage is detected.

Residual current device or residual current circuit breaker

Residual current device (RCD) or residual current circuit breaker (RCCB) is used for the same purpose of ELCB, but it disconnects a circuit whenever it detects that the electric current is not balanced between the phase conductor. RCCB is best because it will detect any earth fault. ELCB only detects earth faults that flow back through the main earth wire so this is why they stopped being used.

The speed of electricity or electrons is 297842 km (186000 miles) per second.

On all alternators, transformers neutral is earthed. Human body is conductor and when touched to the live conductor it completes its shortest root though the body and the body gets electric shock in which its nervous system, the heart, respiratory system may cease to function.

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Fuse is a weakest point in an electrical circuit, which melts when the excess current flows through it in the electrical circuit. The materials, which can be used in fuses, are tin, lead, zinc, silver, antimony, copper, and aluminium, etc.

The ratio of minimum fusing current and the current rating of fusing element is
called the fusing factor. Fusing factor = minimum fusing current / current rating of fusing element. Its value
is always more than 1 (one).

Soldering is the process of joining two metals with an alloy whose melting point is less that of the materials to be soldered. Soldering at high temperature using brass as solder is called brazing or hard
soldering. The composition of the fine solder (soft solder) is tin 60% and lead 40%. Its melting point is 190°C and is widely used.

a. Battery (chemical source)
b. Generator (magnetism)
c. Thermocouple (heat generated)
d. Light (photo electric or solar cell)
e. Pressure (piezo electricity)
f. Friction (static electricity)

a. Physical effect.
b. Chemical effect.
c. Magnetic effect.
d. Heating effect.
e. X-ray effect.

Destructive burning of any material is called the fire. Fire is the result of combining fuel, oxygen and heat. If any one among three is separated the fire will come to end.

a. The resistance of the conductor is directly proportional to the length of the conductor.
b. The resistance of the conductor is inversely proportional to the cross-section of the conductor.
c. The resistance of the conductor depends on the nature of the material by which it is made. That is specific resistance of the material.
d. The resistance of the conductor depends on its temperature. The formula to find the resistance of the substance is below.
R = ρ L Ω/A
Where ρ is the constant for the material called its specific resistance or resistivity.

Specific resistance of the material is the resistance of a piece of unit length and unit cross-section (unit cube of that material). That is the resistance between the opposite faces of unit cube of the material.
Or the specific resistance of any material is the resistance offered by the opposite face of that material. The unit of specific resistance is Ω/cm3 , Ω/inch3, Ω/m3.

Temperature co-efficient of the resistance of any substance is, change in its original resistance due to the change in temperature. The temperature co-efficient of resistance of material is the ratio of increase in resistance of 1°C rise in temperature to the original resistance of the material (strictly at 0°C). Formula for the resistance measurement is, Rt = R0 (1+αt)


Rt Æ Resistance at t°C.
R0 Æ Resistance at 0°C.
α Æ Temperature co-efficient.
t Æ Temperature rise.

The effects of temperature on resistance are
a. In certain pure metals such as gold, copper, silver, aluminium etc. the resistance increases with increasing temperature at fairly regular manner. Such metals possess positive temperature co-efficient of resistance.

b. In certain materials (alloys) such as eureka, nichrome etc. the change in resistance due to increasing temperature is irregular and negligible for a considerable range of temperature.

c. In case of certain materials belongs to insulators, electrolytes such as paper, rubber, glass, mica, carbon, acids, alkalies etc. the resistance decreases with increasing temperature at fairly regular manner. Such materials posses negative co-efficient of resistance.

a. Low voltage: Voltage not exceeding 250V. That is 0 – 250V.
b. Medium voltage: Voltage above 250V upto 650V comes under medium voltage.
c. High voltage: Voltage above 650V upto 33 kV comes under high voltage.
d. Extra high voltage: Above 33 kV voltages are extra high voltages.

It is the unit of charge. One (1) coulomb is the quantity of electricity, which is circulated by a current of one (1) ampere in one second. The letter Q denotes it. So that 1 coulomb = 1 amp * 1 second.

Farad is the unit of capacitance and the letter F denotes it. A condenser has a capacitance of one (1) farad, if it is capable to maintain a charge of one coulomb under a potential difference of one volt between its plates.

1 farad = 1 coulomb / 1 volt. = Q/V.

It is the unit of inductance and the letter H denotes it. A circuit has inductance of one henry, if an electro-motive force of one volt if induced in that circuit, when the current in that circuit changes at the rate of one ampere per second.
1 henry = 1 volt sec / ampere.

Conductance is the property of the conductor, which allows the flow of electric current through it. Conductance is denoted by the letter G and is reciprocal of resistance. The unit of conductance is mho. A substance, which posses conductance as its major property can be called as a good conductor.

A substance, which will not allow the flow of electric current to pass through it is called the insulator. The conductance and conductivity is zero in insulators. Insulators are used to isolate the electric current from neighbouring parts. Insulators will not allow the leakage of current, short-circuiting current, shock to the operator and isolates the electric current safely with out any diversion to any other place.

Qualities of good insulator
a. It should be flexible
b. It should have good mechanical strength
c. It should easily moulded into any shape
d. It should not be effected by acid
e. It should be non-inflammable
f. It should have very high specific resistance to prevent leakage current
g. It should be withstand high temperature. Because insulators posses negative temperature coefficient of resistance. That is resistance decreases with increasing temperature
h. It should have high dielectric strength

A conducting element used for converging (centering) current to and from a medium is called electrode. There are two types of electrode. A positive and other is negative.

Substances such as metals, which have large number of free electrons are said to
offer a low resistance to the flow of electrons under the influence of emf and hence
are called conductors.
Conductors are used to conduct electricity from one place to another place due to its
major property conductance. Conductors are classified into three main groups.
a. Good conductors.
b. Semi conductors.
c. Fair conductors.

Properties of good conductor
a. It posses very low resistance or specific resistance.
b. It posses more conductance and there by conducts electricity readily through it.
c. It is a good conductor of heat.
d. It is highly resistance to corrosion by liquid.
e. It must be malleable and ductile.
f. It must be flexible.
g. It posses better tensile strength.
h. It should not react with climatic conditions.
i. It can be drawn in very fine wires.
j. It must be readily joinable.
k. It must be very low in cost.
l. It must available in plenty.

a. Silver
b. Silver copper alloy
c. Copper (Hard down and Annealed)
d. Gold
e. Zinc
f. Platinum
g. Tin
h. Aluminum
i. Iron
j. Brass
k. Phosphorous bronze
l. Nickel
m. Lead
n. Germanium silver
o. Antimony
p. Platinoid
q. Mercury
r. Bismuth
s. Platinum iridium

Semiconductors posses less conductivity (conductance) than good conductors. That
is semiconductors gives opposition (resistance) to the flow of free electrons than that
of good conductor.
Examples for semiconductor are
a. Dilute acid
b. Metallic ores
c. See water
d. Moist earth
e. Silicone
f. Germanium

Fair conductors are the materials, which have less conductivity than that of semiconductor. Fair conductor gives more opposition to the flow of free electrons than that of semiconductors.
Examples for fair conductors are
a. Charcoal
b. Coke
c. Carbon
d. Plumbago

Resistors posses high resistance, but less conductance. This property is well utilized to convert electrical energy into heat energy. Common application of resistors is production of heaters. Examples are eureka, carbon, nichrome, tungsten, manganin, germanium, and tentalum. In case of heaters, electrical iron and soldering iron etc the heating element are made of nichrome, but in lamps filament is made of tungsten.

Resistor: a fixed resistance connected permanently in the circuit for limiting the current to definite value is called the resistor.
Rheostat: a variable resistance by sliding contacts on it the current can be varied is called rheostat.
Potential divider: when a resistance is used to develop a voltage drop it is called a potential divider.

Solder is an alloy of lead and tin mixed in different proposition as per the work to be done. In some cases certain % of bismuth and cadmium is also added to decrease the melting point of the solder. Antimony increases the melting point of the solder. Bismuth has a special quality in comparing to most of other metals. That is it expands when it cools. This property helps to shrink the solder and there by it allows
the joint become firm. The quality of the solder depends on the % of tin in the solder. To get stronger joint add more tin in the solder. For electrical work fine solder of 1½ part tin and 1 part lead is used and for sheet
metal works soft solder of 1 part tin and 1 part lead is used.

Flux is a cleanser and is used to remove and prevent oxidation of the metals, allowing the solder to flow from and to, to unite the solder more firmly with the surface to be joined.

Electricity has affinity (fondness) to pass through peripheral surface of the conductor. This effect of electricity flowing through the peripheral surface of the conductor is known as skin effect.

In a closed electrical circuit, at a constant temperature, the ratio between the resulting unvarying current or direct current and applied voltage is a constant. That constant is known as resistance.
In simple manner Ohm’s law says that, in a closed electrical circuit the current is directly proportional to the voltage and inversely proportional to the resistance of the
I = V/R or
R = V/I or
E = IR

It is that circuit where two or more electrical consuming devices are connected so as to provide only one path to the flow of current.
Characteristics of series circuit
a. It has only one path for the flow of current.
b. If any breakage happens the whole system will be out of that circuit.
c. It is very difficult to find the fault.
d. Individual voltage drop depends on individual resistance (V = I rn).
e. The total resistance of a series circuit is the sum of the individual resistance.
f. Addition of the resistance increases total resistance and decreases the current.
g. Individual device will not get its full efficiency.

It is that circuit where two or more electrical consuming devices are connected so as to provide as many parallel paths to the flow of current.

Characteristics of parallel circuit
a. As many parallel paths as there are devices.
b. Individual devices will get its full efficiency.
c. Breakage in one circuit will not affect the other circuit.
d. Current in each device is different according to the resistance of the device.
e. If the individual resistance increases the total resistance will decrease
(1/R = 1/ r1 + 1/ r2 +1/ rn)
f. The reciprocal of total resistance is equal to the sum of the reciprocal of
individual parallel resistances (1/R = 1/ r1 + 1/ r2 +1/ rn).
g. Individual conductance is inversely proportional to the individual resistance.
h. If two same value resistors are connected in parallel circuit the total resistance is
the resistance of one resistor. And the total of parallel circuit resistance will be
less than the least resistance in that circuit.

Capacitor or condenser is a device to store electrical energy and to release it into the circuit of which the capacitor forms a part.
Capacity of a capacitor depends on following factors
a. Capacity of the capacitor is directly proportional to the area of the plate.
b. Capacity is inversely proportional to the distance between the plate. That is if the
distance is more the capacity decreases or if the distance is less the capacity more.
c. It depends on the nature of dielectric constant.

The voltage rating of the capacitor depends on the distance between the plates of the capacitor. If the voltage exceeds, the electrons across the space between the plates can result in permanent damage to the capacitor.

a. Paper capacitor.
b. Rolled plastic cover or polyester type capacitor.
c. Mica capacitor.
d. Silver mica capacitor.
e. Ceramic capacitor.
f. Electrolytic capacitor.

In series circuit the resultant capacitance 1/CT = 1/c1+1/c2 + 1cn farad.
In parallel circuit the resultant capacitance CT = c1 + c2 + cn farad.

C = Q/E farad.


Basic Electrical Maintenance Questions and Answers (Part-2) – Coming Soon