Electrical Interview Questions & Answers
ELECTRICAL INTERVIEW QUESTIONS AND ANSWERS.
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1. 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.
2. Why the capacitors works 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.
3. 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 circuit, Arc is extinguished by the Arc Quenching
media like air, oil, vacuum etc.
4. How many types of colling system it transformers?
Answer:
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)
5. What is the function of anti-pumping in circuit breaker?
Answer :when breaker is close at one time by close push button, the anti-pumping contactor
prevent re close the breaker by close push button after if it already close.
6. What is stepper motor? What is its uses?
Answer: Stepper motor is the electrical machine which act 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.
7. 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 circuit where measurements of high voltage and high current is involved, they are used there.
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, 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 circuits. 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\5A and it has
to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be
given to ammeter .and after measuring 8A we can calculate the primary current. Same is the
operation of PT but measuring voltage.
8. 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’s not
consuming. Hence the load current in the transformer is because of core loss so it is minimum.
9. 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)
10. What is the difference between Isolator and Circuit Breaker?
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 operate. 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 specification 1 is rated current and 2 is short circuit
breaking capacity and 3 is instantaneous tripping current .
11. What is Boucholz relay and the significance of it in to the transformer?
Answer: Boucholz 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 Boucholz relay at once gives a horn for some time, if the transformer is
isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own
tripping mechanism.
12. What is SF6 Circuit Breaker?
Answer
SF6 is Sulphur hexa Fluoride gas. If this gas is used as arc quenching medium in a Circuit
breaker means SF6 CB
Page | 2
ELECTRICAL INTERVIEW QUESTIONS AND ANSWERS.
Visit eeekenya.com more resources.
1. 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.
2. Why the capacitors works 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.
3. 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 circuit, Arc is extinguished by the Arc Quenching
media like air, oil, vacuum etc.
4. How many types of colling system it transformers?
Answer:
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)
5. What is the function of anti-pumping in circuit breaker?
Answer :when breaker is close at one time by close push button, the anti-pumping contactor
prevent re close the breaker by close push button after if it already close.
6. What is stepper motor? What is its uses?
Answer: Stepper motor is the electrical machine which act 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.
7. 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 circuit where measurements of high voltage and high current is involved, they are used there.
Page | 1
, 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 circuits. 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\5A and it has
to measure secondary current of 8000A.then ANS=8000*5\5000=8Aand this result will be
given to ammeter .and after measuring 8A we can calculate the primary current. Same is the
operation of PT but measuring voltage.
8. 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’s not
consuming. Hence the load current in the transformer is because of core loss so it is minimum.
9. 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)
10. What is the difference between Isolator and Circuit Breaker?
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 operate. 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 specification 1 is rated current and 2 is short circuit
breaking capacity and 3 is instantaneous tripping current .
11. What is Boucholz relay and the significance of it in to the transformer?
Answer: Boucholz 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 Boucholz relay at once gives a horn for some time, if the transformer is
isolated from the circuit then it stop its sound itself otherwise it trips the circuit by its own
tripping mechanism.
12. What is SF6 Circuit Breaker?
Answer
SF6 is Sulphur hexa Fluoride gas. If this gas is used as arc quenching medium in a Circuit
breaker means SF6 CB
Page | 2
