Table of Contents
electrical interview questions and answers
if I connect a capacitor to a generator load?
electrical interview questions and answers
Answer:
When you connect a capacitor to a generator, it helps to improve the power factor. This means it makes the generator work more efficiently. But this only helps if the generator’s engine is strong enough to handle the extra work. If the engine is too small, the generator can become overloaded because improving the power factor also increases the total energy the generator has to supply.
Also, never connect a capacitor when the generator is just starting up or if there is no other load connected. Doing this can harm the generator or cause it to behave unsafely.
2. Why do capacitors work only on AC (Alternating Current)?
electrical interview questions and answers
Answer:
A capacitor does not allow DC (Direct Current) to pass through. It completely blocks DC like a wall. But it allows AC to pass through easily. That’s why capacitors are used mostly in AC circuits—they work well there. In DC, they just stop the current from flowing after a short time.
3. What is the working principle of a circuit breaker?
electrical interview questions and answers
Answer:
A circuit breaker is a safety device that turns off electricity when there is a problem. It has two parts that touch each other—one is fixed, and the other can move. When everything is normal, these two parts stay connected, and electricity flows smoothly.
But if there is a fault—like too much current due to a short circuit or overload—an electric arc is created. This arc is dangerous, so the circuit breaker quickly pulls the two contacts apart and stops the flow of electricity. The arc is put out safely using air, oil, vacuum, or other materials. This keeps the system safe and prevents damage or fire.
How many types of cooling systems are used in transformers?
electrical interview questions and answers
Answer:
Transformers get hot when they work, so they need a cooling system to stay safe and work well. There are different types of cooling systems used in transformers:
- ONAN – Oil Natural, Air Natural
The oil and air move on their own without any help from fans or pumps. - ONAF – Oil Natural, Air Forced
Oil moves naturally, but air is pushed with the help of a fan. - OFAF – Oil Forced, Air Forced
Both oil and air are pushed using pumps and fans. - ODWF – Oil Direct, Water Forced
Oil is cooled using water, and both move with the help of machines. - OFAN – Oil Forced, Air Natural
Oil is pushed by a pump, and air moves naturally.
Q2: What is the function of anti-pumping in a circuit breaker?
electrical interview questions and answers
Answer:
When we press the close button of a circuit breaker, it closes the breaker. But sometimes, the button may get stuck or someone may press it again and again.
Anti-pumping makes sure the breaker doesn’t keep closing again and again if it’s already closed. It stops extra or repeated closing to avoid damage or mistakes.
Q3: What is a stepper motor? What are its uses?
electrical interview questions and answers
Answer:
A stepper motor is a special kind of motor that moves in small steps, not like a normal motor that spins around freely.
When we give it an electric signal, it takes one step. With more signals, it takes more steps.
It can move forward or backward – one step at a time.
This is useful in machines where we need exact movement, like in printers, robots, and other automatic systems.
Q4: Tell me in detail about CT and PT.
electrical interview questions and answers
Answer:
CT means Current Transformer.
PT means Potential Transformer.
They help us measure very high current or very high voltage safely.
- A CT is used to measure big currents.
- A PT is used to measure big voltages.
Normal meters can’t measure high values because they can get damaged. So, we use CTs and PTs. They work like small transformers.
Example for CT:
If the CT ratio is 5000/5 and the real current is 8000 A, then:
8000 × 5 ÷ 5000 = 8 A
So the meter sees only 8 A, and we can calculate the real current.
PT works in the same way, but for voltage.
Q5: There is a transformer and an induction motor. Both are given the same power supply. Which one will draw more load current? Why?
electrical interview questions and answers
Answer:
The motor will draw more load current than the transformer.
Why?
Because a motor uses real power to do work, like turning a fan or running a machine.
A transformer doesn’t use power like that. It just passes it on. It only needs a small amount of current to keep working, mostly for its internal losses.
So, the motor needs more current, and the transformer needs less.
What is an SF6 Circuit Breaker?
electrical interview questions and answers
SF6 is a special kind of gas called Sulphur Hexafluoride. This gas is used in some circuit breakers to stop electricity when needed. When there is a fault, the circuit breaker quickly cuts off the electric current. The SF6 gas helps cool down and put out the electric arc safely. These circuit breakers are very strong and are used where there is a lot of high-voltage electricity. They help keep things safe and working properly.
What is the Ferranti Effect?
electrical interview questions and answers
Imagine a very long power line. When not many things are using electricity at the end of the line, something strange can happen. The voltage at the far end becomes more than at the beginning. This is called the Ferranti Effect. It mostly happens in very long lines when there is a light load. It’s like sending a ball down a long tunnel, and it somehow comes out faster at the other end!
What is Insulation Voltage in Cables?
electrical interview questions and answers
Every electric cable has a cover around it. This cover is called insulation. It keeps the electricity inside the cable and stops it from leaking out. Insulation voltage means how much electric power the cable can safely carry without the insulation getting damaged. If we try to push more voltage than the cable can handle, the cover might break, and that can be dangerous. So, knowing the insulation voltage helps us choose the right cable for the job.
What is Power Factor? Should it be High or Low? Why?
electrical interview questions and answers
Power factor shows how well electricity is being used. It is the ratio between real power (which does useful work) and total power (which includes wasted energy too).
A high power factor is good. It means electricity is being used properly, and everything runs smoothly.
A low power factor is bad. It means more electricity is wasted, wires get hotter, and machines can get overloaded.
For example:
- If power factor is low, like 0.17, more current is needed to do the same work. This causes more heat and losses.
- If power factor is high, like 0.95, less current is needed, so there are fewer losses.
Ideal power factor is 1. But it should never go above 1, or the system may behave like a capacitor and cause tripping.
What is the Difference Between Isolator and Circuit Breaker?
electrical interview questions and answers
An Isolator is like a safety switch. It is used when machines need to be repaired. It is turned off by hand, and it should only be used when there is no electricity flowing. It is not used during faults.
A Circuit Breaker is a smart switch. It works automatically and can stop the electricity when there is a problem like a short circuit or overload. It helps protect people and machines. It works even when the electricity is flowing.
What is a Buchholz Relay and Why is it Used in a Transformer?
electrical interview questions and answers
A Buchholz Relay protects the transformer from inside problems. If anything goes wrong inside the transformer, the relay detects gas, gives an alarm sound, and trips the circuit if needed. It helps save the transformer from damage.
Why Do We Use Two Types of Earthing in Transformers: Body and Neutral Earthing?
electrical interview questions and answers
There are two types of earthing:
- Body Earthing (Equipment Earthing): This connects the metal parts of machines to the earth. If something goes wrong, the electricity will go into the ground, not into people. This keeps people safe.
- Neutral Earthing (System Earthing): This connects the neutral point of the transformer or generator to the earth. It helps keep the voltage balanced and protects the equipment during faults.
For a 500 kVA transformer and 380 kVA DG set, the earthing should be proper and strong. Normally, earthing resistance should be below 1 ohm. This ensures safety and smooth working.
: What is the difference between MCB and MCCB? Where are they used?
electrical interview questions and answers
Answer:
MCB means Miniature Circuit Breaker. It is used to protect small electric circuits from too much current or short circuits. It works using heat. MCBs are used in homes and small buildings where the current is low.
MCCB means Moulded Case Circuit Breaker. It can handle more current than MCBs. It works using both heat and magnet. The heat part helps when there is too much current for a long time. The magnet part acts quickly if there is a sudden short circuit. MCCBs are used in places like factories or big buildings where the current is more than 100 Amps.
Q2: What is the use of a Lockout Relay in high voltage (HT) systems?
electrical interview questions and answers
Answer:
A lockout relay is used to turn off power quickly and safely from one place. It is connected before or after an emergency stop switch. It works using a key switch. When the key is turned, power is cut off for many machines at once. This keeps people and machines safe. The relay can control many parts at the same time.
Q3: What is the difference between Earth Resistance and Earth Electrode Resistance?
electrical interview questions and answers
Answer:
Earth Resistance means how much the ground stops electricity from flowing. It is the resistance between a metal rod (electrode) buried in the ground and a far-away point in the earth.
Earth Electrode Resistance is made of two parts:
A) The resistance of the metal rod itself.
B) The resistance between the rod and the soil around it.
Both are important for safe grounding in electrical systems.
Q4: Which power plant has a high load factor?
electrical interview questions and answers
Answer:
Power plants that run all the time (called base load plants) have a high load factor. This means they work most of the time and produce electricity steadily. Hydro power plants have better efficiency compared to thermal or nuclear plants, so they are good for base load and help lower the cost of power.
Q5: Why does an AC solenoid still pull the plunger when we change the wires? Do the poles change?
electrical interview questions and answers
Answer:
Yes, in AC (Alternating Current), the direction of current changes many times every second. So even if you switch the wires, the solenoid works the same. The magnetic poles also keep changing. That’s why the AC solenoid always pulls the plunger, no matter how the wires are connected.
FAQ
What is an IDMT Relay?
An IDMT relay means Inverse Definite Minimum Time relay.
It is a special type of protective relay used in electrical systems. This relay works in a smart way. The bigger the fault current, the faster it trips. That means, when a small fault happens, the relay waits a bit before working. But if the fault is very big, it acts quickly.
So, the relay time is inversely related to the fault current.
Also, it has a minimum time limit, which means it won’t trip before