# Short Interview Questions for Electrical Engineers

### Question

Why capacitor is used for power factor correction?

Capacitors are used for power factor correction because they can store energy in an electric field. This energy can then be used to compensate for the lagging current that occurs when inductive loads are connected to an AC power source.

### Why do Transformers Work only on AC but not on DC?

Transformers work with AC because they rely on the principle of electromagnetic induction, which requires a changing magnetic field. In AC, the voltage continuously alternates direction, creating a fluctuating magnetic field that induces voltage in the secondary coil. In contrast, DC maintains a constant voltage, resulting in a static magnetic field, rendering transformers ineffective.

### How to Check Earthing and Voltage with Multimeter?

To Check the earthing of the circuit by using a multimeter you need to put black Probe into the earth point and as well as red probe into the phase point. if you get voltage your earthing is working properly.

### Why can’t we Store AC like DC ?

Because AC changes its direction of flow, thereby leading to charging and discharge of a battery, It can’t charge a battery directly. Key points: Alternating Current (AC) cannot charge a rechargeable battery because its positive half cycle charges the battery whilst the negative half cycle discharges it.

### What is the Safe Limit of DC Voltages for Humans to Touch?

The safe limit of DC voltage for humans to touch is generally considered to be 30 volts or lower. Beyond this threshold, there is an increased risk of electric shock, though factors like skin condition and moisture can influence safety. Always prioritize electrical safety.

### Why the Core of Transformer is Laminated?

This design minimizes the flow of eddy currents, which are circulating currents induced in the core by alternating magnetic fields. Eddy currents can cause energy losses in the form of heat, and by using laminations, these losses are significantly reduced.
Laminations help maintain the magnetic properties of the core material. A continuous solid core would saturate quickly, but the laminated design allows for a higher magnetic flux density without saturating, improving the transformer’s performance.
Laminations are easier to manufacture and assemble into the core. They can be stacked together and insulated quickly, allowing for mass production of transformer cores.
In case of core damage or the need for repairs, individual laminations can be replaced or repaired more easily than a single solid core.

### What is the Difference between Circuit Breaker and Isolator?

A circuit breaker is a device that automatically interrupts electrical current in case of a fault, protecting the circuit. An isolator, also known as a disconnect switch, is a manual switch used to physically disconnect a circuit for maintenance, without interrupting current automatically.

### How can we size a cable?

To calculate the Cable Sizing one needs to divide the voltage running through the cable by the target current. For instance, If your wire has a voltage current of 150 V and your target is 30 then you divide 150/30. This gives you your target resistance of 5 which is required.

### What is the difference between contactor and relay?

The contactor belongs to the main circuit element, and its task is to control the on-off of the main circuit. The relay is an auxiliary circuit component, and its task is to perform signal transmission and control the main circuit components.

### What is the good ground resistance value?

5.0 ohms however, the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less. According to the NEC, make sure that system impedance to ground is less than 25 ohms specified in NEC 250.56. In facilities with sensitive equipment it should be 5.0 ohms or less.

### Why Solar Cells generate DC not AC?

Cell itself works on charge negative ( free electrons) pass their power to other outer orbit electrons of nearby atom and make them free, and this charge movement is always from negative to positive this is how DC get generated. Same happens when solar cell get solar energy and it’s electrons get activated.

### Why Generators are Connected in Parallel but not in Series?

When they are in parallel, if one of them gets faulty, the other one can continue working providing the same output voltage. Also, they can be loaded differently with both active and reactive power.

To optimize the system and dispatch economic power also protection from blackout that’s why generators are parallel.

In the parallel connection , voltage will be fixed on the rated voltage, which it should be, but the current is increased or doubled to share the higher load.

U can put them in series, it’s actually, pretty cool, kind of  like stacking batteries until u get 400vdc and then zap your friend with it.

### What is more Dangerous? 230V AC or DC?

Both are dangerous (AC and DC can be lethal, and the level of danger depends on various factors, including the current level, the duration of exposure, the path the current takes through the body, and individual factors such as skin condition and resistance)
In general, 230V AC is often considered to be more dangerous than 230V DC under the same conditions due to some specific characteristics of AC (frequency, cardiac effects)

### What is NO and NC on a Contactor?

Normally open (NO) contacts mean that current does not flow through them in its normal state. When normally open contacts are activated they will allow current to flow through them. Normally closed (NC) contacts mean that current can flow through them when the switch or button has not been activated. When normally open contacts are activated it will break the circuit and stop current flow.

### What is the maximum voltage that human can withstand?

The maximum voltage a human can withstand depends on several factors, including the duration of exposure, the current flowing through the body, and individual differences in resistance. However, as a general guideline, electric shocks above 50 volts can be dangerous and potentially life-threatening. Currents as low as 100 milliamperes (0.1 amperes) can disrupt the heart’s rhythm and lead to ventricular fibrillation, which can be fatal. It’s important to avoid electrical hazards and take safety precautions when working with electricity to minimize the risk of electric shock.

Dry and thick skin can withstand up to 400 Vdc depending on pain tolerance and if the power source is somehow limited. Yes, the key word is pain tolerance and power source limitation.
However, 19Vac 50/60Hz is dangerous already due to body’s natural capacitance and low frequency. At higher frequencies (kHz range) though, we may tolerate higher AC voltages.
It’s approximately 70 volt & that’s why we use 110 volt transformer & that also the potential difference +55 to -55 volts i.e. center tapped & earthed for electrical hand tools & the current is 50 mA & we use 30mA RCCB for safe practice.