Electrical Interview Questions: Series-5

Electrical Interview Questions

In this series of Interview questions, we will see the following basic Electrical interview questions.

1. Why do we need a transformer?
2. How can you define a short circuit?
3. What is the difference between power and control circuits?
4. What is the difference between an isolator and a switch?

1. Why do we need a transformer?

For reasons of economy and efficiency, it is preferable to transmit electricity at High Voltage, the explanation is given below.

• The Input power transmitted: P = V x I (Watts)
• However, certain losses occur during the conducting process which amount to: I² x R (Watts)
• After removing losses, the Power output= V x I (Watts) – I² x R (Watts)
• I² x R losses must therefore be kept as small as possible.
• This means that either I or R must be kept small.
• The resistivity of a conductor is ρ=Ra/l ohm-m
• Conductor Resistance is given as R= ρl/a ohms
• Here cross-sectional area of the conductor is a=πd²/4
• Hence R= 4ρl/ πd² ohm
• To keep R small, metal with Low Resistivity (Copper or Aluminum) could be used in cables with a large diameter. Since Resistivity is directly proportional and diameter is inversely proportional to the resistance.
• If the diameter of the cable increases its thickness also increased.
• But a thick cable over long distances would be very expensive.
• It is preferable therefore to reduce “current, I”.

This can be done, but to maintain the same power transmitted the voltage must be increased. To do this, a Transformer is used.

2. How can you define a short circuit?

A short circuit is a circuit in which the electricity has found an alternative path to return to the source without going through an appropriate load.

We can demonstrate this easily by taking a fine piece of wire and connecting it to both the positive and negative terminals of a small battery as shown in the below figure.

The wire will heat instantly and probably melt. In most circuits, this high amperage represents a dangerous situation that could cause a fire or electrocute someone.

In power circuits, huge amounts of currents due to short circuits are produced. Estimation of short circuit currents value is important to design protection equipment.

In order to avoid short circuits occurring go through short circuit preventive measures.

3. What is the difference between power and control circuits?

Power Circuits:
A power circuit is defined as any circuit that carries power to electrical loads. Power circuits
often carry high voltages and consist of incoming main power, a motor starter, and the motor.

Control Circuits:
A control circuit is a special type of circuit that uses control devices like relays to determine when loads are energized or de-energized by controlling current flow. Control circuits usually carry lower
voltages than power circuits.

4. What is the difference between an isolator and a switch?

Isolator:

An isolator is a mechanical device that is opened manually and is provided so that the entire
installation, or one circuit or one piece of equipment may be cut off from the live supply. In
addition, a means of switching off for maintenance or emergency switching must also be
provided.

An Isolator can merely establish the circuit under no load. It cannot establish or break any
load current.

Switch:

A disconnecting switch is used as a disconnecting device that isolates either a part of the circuit or the whole of the circuit.

 A switch can make or break an electrical circuit under rated load current.