Top electrical engineering interview questions with short answers for site, maintenance and design jobs.
1. Why is a transformer rated in kVA and not in kW?
Transformer losses (copper and iron loss) depend on voltage and current, not on the power factor of the load. Since the load power factor is not known in advance, the rating is given in kVA.
2. Why can a transformer not work on DC supply?
A transformer needs a changing magnetic flux to induce voltage in the secondary. DC gives a constant flux, so no voltage is induced, and the primary draws a very high current that can burn the winding.
3. What is power factor and why should it be high?
Power factor is the ratio of real power (kW) to apparent power (kVA). A low power factor means more current for the same work, causing higher losses, voltage drop and larger cables and equipment. It is improved using capacitor banks.
4. What is the difference between star and delta connection?
In star, line voltage is √3 times phase voltage and line current equals phase current; it has a neutral point. In delta, line voltage equals phase voltage and line current is √3 times phase current; there is no neutral.
5. What is the difference between MCB, MCCB and RCCB?
An MCB protects against overload and short circuit at low currents (usually up to 125 A). An MCCB does the same for higher currents and often has adjustable trip settings. An RCCB detects earth leakage current and protects people from electric shock.
6. What is the purpose of earthing?
Earthing gives fault current a safe low-resistance path to the ground, so protective devices trip quickly and people are protected from electric shock. It also stabilises system voltage.
7. What is slip in an induction motor?
Slip is the difference between synchronous speed and rotor speed, expressed as a fraction or percentage of synchronous speed. A motor needs some slip to produce torque.
8. Why is a starter used for an induction motor?
At start, a motor can draw 5 to 7 times its full-load current. A starter (such as DOL, star-delta or soft starter) limits this starting current and protects the motor and supply.
9. How do you test the insulation of a motor or cable?
With an insulation resistance tester (megger). The insulation resistance between windings and earth, or between conductors, is measured at a set test voltage and compared with the required minimum value.
10. What is the function of a relay in a substation?
A protective relay senses abnormal conditions such as over-current, earth fault or differential fault and sends a trip signal to the circuit breaker to isolate the faulty section.
These are common questions asked in interviews for freshers and experienced candidates. Read each topic in your textbook for full details. See the latest jobs on our homepage.