Electrical Interview Questions - Part 1

Electrical Interview Questions-Series

1. How the over load and short circuit protections will provide in a Molded Case Circuit Breaker (MCCB)?

Molded case circuit breakers are used to provide circuit protection for low voltage distribution systems. They will protect connected devices against both overloads and short circuits.

  • Over current protection is provided by thermal trip unit of MCCB.

Thermal trip unit:

It is made up by a bimetallic thermal device which actuates the opening of a circuit breaker with delay depending on the over current value. Thermal trip unit allow high but not too much currents for the given delay time. So it is possible to avoid unnecessary tripping due to current spikes that last for a brief moment.

  • Short circuit current protection is provided by magnetic trip unit of MCCB.

Magnetic trip unit:

Made up by an electromagnetic device, with fixed or adjustable threshold, which actuates the opening of a circuit breaker on a predetermined over current value. If the currents are very high even for a short time it will trip the breaker.

2. Why DC MCCBS are having less breaking capacity than AC MCCBS for the same voltage and current ratings?

Switching of DC currents is much severe phenomenon than switching AC currents due to non occurrence of natural zero.

3. What are different types of C.T testing methods?

  • Polarity test
  • Winding and Insulation resistance tests
  • Ratio test
  • Knee point voltage test

4. What are IPBD and SPBD?

These are the different bus ducting methods.

IPBD: Isolated Phase Bus Duct

Application is to form alternator power lead out. IPBD is used because of its highest reliability and short circuit resistance.

SPBD: Segregated Phase Bus Duct

The three phase conductors are inside an enclosure but segregated from each other. SPBD is used to avoid the occurrence of phase to phase short circuit.