Topic: MCB (Miniature Circuit Breaker)

1. What is MCB?

Ans.: Miniature Circuit Breaker

2. Why MCB is used?

Ans. : MCB is used to protect electrical wire and Equipment from short circuit and earth fault

3. What are different types of MCB?


  1. B,C,D,K and Z as per tripping class
  2. Single Pole, Double Pole, Three Pole, Four Pole, Single pole with Neutral, Three Pole with Neutral as per no of poles

4. What is the difference between RCCB and MCB.

Ans. MCB protects from overload and short circuit while RCCB gives protection against earth fault.

5. What is difference between MCB and MCCB ?


(i) MCB is available upto 125 Amp only while MCCB is available upto 2500 Amp.

(ii) MCB trip settings( current and time pick up) is fixed and cannot be adjusted while MCCB’s trip settings can be adjusted.

6. What is difference between MCB and fuse ?

Ans. On short circuit, Fuse melts and replacement of fuses is done while MCB can be used again after tripping on fault.

Topic: MCCB(Molded Case Circuit Breaker)

1. What is MCCB ?

Ans. MCCB is Moulded Case Circuit Breaker.

2. Why MCCB is used?

Ans. MCCB is used for overload and short Circuit as MCB is only used upto 125 Amp.

3. What are types of MCCB?


(i) as per trip class B, C, D, K and Z. (ii) as per trip release, Thermal Magentic and Electronic

4. Rated impluse withstand voltage(Uimp) of MCCB?

Ans. The transient over voltage of short duration that MCCB can withstand. duration 1.2/50 micro sec.

5. What is operating short circuit breaking capacity(Ics) of MCCB?

Ans. Maximum short circuit current, MCCB can break successfully. No damage if fault current is less than or equal to Ics .

6. Ultimate short circuit breaking capacity(Icu) of MCCB?

Ans. Highest short circuit current , MCCB can break. Keep in mind, if short circuit current is higher than Ics but lower than Icu than MCB will break the current but in doing so it will damaged ..

Topic: Air Circuit Breaker (ACB)

1. What is ACB?

Ans. Air Circuit Breaker

2. What are types of ACB?


  • With protective release and without release
  • With thermal magentic release or with electronic release
  • AC and DC breaker
  • Fixed type and draw out type
  • Manual charging and motor charging

3. Highest ACB rating or upto what capacity or Ampere ACB is available?

Ans. ACB is available from 400 Amp to 5000 Amp.

4. What is manual charging in ACB?

Ans. A motor is provided in the breaker to charge the closing spring.

Topic: RCCB ( Residual Current Circuit Breaker)

1. What is RCCB?

Ans. RCCB means Residual Current Circuit Breaker

2. Why RCCB is used?

Ans. To detect earth faults and isolate and save the equipment and person from any shock or damage.

3. Ratings of RCCB ?

RCCB is available in 30 mA, 100 mA and 300 mA sensitivity.

4. What is IEC standard for RCCB?

Ans. IEC 61008-1