Mini Circuit Breaker , MCCB , Selection of MCB, MCCB

Miniature Circuit Breaker and Modular case circuit Breaker

MCB ( Miniature Circuit Breaker or mini circuit breaker ) is an automatic switch that opens during abnormal condition (i.e. overload condition or faulty condition) in the circuit. MCBs are typically rated up to a current of 125 A. Handling MCB is relatively safe, and it quickly restores the supply. MCB – Miniature Circuit Breaker can be reset quickly and does not demand more maintenance costs. MCB works on a bi-metal respective principle that protects against overload current and solenoid short circuit current. It is mainly used for domestic purposes. The symbol of MCB is shown below. \\

MCB can be manually switched ON and OFF as similar to normal switch if necessary. MCBs are time delay tripping devices, to which the magnitude of overcurrent controls the operating time. This means, these get operated whenever overloads exist long enough to create a danger to the circuit being protected. MCBs do not respond to transient loads such as switches surges and motor starting currents. Generally, these are designed to operate at less than 2.5 milliseconds during short circuit faults and 2 seconds to 2 minutes in case of overloads (depending on the level of current).

MCCB (Modular Case Circuit Breaker) is another type of electrical protection device which is used when load current exceeds the limit of a miniature circuit breaker (MCB). it is rated upto 10 to 2500 amps. It is generally used where normal current is more than 100 Amps i.e. industrial purposes.

Selection of MCB or MCCB Rating

For Normal load the current rating of the MCCB is 125% of full load current. \\ For Example the full load current is 20A. \\ MCCB or MCB rating = 1.25 x 20 = 25A

For Motors instead of full load current, starting current is considered for calculating the rating of MCCB or MCB, because the starting current of motor is greater than the full load current. If we considered the full load current for calculating MCCB or MCB rating, then MCCB or MCB will trip, when we try to start the motor. \\MCCB or MCB current Rating for Motors = 125% of Starting current or Inrush current.

MCB or MCCB rating for motors depend on starting current of motor instead of full load current. For calculating the starting current of motors, we should know the KVA per HP of a motor.

On the name plate of motor, there is Design code letter which indicates its KVA per HP value, as shown in table below.

Starting Current or Inrush Current or Locked rotor current = \frac{\textrm{Design Code Letter Value} \times HP \times 1000}{\sqrt{3} \times \textrm{Volatge (in Volts)}}

Example: A 50HP, 480 Volt, 3 phase induction motor is designated on its nameplate as class H motor. How much inrush current or starting current will this motor draw at starting.

Design code value of Letter “H” = 7.09 (I have Taken Max. Value, You can take average value also)\\ Horse Power (HP) = 50\\Voltage = 480 \\\;\\ Inrush current = \frac{\textrm{Design Code Value}\times HP \times 1000}{\sqrt{3}\times \textrm{Volatge (in Volts)}}

Inrush Current = \frac{7.09 \times 50 \times 1000}{\sqrt{3} \times 480} \textrm{= 246.4 A}

Thumb Rule: Generally starting current of motor 4 to 6 times of full load current.

Types of MCCB or MCB

The various types of MCCBs are given below.

1. Type B: They are operating when fault current reaches 3 to 5 times of full load current with the delay of 0.04 to 15 seconds. It is mainly used for domestic applications such resistive loads, lighting loads etc.
2. Type C: They are operating when the fault current reaches 5 to 8 times of full load current, with the delay of 0.04 to 5 seconds. They are used for inductive loads such as transformers, Welding machines, electromagnetic etc.
3. Type D: They are operating when fault current reaches 10 to 15 times of full load current, with delay of 0.04 to 3 seconds. They are suitable for heavy starting currents such as motor pumps , lifts etcs.
4. Type K: They are operating when fault current reaches 8 to 10 times of full load current, with delay of 0.04 to 5 seconds. They are suitable for feeder protection.
5. Type Z: They are very sensitive MCCBs. They are operating when fault current reaches 1.5 to 3 times of full load current. They are suitable for electronic loads.

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