What is MCB?
A miniature circuit breaker is an automatically operated electrical switch used to protect low voltage electrical devices from fault due to overload or short circuit. The typical current rated upto 125A, above 125A of typical current MCCB can be applied. Fault zone can be easily identified. MCB are protective than the fuse.
Working Principle of MCB:
Miniature circuit breaker works on the principle of the two effects:
- Thermal effect
- Electro-magnetic effect
Working based on thermal effect in MCB:
The thermal effect of the miniature circuit breaker works is based on a bimetallic strip, when an overload current occurs in an electrical path through MCB, the bimetallic strip gets overheated, a deflection occurs and the bimetallic strip gets bending. This bimetallic strip bend creates a mechanical latch and the latch opens the MCB contact and open the closed circuit.
Working based on electro-magnetic effect in MCB:
This type of circuit breaker works on basis of a magnetic actuator. When electric current in a circuit flow through the MCB, the actuator acts when short circuit occurs and the breaker trips and the circuit is opened. The electro-magnetic circuit breaker acts quickly. The opening of the trip unit is 0.5 milliseconds.
Construction of the MCB:
The miniature circuit breaker is easy to install and maintenance free device. It can be applied in domestic and industrial applications in replacement of fuse.
It consists of 3 major constructional parts.
1.frame of the MCB:
The frame of the miniature circuit breaker is completely molded. The molded frame is completely strong and insulated one which prevents external source contact.
2.Mechanism of the MCB:
Normally, the MCB have three categories of mechanism. They are “ON”, “OFF” and “TRIPPED”.
- ON- In closed circuit the MCB will be in ON condition for the flow of current.
- TRIPPED- When overcurrent or short circuit occurs the trip coil gets overheated and latches automatically tripped and open the circuit.
- OFF- When to carry out any electrical work the MCB can be manually tripped off.
Based on the switch latch condition we can be able to determine whether the MCB is in switch ON, OFF or TRIPPED condition.
Trip operation works main mechanism in the MCB. Two types of mechanism provided to trip the circuit.
- Bimetal trip – For overload Current
- Electromagnet – For Short Circuit
Different types of MCB based on Fault Current:
MCB are different types based on the tripping during fault current. They are:
- TYPE B MCB
- TYPE C MCB
- TYPE D MCB
- TYPE K MCB
- TYPE Z MCB
Tabular representation of types of MCB:
|TYPE B MCB
|TYPE C MCB
|TYPE D MCB
|TYPE K MCB
|TYPE Z MCB
|3 to 5 times of full load current
|5 to 10 times of full load current
|10 to 20 times of full load current
|8 to 12 times of full load current
|2 to 3 times of full load current
|small inductive load
|high inductive load
|very high inductive load
|motor and inductive load
|Domestic and light commercial appliances
|for welding work and large winding motors
|motor moment start up and protection of motor from short circuit
The above table shows the type of MCB’S based on fault or overload current. MCB’s are applicable in:
Different types of MCB based on Poles:
Based on the number of poles connected to the circuit, MCB is classified as:
- Single Pole MCB
- Double Pole MCB
- Triple pole MCB
- Triple pole MCB With Neutral
- Four pole MCB
Single pole MCB:
For the purpose of the single phase circuit, single pole MCB are used. It’s denoted as SP MCB.
Double Pole MCB:
For the protection of the phase and neutral, the double pole MCB are used. It’s denoted as DP MCB.
Triple pole MCB:
For the protection of the three phase without neutral, the triple pole MCB used. It’s denoted as TP MCB.
Triple pole MCB with Neutral:
For the protection of the three phase with neutral connection, the triple pole MCB used. It’s denoted as TP(3P+N) MCB. In this MCB, 3 pole connected with protection and neutral have separate pole without protection only for switching the power supply.
Four pole MCB:
It is similar to Triple pole MCB with neutral pole connected, but, neutral pole is protected separate pole.
Advantages of MCB:
- Fault occurred in electrical circuit can be easily identified.
- MCB is electrically safe compared to fuse.
- By switching operation, quick power supply restoration possible.
- MCB knobs are efficient interface than fuse.
- MCB are shorter tripping time than fuse.
Disadvantages of MCB:
- When compared with cost wise, MCB is costlier than fuse.
- The panel board of the MCB are different than the simple fuse which can be rewired.
- If the trip switch is fault due to frequent trip or manually tripped, then the MCB to be completely replaced.
- For high rated current, MCB cannot be applicable.
- MCB to be wired in proper wiring to avoid short-circuit in panel.