A circuit breaker is an automatically-operated electrical switch. It is designed to protect an electrical circuit from damage caused by overload or a short circuit. Its basic function is to detect a fault condition and, by breaking continuity, to immediately discontinue electrical flow.
- As the name implies it is a miniature form of the circuit breaker.
- It can be operated as an ordinary switch in making/breaking the circuit on which it has been applied on.
- It is easy to handle and provides complete safety to human beings.
- They have a vast range and small steps between rated current, allow cables to utilized fully.
Why MCB is preferred over fuse?
A fuse, which operates only once and then have to be replaced, a miniature circuit breaker can be reset (either manually or automatically) to return back to normal operation.
MCB and its Purpose:
Miniature circuit breakers are intended to prevent damage to an electrical circuit as a result of excess current. They are designed to trip during an overload or short circuit to safeguard against electrical faults and equipment failure.
Miniature circuit -breakers are only used at low voltage (LV), mainly in domestic or light-industrial, or commercial applications.
- In general, they are used in the same applications as cartridge fuses.
- They offer an alternative for protecting radial or ring circuits.
- They are usually only single-phase devices.
- Normal rated load current range of up to 100 A.
- Maximum short-circuit rating of 16 kA/240 V.
MCB Working Principle
There are two arrangements of operation of the miniature circuit breaker(MCB).
One is due to the thermal effect of over current.
The other one is due to the electromagnetic effect of over current.
The thermal operation of the miniature circuit breaker is accomplished with a bimetallic strip.
Whenever overcurrent flows continuous flows through MCB, the bimetallic strip is heated and deflects by bending.
The bending of the bimetallic strip releases a mechanical link. This mechanical link is attached to the operating mechanism. It causes to open the miniature circuit condition, the sudden rising of electric current, causes electromechanical displacement of plunger associated with tripping coil or solenoid of MCB.
The plunger strikes the trip lever gives rise to the immediate release of the latch mechanism, therefore, open the circuit breaker contacts. This is a simple explanation of the working principle of the miniature circuit breaker.
Miniature circuit breaker construction is very simple, rugged, and maintenance-free. Normally, an MCB is not repaired or maintained, it just replaced by the same class of new ones when required.
A miniature circuit breaker has normally eight main constructional parts.
- Actuator lever: It is used to trip manually and reset the circuit breaker. Also indicates the status of the miniature circuit breaker (ON or OFF/Tripped).
- Actuator mechanism: forces the contacts together or apart.
- Contacts: Allows current when touching and break the current when moved apart.
- Bi-metallic strip
- Calibration screw: It allows the manufacturer to precisely adjust the trip current of the device after the assembly.
- Arc divider/extinguisher.
Frame of MCB
The frame of the Miniature Circuit Breaker is a molded case. This is a strong, solid, insulated housing in which the other components are mounted.
MCB Types (based on phase voltage or Voltage rating):
Depending upon phase system or voltage rating there are various types of MCBs available in the market.
1 pole, 2 poles, 3 poles, 3 poles with neutral, and 4 pole MCB.
1 pole MCB is for a single-phase ac system connected to the phase line of th system. Mostly uses in our houses.
2 pole MCB is also used for a single-phase ac system, but it protects both phase and neutral lines.
3 pole MCB is used for the 3 phase ac system. It protects the only RYB of 3 phase line.
4 pole MCB is used 3 phase ac system, but it protects RYB and the neutral line also.
What is a kA rating of MCB?
kA rating actually confirms how much current the circuit breaker can withstand or withhold under any type of fault or short circuit conditions.
A value of 10 kA means that the circuit breaker can withstand 1000 amperes of current under fault conditions during the brief time it takes to trip the circuit breaker.
TRIP CURVE CLASSES:
Trip curve class B:
The MCBs with this class characteristics experience immediate trips whenever the currents flowing through them reach between 3 to 5 times their rated current. These MCBs are used mostly for cable protection.
Trip Curve Class C:
Usually, MCBs that show this class characteristics have immediate trips when the current flowing through them goes between 5 to 10 times its rated current.
So they are acceptable in domestic and residential applications and electromagnetic starting loads that require medium starting currents.
Trip Curve Class D
MCBs with these class characteristics trip immediately whenever the current flowing through it rates range 10.1 to 20 times its rated current.
MCBs in this class are recommended for use in inductive loads and motor loads with high starting currents.
Trip Curve Class K:
The MCBs with this type of class characteristics, experience immediate trips whenever the currents flowing through them reach between 8 to 12 times their rated current.
The trip curve class K-type MCBs can be used for inductive and motor loads with high inrush currents.
Trip Curve Class Z:
The MCBs with class Z characteristics experience immediate trips when the currents flowing through them reach between 1 to 3 times their rated current.
This class MCBs are normally very highly sensitive to short circuits and can be used for protecting highly sensitive devices such as semiconductor devices.
Difference between MCB and MCCB:
The main difference between the MCB and MCCB is their capacity.
With MCB, rated under 100 A with an interrupting rating of under 18000 A.
MCB trip characteristics may not be adjusted since they basically cater to low voltages.
MCCB would provide amps as high as 2,500 or as low as 10 depending on what is necessary. Their interrupting rating ranges from around 10,000A to 200,000 A.
Trip current can be adjusted in MCCB.
- Operation Mechanism Of MCB
The Operating Mechanism of Miniature Circuit Breaker provides the means of manual opening and closing operation of miniature circuit breaker. It has three-positions “ON,” “OFF,” and “TRIPPED”. The external switching latch can be in the “TRIPPED” position, if the MCB is tripped due to over-current. When manually switch off the MCB, the switching latch will be in “OFF” position. In close condition of MCB, the switch is positioned “ON”. By observing the positions of the switching latch on can detemine the condition of MCB whether it is closed, tipped or manually switched off.
Choosing a circuit breaker In conclusion
A Multi 9 circuit breaker is determined by:
- Breaking capacity
- Number of poles
- Tripping curve
- Type of load
Applications of MCB for home electrical panel
As with all breakers, the miniature circuit break is designed to protect the house from circuit overload. They are much safer than the typical fuses, because they can be reset manually and because they handle much larger amounts of power. The breaker can manage the flow of energy, distributing properly the voltage even when many devices run off the same power circuit.
Applications of MCB for ground fault trip mechanism
In some cases, you can use a minature circuit breaker with ground fault, or arc fault mechanisms, because the breakers consist of a system that opens the contacts if a line to ground fault occurs.
Applications of MCB for lights
Using miniature circuit breakers in the lighting system of the house, because they can deal with the amount of power needed to light a house, especially if using specific types of lamps, such as fluorescent lights. When switching on the lights, there is a need for additional power which might cause problems to the circuit, especially when lights are used extensively in the entire house.
Application of MCB for industrial appliances
There are many small scale industrial buildings that use the miniature circuit breakers instead of the old fuses. These installations require up to 30 kA capacity, therefore the miniature breakers are ideal for this type of applications and devices. Some withe commercial installations and applications. Some industrial applications that use miniature breakers are large stoves, and appliances used in restaurants,bakeries and food commercial stores.