What is MCB (Miniature Circuit Breaker)
Miniature Circuit Breakers (MCBs) are very important components in electrical systems nowadays, it offers enhanced safety and operational convenience compared to electrical fuses. Unlike fuses, which require manual replacement after they melt due to overcurrent, Miniature circuit breakers work as automatic switches that trip (open), when they detect an excessive amount of “current flow” through a circuit. When the problem causing the over-current is solved, MCBs can be easily reset without a replacement.
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Miniature circuit breakers are used as efficient alternatives to fuse across various applications, such as domestic, commercial, and industrial uses. They are available in a wide range of configurations with breaking current capabilities ranging from 10KiloAmps to 16KiloAmps. The breaking current capacity is the maximum current that an MCB can safely interrupt or break in the event of a fault, ensuring reliable protection against electrical hazards and fire.
Miniature circuit breakers provide robust protection by swiftly disconnecting the electrical circuit in case of any abnormal conditions or faults, thereby preventing damage to the equipment, it highly reduces the fire risks, improving overall safety in electrical installations. Miniature circuit breaker’s versatility and reliability make them indispensable electrical components in modern electrical systems.
FUNCTIONS OF MINIATURE CIRCUIT BREAKER
An MCB (Miniature Circuit Breaker) is a component used in an electrical system that is designed with an enclosure made of molded insulating material. Its primary function is to open/trip a circuit when the current flowing through it exceeds the predetermined value, thereby protecting the connected electrical loads from damage due to overcurrent. The key features and functions of MCB are discussed below:
- Switching Function: Miniature circuit breaker or MCB acts as an automatic switch that can be manually operated to turn the circuit on or off when necessary, similar to a standard switch.
- Time Delay Tripping: Miniature circuit breakers are equipped with a time delay mechanism that controls their response to overcurrent conditions. They react to sustained overloads that could pose a danger to the circuit being protected rather than transient loads like switching surges or motor starting currents.
- Operating Time: During short circuit faults, a Miniature circuit breaker or MCB typically operates very quickly, within less than 2.5 milliseconds to disconnect the circuit. For overloads, the operating time can range from 2 seconds to 2 minutes, depending on the level of current and the specific MCB design.
- Pole Type: MCBs are manufactured in various pole configurations, including single-pole, double-pole, triple-pole, and four-pole type. These different configurations accommodate various applications, including single-phase and three-phase systems.
- Voltage Rating: Miniature circuit breakers are rated for different voltage supplies, such as 110V DC, 220V DC, and 240/415V AC (single-phase and three-phase), with varying short circuit current capacities.
- Current Rating: Miniature circuit breakers are available with different current ratings, typically ranging up to 100 A for single-phase devices. Some MCBs allow adjustment of their tripping current capacity, while others are fixed for specific load currents and short circuit ratings.
- Functions: MCBs serve multiple purposes, including:
- Local control switches for electrical circuits.
- Isolating switches to disconnect circuits during faults.
- Overload protection devices for installations, equipment, or appliances.
- Fault Isolation: Miniature circuit breakers linked in two or three-pole types ensure that a fault in one line interrupts the entire circuit, providing complete circuit isolation. This feature is crucial for protecting equipment, particularly in scenarios like single-phasing in three-phase motor protection.
Construction and operation of MCB
An MCB (Miniature Circuit Breaker) is constructed ensuring mechanical strength and strong electrical insulation. Within this housing, the switching system comprises fixed and dynamic contacts to which incoming and outgoing wires are connected. The material used for the current-carrying parts varies, typically being electrolytic copper or silver alloy, depending on the MCB’s rating.
During overload or short circuit situations, when the contacts separate, forming an electric arc. Modern MCBs are equipped with arc interruption systems designed to safely handle these arcs. Metallic arc splitter plates are placed to extract arc energy and provide cooling. These plates are held in place by insulating materials, with an arc runner provided to guide and extinguish the arc produced between the main contacts.
Inside the insulated case of MCB, there are
- Incoming and outgoing terminals
- DIN rail holder
- Arc chute and Arc chute holder
- Fixed and dynamic contacts
- Bimetallic strip and strip carrier
- Latch and Plunger
- Solenoid
- Switching mechanism.
Let us break down the components and their functions step by step:
Incoming Terminal and Outgoing Terminal:
These terminals are where the incoming and outgoing electrical wires are connected, respectively. The incoming terminal receives the electrical supply, and the outgoing terminal connects to the load or next segment of the electrical circuit.
Din Rail Holder:
The Miniature circuit breaker is typically mounted on a DIN rail in electrical distribution panels. The DIN rail holder securely holds the MCB in place within the panel.
Arc Chutes Holder and Arc Chutes:
Arc chutes are essential for extinguishing the arc that forms between the contacts when the MCB opens during a fault condition (like overload or short circuit). The arc chute holder houses these arc chutes in a way that directs the arc into the chutes where it is cooled and extinguished safely.
Fixed Contact and Dynamic Contact:
The Miniature circuit breaker’s switching mechanism involves these two contacts. The fixed contact remains stationary, while the dynamic contact moves when the MCB trips. These contacts are where the electrical current flows through when the circuit is closed.
Bi-metallic Strip Carrier and Bi-metallic Strip:
The bi-metallic strip is a crucial component in the thermal tripping mechanism of the MCB. It consists of two metals bonded together with different coefficients of thermal expansion (typically brass and steel). When current flows through the MCB, the bi-metallic strip heats up. As it heats, the differential expansion of the metals causes the strip to bend, which triggers the tripping mechanism.
Latch and Plunger:
The latch and plunger are part of the mechanical trip mechanism in the MCB. When the MCB trips due to an overload or short circuit, the plunger releases the latch, allowing the contacts to open and disconnect the circuit.
Solenoid:
The solenoid is a component of the magnetic tripping mechanism in the MCB. During a short circuit or heavy overload, a magnetic field is generated by the solenoid, which attracts the trip lever, causing the contacts to open rapidly and disconnect the circuit.
Switching Mechanism:
The MCB can be manually operated as a switch to turn the circuit on or off when needed. This operation is similar to a standard switch and provides local control over the electrical circuit.
The MCB’s operating mechanism involves both magnetic and thermal tripping arrangements:
Magnetic Tripping: This mechanism utilizes a composite magnetic system that includes a spring-loaded dashpot with a magnetic slug immersed in silicon fluid. A current-carrying coil within the trip arrangement moves the magnetic slug against the spring towards a fixed pole piece. When a sufficient magnetic field is generated by the coil—typically during short circuits or heavy overloads—it attracts the armature of the trip lever, causing the MCB to trip.
Thermal Tripping: In this arrangement, a bimetallic strip is crucial. It includes a heater coil wound around it to generate heat proportional to the current flow. The heater design can be direct, where current flows directly through the bimetallic strip, or indirect, where a coil of current-carrying conductor surrounds the strip. As the current increases beyond normal operating levels, the bimetallic strip deflects due to the differential expansion of its constituent metals (typically brass and steel). This deflection activates the tripping mechanism, thereby disconnecting the circuit.
The bimetallic strips are carefully designed to ensure they do not trip under normal operating currents but respond appropriately to overloads by heating and bending to move the latch. This design provides specific time delays under certain overload conditions, enhancing the MCB’s reliability and performance in protecting electrical circuits.
Operation Sequence:
Normal Operation:
When the electrical current is within safe limits, the MCB’s contacts (fixed and dynamic) remain closed, allowing current to flow through the circuit uninterrupted.
Overload or Short Circuit:
If the current exceeds the rated capacity of the MCB, several things can happen:
The bi-metallic strip heats up, causing it to bend and activate the thermal tripping mechanism.
The solenoid generates a strong magnetic field during a short circuit, which triggers the magnetic tripping mechanism.
Tripping of MCB:
When either the thermal or magnetic tripping mechanism is activated. The plunger releases the latch, allowing the contacts to open rapidly. The arc that forms between the contacts is directed into the arc chutes, where it is safely extinguished.
Resetting the MCB:
After the fault condition is cleared and the MCB cools down (in the case of thermal tripping), the MCB can be manually reset by toggling the switch to the ‘off’ position and then back to the ‘on’ position. This process restores the MCB to its operational state.
How to select the proper MCB for different load ratings?
Understanding the specifications of Miniature Circuit Breakers (MCBs) is crucial to selecting the right one for your application. Here’s a breakdown of the key specifications:
Current Rating: This indicates the maximum current that the MCB can handle without tripping. Choose an MCB with a current rating that matches the load requirement of your circuit. If the current rating is too low, the MCB will trip frequently, causing interruptions. If it’s too high, it might not provide adequate protection during faults.
Tripping Curve: MCBs come with different tripping curves. The most commonly used are B, C & D which indicate different levels of sensitivity to instantaneous over current.
B-curve: It is mostly used in domestic or light commercial loads like heaters and lights. These MCBs generally trip in between 3 to 5 times the rated current capacity.
C-curve: It is mostly used in commercial and industrial loads that have moderate surges. These MCBs generally trip in between 5 to 10 times the rated current capacity.
D-curve: It is mostly used in motors and transformers where there is a high inrush current. These MCBs generally trip in between 10 to 20 times the rated current capacity.
Few more specialized MCB models are available. These include:
- Type K MCBs – these will trip when the current reaches eight to twelve times the rated current carrying capacity. They are a good choice for motors
- Type Z MCBs – these are highly sensitive MCBs, that trip when the current exceeds the rated load by only two to three times. They are used with more delicate devices prone to short circuits, such as semiconductors
Choose the curve based on the type of load and its characteristic current surges.
Breaking Capacity: This is the maximum fault current that the MCB can safely interrupt without damaging any equipment. Ensure the breaking capacity (expressed in kA) exceeds the maximum fault current expected in your installation to ensure safety.
MCB Pole types:
single pole type: used to protect the single live wire.
2-pole type: used to protect 2 live wires in industrial and commercial settings.
3-pole type: It protects 3 live wires, it is least common and seen in industrial applications only.
4-pole type: used to protect 3 live and 1 neutral wire, used extensively in industrial and commercial purposes.
MCB Voltage rating:
Voltage rating is also a very important factor when selecting an MCB. 110Volts, and 220V DC MCBs are used in industries while residential and commercial application sees 240Volts and 415Volts AC MCBs extensively.
