What is MCB (Miniature Circuit Breaker) and How Does it Work

Safety is a major consideration in selecting electrical equipment, appliances, or devices. Every piece of equipment and machine at homes, offices or factories needs a well-established electrical system. Without it, electrical devices can become vulnerable to damage from overloads and short circuits in the event of power fluctuations or high inrush currents.

Circuit breaker technology has advanced in recent years, and new techniques have been developed to curb power fluctuations and provide overload protection. One such technique is the use of the MCB (Miniature Circuit Breaker). To get you started, we'll go over some of the basics of a mini circuit breaker in this article.

What is a MCB (Miniature Circuit Breaker)?

MCBs are small electromechanical devices mainly used to protect circuits from overcurrent failures and ensure electrical safety in residences, offices, and industrial sites.

Most circuits now employ MCBs instead of fuse switches because they don't need to be replaced if an overload is detected. Unlike a fuse, an MCB operates as an automatic switch—opening and closing in the event of circuit fluctuations, providing increased operational safety and convenience without increasing running costs.

Standard Circuit Breaker vs. Miniature Circuit Breaker

There are three major differences between a standard circuit breaker and a miniature circuit breaker:

●Tripping Mechanism: Circuit breakers use a relay mechanism, whereas MCBs use a release mechanism.

●Breaking Capacity: Circuit breakers have a high rupturing capacity, while MCBs have a low rupturing capacity.

●Voltage: Circuit breakers are used in systems involving high voltage, while MCBs are used in systems involving low voltage.

How Does a MCB (Miniature Circuit Breaker) Work?

There are two types of tripping mechanisms in MCB (Miniature Circuit Breaker):

1. Thermal Tripping

2.Magnetic Tripping

Products and reference designs of Miniature circuit breaker

As shown in the figure below, AFCI consists of seven parts, each part is Isolated AC/DC power supply、Input power protection、Signal input protection、Analog front end、Self-diagnostics/monitoring、Position monitoring、MCU、Wireless interface、Optocoupler、HMI、Serial interface、Motor driver power stage.

Isolated AC/DC power supply

DC voltage is derived from 230V/110V AC lines to power the breaker. Isolated AC/DC converter is used to derive necessary power supply from the AC lines. Isolated AC/DC power supply includes Linear & low-dropout (LDO) regulators、Flyback controllers, the corresponding products are TLV773、TPS7A20、TLV709

Input power protection

Current drawn by the motor is monitored to prevent any over current due to fault. Input power protection includes eFuses & hot swap controllers、Ideal diode/ORing controllers、Load switches, the corresponding products are TPS25983、LM74703-Q1、TPS22996

Signal input protection

Miniature circuit breaker interface with sensors for measuring currents and voltages. The signal path is subject to electromagnetic disturbances (EMC) that can either damage sensitive electronic or create false. Signal input protection includes TVS diodes、ESD protection diodes, the corresponding products are TVS3300、TVS0500、TVS1400、TPD2E009、TPD4E05U06

Analog front end

Current sensing analog front-end provides signal conditioning of the current signal by giving dc offset and removing any high frequency noise. This also helps in driving input signals feeding to the ADC. Analog front end includes Series voltage references、Shunt voltage references、Precision op amps (Vos<1mV)、Analog current-sense amplifiers、General-purpose op amps、Instrumentation amplifiers、Programmable & variable gain amplifiers (PGAs & VGAs), the corresponding products are REF2025、TL431、LM4041B、OPA2317、INA281、TLV9102、TLV9004、INA826、PGA117

Self-diagnostics/monitoring

A supervisor IC is used to monitor the power supply levels. It provides a fixed delay during a power glitch or when the reset function is forced on the MCU. This subsytem also monitors the temperature. Self-diagnostics/monitoring includes Supervisor & reset ICs、Analog temperature sensors、Digital temperature sensors, the corresponding products are TLV803E、TPS3435、LM62、TMP103、TMP116

Position monitoring

Position of the breaker is sensed using magnetic sensors which are placed physically placed closer to the plunger of the MCB. This is used to obtain the status of the switch and also for self-diagnostics in the event of motor failure. Position monitoring includes Hall-effect latches & switches, the corresponding products are DRV5032

MCU

Line current signal is sampled and monitored continuously by the main CPU to take further action in case of any fault conditions. This also takes input signals from e-meter to subsequently control the motor to break open the circuit. MCU includes MSP430 microcontrollers, the corresponding products are MSP430FR2311、MSP430FR2111、MSP430F6776

Wireless interface

Wireless interface with the MCB enables communication between other breakers in the vicinity and also with the bluetooth enabled devices providing smart features. Wireless interface includes Low-power 2.4-GHz products、Wi-Fi products、Sub-1 GHz wireless MCUs, the corresponding products are CC2652RSIP、WL1835MOD、CC1312R

HMI

LED indication is used to show the status of the circuit breaker. HMI includes RGB LED drivers, the corresponding products are LP5812、LP5810、LP5811

Serial interface

Wired interface is provided to communicate with the MCB. Typically RS-485 interface is used. Serial interface includes RS-485 & RS-422 transceivers, the corresponding products are THVD1500、THVD1410、SN65HVD1785

Motor driver power stage

In case of either fault condition or in the event of signal from e-meter, breaker is operated using a motor driver from a 10V or 12V supply. Motor driver generates switching pulses to drive the main DC motor. Motor driver power stage includes Stepper motor drivers, the corresponding products are DRV8812、DRV8803、DRV8848