Example analysis of circuit breaker curve selection

When an electric motor (such as a dryer or vacuum cleaner) starts, surge current is the reason why the lights inside the house become dim when the cleaner starts.

Figure 1 (below) shows an example of surge current in an AC motor.

As shown in the figure, the surge current generated by connecting the motor is 30A. It is much higher than the steady-state current during operation. The surge current reaches its peak and then begins to decay as the motor rotates.

We need different trip curves to balance appropriate overcurrent protection with optimal machine operation. Choosing a circuit breaker with a tripping curve that trips too early may result in tripping. Choosing a circuit breaker that trips too late will result in catastrophic damage to the machine and cables.

How does MCB work?

It would be helpful to understand the tripping curve and the methods of small circuit breakers or overcurrent protection. Equipment, functionality. The following diagram shows the internal structure of a miniature circuit breaker (MCB).

Simultaneously using bimetallic strips (2) and electromagnetic coils/valves (6), miniature circuit breakers can be a combination of two separate circuit protection devices. Bimetallic strips can provide overload protection against overcurrent in small situations, typically up to 10 times the operating current. A metal strip is composed of two strips of different metals, which expand at different speeds when heated together. In case of overload, the bimetallic strip will bend and this action will trigger the trip mechanism and disconnect the circuit. Mechanical displacement with temperature change conversion.

Magnetic coils or solenoid valves (6) respond to rapid, high overcurrent caused by short circuits, typically exceeding 10 times the operating current - up to tens or hundreds of thousands of amperes. High current causes a magnetic field generated by the coil, which rapidly moves the internal piston (within microseconds) to trip the actuator mechanism and disconnect the circuit.

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