Understand MCB B, C & D Trip Curves

An MCB trip curve defines how quickly a Miniature Circuit Breaker responds to different levels of overcurrent over time. It is represented by a time–current characteristic curve, which shows the relationship between fault current magnitude and tripping time. This curve is crucial for selecting the right MCB to ensure protection without unnecessary tripping.

MCBs operate using two tripping mechanisms. The thermal trip protects against overloads by responding slowly to moderate overcurrent, allowing temporary surges without interruption. The magnetic trip responds instantly to high fault currents caused by short circuits, disconnecting the circuit in milliseconds.

The most common trip curves are B, C, and D.

Type B trips at 3–5 times the rated current and is used in residential circuits such as lighting and socket outlets where inrush current is low.

Type C trips at 5–10 times the rated current, making it suitable for commercial and light industrial applications with moderate inrush currents, such as motors and control panels.

Type D trips at 10–20 times the rated current and is designed for high inrush loads like transformers, compressors, and heavy machinery.

Choosing the correct MCB trip curve improves system reliability, safety, and coordination, ensuring sensitive circuits are protected while preventing nuisance tripping in applications with higher starting currents.