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What does B,C,D curve mean for Acti 9 MCB?

B Curve means the MCB trips between 3-5 times full load current. It is mainly used in residential applications where loads are resistive e.g. lighting fixtures, domestic appliances with low surge Levels.
C Curve means the MCB trips between 5-10 times full load current. It is used in commercial/industrial applications where there is greater chances of higher short circuit currents e.g. mainly inductive loads , fluorescent lighting.
D Curve means the MCB trips between 10-14 times full load current. It is used where inrush currents can be very high e.g. motors, X-ray machines.
Here is the curves diagram:
Curve Diagram