The calculation of Conventional non-tripping current and Conventional tripping current of MCB
You wouldn't expect a 10A fuse to blow at (say) 10.5 amps but 13% above this at 11.3 amps you might expect a fuse to eventually blow. It's exactly the same with circuit breakers and they both have the 13% overload point meaning the CBs/fuses should not trip if they were at this level all day. On the other hand, the 45% overload level should trip after 1 hour and at 155% overload (2.55 times rated current) it should trip after one minute.

However there are different breaker technologies that will accelerate this: -



Data taken from here and note that is just shows the thermal part of the breaker's characteristic. The magnetic part handles the much bigger overload currents.