Why 15A MCB not operate at 16 Awhy it operate at 29A or 30 A?

A 15A MCB will still trip at 16A, just it will take a long time as it should not operate at 15A. Will usually trip within 10s at 200% overload (C curve) and 1 s at 500%, you need to look at the trip time curve which is different for different CB types (A,B,C or D curve)