I think you are talking about the "acceptable range" that defines whether an mcb meets the standard; a real mcb will operate somewhete in the region between the two curves, but we don't know where without testing it. I.e. for a given current, any mcb complying with the standard will operate more quickly than the 1.45 x In line but more slowly than the 1.13 x In line.
Another way of looking at this, is that one Type B mcb may trip in 0.1 seconds with a current of 3 x In, whereas another (at the other end of the acceptable range) may require 5 x In. When we are looking at the properties for safety, we assume the Type B mcb we have is a "5 x In" one , but if we are looking at the effects of "inrush" currents that may occur at switch-on or after a power dip, we need to consider that the Type B mcb may well be a 3 x In one.
A manufacturer is likely to make their design somewhere in the middle of this (e.g. a 4 x In design for Type B), and in reality there will be a "standard distribution" around this for all qualifying Type B mcb's (i.e. statistically, many more mcb's in a batch in this example will be around the 3.7 to 4.3 x In range, than would be found nearer 3 x In or 5 x In).
Another thing which often confuses people is the "discontinuity" in the curves. The mcb has two components that can trigger a trip: the Magnetic element, and the Thermal element. The Thermal element takes more time to operate, and is generally there to protect against "overcurrent". The magnetic element operates from the large magnetic fields that occur with large currents, e.g. in a fault. The "discontinuity" in the curve, is where the magnetic element "takes over" from the thermal element.
EUR ING Graham Kenyon CEng MIET TechIOSH
Principal and Proprietor,
G Kenyon Technology