The two conditions of 433.1 - and the one everybody forgets
Overload protection in IEC 60364-4-43 is two inequalities, not one. The second is about the device conventional operating current, and it is the one that fails.
Two inequalities, not one — A device protecting a cable against overload has to satisfy both conditions of IEC 60364-4-43, 433.1. Most submissions check the first and assume the second.
The conditions — I_B is the design current of the circuit; I_Z the continuous current-carrying capacity of the cable; I_n the rated current of the device - for an adjustable device, the current setting selected; I_2 the current ensuring effective…
The same thing on one axis — Redrawn from Figure B.1 of IEC 60364-4-43. The left group belongs to the circuit and the cable; the right group belongs to the device. Nothing links them automatically.
Where it goes wrong
Condition 2 is a statement about the device you bought — Where the device conventional operating current is at or below 1,45 times its rated current, condition 2 follows from condition 1 and needs no separate arithmetic. Where it is higher - and some devices are - the cable has to be larger…
The review checklist — A schedule that shows the four currents in four columns is checkable in seconds. One that shows only "cable size" and "MCB" is not checkable at all.