Regulation 433.1.201 indicates that a BS 88-2 or BS 88-3 fuse, with a rated current (I_n) not exceeding the current carrying capacity of the circuit conductors, will comply with condition (iii) of Regulation 433.1.1. [i.e. the current (I₂) causing effective operation of the protective device does not exceed 1.45 times the current carrying capacity of the circuit conductors].

However, the product standard for those fuses (BS EN 60269-1:2007+A2:2014), along with Table 4.1 of the Electrical Installation Design Guide, gives I_f (I₂) as 1.6 I_n, which clearly exceeds the 1.45 of Regulation 433.1.1. Furthermore, the conventional fusing time for fuses rated in excess of 63 A is greater than 2 hours, as opposed to 1 hour for a BS EN 60898 circuit breaker, which would surely increase the likelihood of a cable overheating under overload conditions.

Does anyone have an insight into this apparent inconsistency, or am I misunderstanding something?

Geoff