Regulation 433.1.201 indicates that a BS 88-2 or BS 88-3 fuse, with a rated current (In) 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 If (I2) as 1.6 In, 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
