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Overload protection - Regulation 433.1.201and BS 88 fuses

Geoff Lyons

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

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  • I think we've had this come up before:  Cable overloads and fusing factors (although without a clear solution).

    It does sound like a good question - maybe we should have a 1.45/1.6 = 0.9 correction factor. Of maybe it's like MCBs (following the energy let-though values of the standard suggest we'd min 2.5mm² c.p.c.s on a 6kA rated B32 and some D types are allowed up to 8s rather than 5s to disconnect at 10x In) - where there's an understanding that (most?) manufacturers do rather better in practice than the minimum requirements of the standard.

       - Andy.

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  • I think we've had this come up before:  Cable overloads and fusing factors (although without a clear solution).

    It does sound like a good question - maybe we should have a 1.45/1.6 = 0.9 correction factor. Of maybe it's like MCBs (following the energy let-though values of the standard suggest we'd min 2.5mm² c.p.c.s on a 6kA rated B32 and some D types are allowed up to 8s rather than 5s to disconnect at 10x In) - where there's an understanding that (most?) manufacturers do rather better in practice than the minimum requirements of the standard.

       - Andy.

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