I'm fairly sure the correction factor of 1.6 is for a re-wirable fuse. (Not a BS-88 HRC fuse.)

I'm sorry - I've out clevered myself - I wish I knew how to delete a silly reply - but I dont - 0.725 is the correction factor for re-wireable fuses, not 0.6.....

Please pass over my above reply. 

I've just checked p425 5.1.1 - this references back to 433.1.201 - does this help? Probably not and I'd need access to the product standard.........

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.

product standard for those fuses (BS EN 60269-1:2007+A2:2014)

I wonder if it was the same in the older British versions of the standards? (e.g. BS 1361 before they were subsumed into EN/IEC common ones)

  - Andy.

AJJewsbury said:

maybe we should have a 1.45/1.6 = 0.9 correction factor.

The factor would actually be C_f =0.925 (using the same significant figures as C_f for rewireable fuses) , and therefore within 10 % of 1.

Why is this relevant?

I think the answer likes in an explanation given in Guidance Note 6 (Section 2.6, page 28 in the 9th Ed):

So, 1.45 being a 'good estimate' perhaps allows some lee-way ... maybe 10 % ... which would therefore OK for I₂=1.6I_n

Geoff Lyons said:

urthermore, 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.

1 hr vs 2 hr probably not an issue ... again the extract from GN6 ends with the statement 'However, such currents must not persist for long periods' ... this is of course related to the first requirement of Regulation 433.1

Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

gkenyon said:

The factor would actually be C_f =0.925 (using the same significant figures as C_f for rewireable fuses) , and therefore within 10 % of 1

1.45 / 1.6 = 0.906  - but still within 10% of 1

I appreciate the empirical nature of the 1.45 factor, but if we believe that a 10% leeway is reasonable, would it not be more correct for Regulation 433.1.1 condition (iii) to say “does not exceed 1.595 (1.45 x 1.1) times the lowest…”? Or even round it up to 1.6 for consistency with Regulation 433.1.201.

gkenyon said:

1 hr vs 2 hr probably not an issue ... again the extract from GN6 ends with the statement 'However, such currents must not persist for long periods' ... this is of course related to the first requirement of Regulation 433.1

Every circuit shall be designed so that a small overload of long duration is unlikely to occur.

Fair point, though how long is a long duration? Up to 2 hours for a 25% overload (I_n = 100 A) perhaps.

Anyway, I feel my question has been satisfactorily answered; thanks everyone for your contributions.

Geoff, I agree, if I can find out some more I will get back to you.

Thanks very much Graham; there's no urgency though, my question was just out of curriosity prompted by the EIDG.

or even curiosity!