But that's not really 'derating' is it?
If that were/is the case there should be no commas.
Why are unfused spurs allowed in the regs?
- Thread starter piggoy
- Start date
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What is it then? (or, if you prefer, what do you understand by "derating (below 'ideal')"?). Whatever, that's a purely semantic issue!
Not to my mind. Whether that phrase appears between commas, between brackets or without any punctuation, it would still convey the same meaning to me. I suspect we may have to disagree about this one!
Kind Regards, John
Or is it that folk get too tied up with absolutes?
Face it that whatever installation situations used a piece of 2.5 T&E is NOT going to ignite, overheat or other wise fail if a load of 26A is applied to it!
Why is this?Perhaps due to a little thing common to all engineering solution in this country the FOS.
The authors of advice know this and choose to accept that in this specified situation a lower FOS is acceptable, a decision no doubt reached after discussion and after carrying out a detailed risk assessment! The only condition specified is that it should not be prolonged (though that word is not defined, though in terms of the DNOs and the work we have done on cable ratings a time span of 8 hours would possibly fit)
Now, of course you may ask why is this information not generally available?
Perhaps because there are those without sufficient technical knowledge to correctly modify circuit design to ensure safety.
Face it that whatever installation situations used a piece of 2.5 T&E is NOT going to ignite, overheat or other wise fail if a load of 26A is applied to it!
Why is this?Perhaps due to a little thing common to all engineering solution in this country the FOS.
The authors of advice know this and choose to accept that in this specified situation a lower FOS is acceptable, a decision no doubt reached after discussion and after carrying out a detailed risk assessment! The only condition specified is that it should not be prolonged (though that word is not defined, though in terms of the DNOs and the work we have done on cable ratings a time span of 8 hours would possibly fit)
Now, of course you may ask why is this information not generally available?
Perhaps because there are those without sufficient technical knowledge to correctly modify circuit design to ensure safety.
As you know, a good few of us here sympathise with your views. However, I think you hit the nail on the head in the last thing you say that - it appears that, unfortunately seemingly increasingly, there are a good few people practising in this field who need 'absolute' rules or, at least, something approaching 'absolute guidance.' This is probably one of the reasons why BS7671 generally does deal with 'absolutes', rarely leaving much scope for intelligent judgement/discretion on the part of designers.
I think the OP's point in this thread was largely about an apparent inconsistency in BS7671. In general, BS7671 is 'absolute' in relation to cable CCC, in that it does not allow a cable's 'tabulated CCC' (no matter how conservative that may be) to be exceeded either by the In of protective device(s) or the potential load current, no matter how unlikely the designer considers it that the CCC of the cable will be exceeded for a 'long time'. However, it seems that the designer is suddenly given such discretion if the cable in question in an unfused spur from a ring final circuit. For those who 'need' absolute rules/guidance, this must be a bit confusing!
Kind Regards, John
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What is the definition of fixed and why should fixed be any different from non fixed? Washing machines, dish washers, and kettles are all over 2 Kw and are always fitted with a 13A plug. I have all three plugged into my kitchen ring final with no problems. The chances or using all three at once, no way.
BS7671 defines 'fixed equipment' as that which is "designed to be fastened to a support or otherwise secured in a specific location" (which does not necessarily include washing machines, dishwashers etc.) - but I agree that it's a totally irrelevent distinction.
Despite what eric says, AFAIAA there is no requirement for fixed loads >2kW to have a dedicated supply. The nearest of which I am aware is the guidance in ('informative') Appendix 15 of BS7671 which says that, as one of the design measures to avoid overloading of a ring final circuit, water heaters, space heaters and cooking appliances >2kW should not be fed from a ring final circuit.
Kind Regards, John
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