That doesn't matter - the fuse/mcb is there to protect the cable.
3A MCB/RCBOs
- Thread starter mikhailfaradayski
- Start date
It all comes back to the age-old discussion as to what the purpose of an MCB is. Many of those here frequently assert that they are there only to protect the cable of the fixed wiring. If you take that view, and are wiring in T&E (of which 1mm² is the smallest available), there's never going to be a reason to have an MCB less than 6A (well, actually 8A, provided grouping or other de-rating factors are not at work).
Others (seemingly including yourself) believe that OPDs can sometimes have a useful role in protecting things in addition to the cable, but they/we tend to get shot down
Kind Regards, John.
Bang, bang. 
So what about the alleged 'unacceptability' of having a 50A MCB 'protecting' a 40A switch (connected to a <40A fixed load) which I seem to remember discussing in the last day or two?Bang, bang.
...does 'cable' include switches??
Kind Regards, John.
Others (seemingly including yourself) believe that OPDs can sometimes have a useful role in protecting things in addition to the cable, but they/we tend to get shot down
Kind Regards, John.
I think its more than a belief, its a fact! Whilst I appreciate it provides the required protection for 1mm T&E, there are circuits where the hard-connected devices do not have a fuse, and a short circuit getting up to 6amps before tripping could be hazardous!
There are instances where the MCB for a circuit is set for the appliance on the end, such as immersion and storage heaters - wired in 2.5mm T&E, and MCB rated at 16amps
I refer the honourable gentleman to my previous answer
Technically there is no problem - but a >50A switch would, of course, be required.
However, if new, it would be considered bad design.
Yes. An even earlier thread deduced that that was why lighting circuits were protected by 5A/6A devices although the cable was capable of handling more....does 'cable' include switches??
In case you don't realise, I'm on your side! The 'others' will, of course, argue that whatever is on the end of the cable should have it's own fuse, appropriate in rating to protect that 'something' - but, as we all know, that's very often not the case.
Whatever one's view, I really can't see that it can ever do any harm to use a fuse (or other OPD) of the minimum rating that is suitable for a fixed load - so, given that it might 'do some good', it seems only sensible to me to adopt that approach.
Kind Regards, John.
OK - for "alleged 'unacceptability" please read "alleged poor design for new circuits".
Exactly - so I don't really understand your 'yes'. Or, rather, I don't understand all these assertions that the OPD is there only to protect cables. If they wish to avoid confusing readers, those who make such assertions should surely at least qualify/clarify their statement with an indication that, when they say 'cable', that also includes accessories (but not 'end loads').[Yes. An even earlier thread deduced that that was why lighting circuits were protected by 5A/6A devices although the cable was capable of handling more....does 'cable' include switches??
Kind Regards, John.
I suppose that is fair comment although I don't know if I have ever inserted "only" in the statement to indicate 'and nothing else'.Exactly - so I don't really understand your 'yes'. Or, rather, I don't understand all these assertions that the OPD is there only to protect cables. If they wish to avoid confusing readers, those who make such assertions should surely at least qualify/clarify their statement with an indication that, when they say 'cable', that also includes accessories (but not 'end loads').[Yes. An even earlier thread deduced that that was why lighting circuits were protected by 5A/6A devices although the cable was capable of handling more....does 'cable' include switches??
Although, to cover every eventuality in an answer (for the uninitiated) would seem a tad onerous and lead to even longer threads.
Have you any examples?I think its more than a belief, its a fact!Others (seemingly including yourself) believe that OPDs can sometimes have a useful role in protecting things in addition to the cable, but they/we tend to get shot down
A short-circuit will be a lot more than 6A.
Indeed it has to be but we are talking about overload not short-circuit
If the manufacturers of a 1A appliance consider it necessary they should install an integral 1A fuse - and not demand a 3A fuse (still three times larger than necessary) be fitted in the circuit.
Not sure why immersions (13A) are always wired with 2.5mm² (max.27A) conductors - 1.5mm² would do. See other thread.
How do you think the 16A mcb protects the immersion element?
Maybe you don't (I really can't remember) but some certainly do include the 'only', and the context in which the assertion is made is usually such as to make it appear that there is an implied 'only'/
That's my worry, even if it makes things a little onerous for you. If 'the uninintiated' read these assertions, I think that (whether or not the word 'only' appears) they are very likely to assume that it means 'only the cable'.
Kind Regards, John.
Would it suffice if, in future, I said 'the circuit' instead of 'the cable'?
Well, I certainly think that would be less likley to confuse/mislead DIYers than just saying 'the cable'.
However, I'm not sure how such people would interpret 'the circuit' - and the danger from your point of view would be that they might interpret it as including things which you don't believe the OPD is there to protect!
Kind Regards, John.
Did you think about volt drop?
I did not.
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