Wiki article on MCB/cable size

[JohnW2]

In view of the fact that cable has been around longer than mcbs, could it be that mcbs, when invented, had to be manufactured to these values to be able to be incorporated into an installation.

That is certainly a possibility. I don't know when the standard for Type B MCBs was first created, and the crucial question is whether or not the 45% acceptable temporary overload for cables had been established by that time. No doubt someone will know the chronology of those two events.

So it was the mcb that was matched to the cable even though now, apparently, we seem to be selecting cable to match the mcb.

Well, an appropriate MCB has to be selected to adequately protect the cable, but the cable size in turn should have been determined according to the design current (i.e. load) - so we shouldn't really do things the way around you suggest. If one starts by selecting an MCB to suit then design current and then selects cable size on the basis of the In of the MCB, then one obvioulsy should end up in the same place - but it's still the design current that has really dictated the cable (and MCB)selection. What is not acceptable is to go for an MCB whose In is less than the design current, even if the cable is matched to that MCB.

Kind Regards, John

 

[NotHimAgain]

Some of the confusion here may be due to not fully appreciating what a cable rating, as defined in Appendix 4, is.

It is important to realize that it is the maximum 'continuous' rating. A cable can transmit much higher quantities of energy for short periods of time without any detrimental effect.

Talking about temporary overloads is not helpful here as the cable is, in fact, not overloaded unless the elevated level of energy transmission continues beyond a limiting value. This value is determined by the temperature attained by the cable, as it is ultimately this that can cause damage to the insulation. The factors mentioned above are just the application of this.

The other point to note is that thermal protective devices cannot be set to provide an 'instant snap action' shut off when some notional setting is just exceeded. The curves in Appendix 3 for fuses, and for the mcbs operating in their thermal region indicate this.

A fuse element takes time to melt and a bi-metal strip takes time to bend. Contrast this with the operation of the short circuit sensor used in mcbs - this is a magnetic device and it does provide a 'snap' action - the graphs in Appendix 3 indicate this as the curve descends to zero as soon as a limit value is passed.

 

[JohnW2]

Some of the confusion here may be due to not fully appreciating what a cable rating, as defined in Appendix 4, is.
It is important to realize that it is the maximum 'continuous' rating. A cable can transmit much higher quantities of energy for short periods of time without any detrimental effect.
Talking about temporary overloads is not helpful here as the cable is, in fact, not overloaded unless the elevated level of energy transmission continues beyond a limiting value. This value is determined by the temperature attained by the cable, as it is ultimately this that can cause damage to the insulation. The factors mentioned above are just the application of this.

Yes, I now understand all that, and I think the only issue here is my rather sloppy use of the phrase 'temporary overload' in my previous message. I was using phrase to refer to the extent by which is was deemed to be not detrimental for current to exceed Iz for a limited period of time (i.e. the now infamous 1.45 factor).

The other point to note is that thermal protective devices cannot be set to provide an 'instant snap action' shut off when some notional setting is just exceeded. The curves in Appendix 3 for fuses, and for the mcbs operating in their thermal region indicate this.
A fuse element takes time to melt and a bi-metal strip takes time to bend. Contrast this with the operation of the short circuit sensor used in mcbs - this is a magnetic device and it does provide a 'snap' action - the graphs in Appendix 3 indicate this as the curve descends to zero as soon as a limit value is passed.

Indeed so, but this is presumably primarily of relevance in relation to fault ('short circuit') protection, rather than overload protection - for example, the 'snapping' of a Type B MCB occurs at 5 times the rated In, which is not the sort of current one would usually expect to see as a result of user-created overloads - one would need a 7.5kw shower connected to a 6A lighting circuilt or 20 2kw fan heaters (or half a dozen showers) plugged into a 32A RFC to achieve that degree of overload!

Kind Regards, John.

 

[NotHimAgain]

The other point to note is that thermal protective devices cannot be set to provide an 'instant snap action' shut off when some notional setting is just exceeded. The curves in Appendix 3 for fuses, and for the mcbs operating in their thermal region indicate this.
A fuse element takes time to melt and a bi-metal strip takes time to bend.

Overload protection is provided by operation in the thermal region of an mcb (a fuse is all thermal ).

 

[JohnW2]

Overload protection is provided by operation in the thermal region of an mcb (a fuse is all thermal ).

Quite so. As I said, the 'snapping' of an MCB only relates to fault/short circuit protection. As you say, overload protection is 'thermal' for both fuses and MCBs.

I'm not so sure whether the MCB/fuse difference in fault/short circuit protection is all that important in practice, is it? As we have all seen, given a full blown (excuse pun!) short circuit, a fuse will operate essentially 'instantaneously' (with or without a bang!).

Kind Regards, John.