6.3.3??

[sparkticus]

But whatever design, in reality I wonder how many RFCs there are and what infinitesimal cases there are of the fuse or MCB operating through overload?

Not too many overloaded I suspect, at least not for more than 10 minutes or so/two three times per day.

Yes we are aware of what limits we can "push" our kit too and over what limits we are in an "at our own professional risk" area. But if we didn't I would expect lots of cold Christmas Dinners every year!

Yes, there was a time when I used to respond to such calls on Xmas day though few and far between. These day I leave that to the next generation - who appear to switch their phones off anyway

 

[westie101]

If I'm on call (and, damn, I am this year) I gotta go!
Oddly our biggest "at risk" of our fuses operating due to load is caravan sites if Easter is early and cold!

Though I do recall spending a good part of New Year's Eve in a local town where the load from bars and air conditioning pushed loads on one circuit to 150% of the fuse rating (400A)

 

[bernardgreen]

Looking at it with my version of common sense.

Provided the over current protection device is rated such that overloads are disconnected before there is damage to the cable at its weakest point. ( where it is encased in thermal insulation ) then does it really matter what the predicted / calculated / estimated / guestimated current load might be.

If too many appliances are used on the circuit the over current protection device will operate before the cable is damaged by the current exceeding the desgin current.

In short if some one plugs in and turns on too many devices it is a fault and protection devices operate.

 

[sparkticus]

Looking at it with my version of common sense.

If too many appliances are used on the circuit the over current protection device will operate before the cable is damaged by the current exceeding the desgin current.

Of course, that is without question the thinking behind the design of a ring final and a radial for that matter. Diversity factored in the design anticipates that not all sockets will be fully loaded at the same time, indeed for many ring finals it would need to anticipate that perhaps only 50% were "partially" used at any one time with anything above that being a fault. But I think the implication thus far is that it is becoming more difficult (in a domestic environment at least) to factor the diversity.

 

[JohnW2]

Exactly, and as I tend to say much too often, domestic electrical is a bit of a nightmare these days. I also say (far too often) that I rarely design ring finals anymore. I try to plan for radials and plenty of them.

Precisely - but I don't really think that using radial, rather than ring, final circuits alters the problem significantly. The designer's nightmare arises becasue one has many socket outlets without any certainty as to what, and how much, will be plugged into them.

Kind Regards, John.

 

[JohnW2]

with the 32amp MCB perspiring in its zone between max Ib & In

But whatever design, in reality I wonder how many RFCs there are and what infinitesimal cases there are of the fuse or MCB operating through overload?

Exactly. In practice, the designer's 'judgement' usually works out fine. However, what one doesn't know is how often circuits are pushed into that zone between In and I2 for appreciable periods of time (theoretically in violation of BS7671). I get the impression that 'your' regs probably accept that getting into that zone for one of your fuses is a perfectly acceptable and common situation; in tyerms of BS7671, that would be called 'bad design', and non-compliant if it happened fairly often.

Kind Regards, John.

 

[JohnW2]

Looking at it with my version of common sense. ... Provided the over current protection device is rated such that overloads are disconnected before there is damage to the cable at its weakest point. ( where it is encased in thermal insulation ) then does it really matter what the predicted / calculated / estimated / guestimated current load might be.

Yes, at least conceptually, that also corresponds to my version of common sense. However, BS7671 (433.1) seems to be working on the assumption that there is something undesirable in having the load current exceeding In (but not exceeding it by enough to result in the device operating) for more than occasional brief periods. In as much as the devices were not designed for such a situation, and therefore may not have been tested under such conditions, I suppose they may have a point - at the very least, operating temperature would presumably rise at least a little beyond those of 'normal test conditions'.

As I wrote last night, even forgetting BS7671, I don't think it would be good design practice to protect a circuit supplying a fixed load of 17/18A with a 16A MCB, even if one knew that one would almost certainly 'get away with it' (in the sense of the device not operating).

Kind Regards, John.

 

[Paul_C]

I was suggesting that eyebrows could be raised at the suggestion that the design current was 6A for a circuilt supplying multiple 13A socket outlets. Of course, there are some situations in which one might be pretty confident that it was unlikley that the 6A total demand would ever be exceeded (despite the multiple 13A sockets), but I personally feel that it would be difficult to justify that in most 'normal' socket circuit situations;

This was reflected in older editions of the Wiring Regs., which stated that a single 13A socket must have an assumed demand of 13 amps. Thus for a circuit containing any 13A socket outlets, the minimum fuse/MCB size required was 15A.

 

[JohnW2]

This was reflected in older editions of the Wiring Regs., which stated that a single 13A socket must have an assumed demand of 13 amps. Thus for a circuit containing any 13A socket outlets, the minimum fuse/MCB size required was 15A.

That would make sense, but that still calls for serious application of discretion/diversity when there are multiple sockets! To assume that a circuit with multiple 13A outlets is never going to be loaded beyond 15/16A is clearly quite 'optimistic' - so, in that sense, the reg you are referring to was actually not all that 'conservative'!

Kind Regards, John.

 

[Paul_C]

To assume that a circuit with multiple 13A outlets is never going to be loaded beyond 15/16A is clearly quite 'optimistic'

The rule I referred to above was just in relation to making a 15A circuit the minimum rating acceptable to feed a single 13A socket, but for higher-rated circuits there were more rules which provided for diversity but which were more restrictive than today.

Under the 14th edition, a 20A circuit could feed a maximum of six 13A sockets, in a single room only, of up to 300 sq. ft. For a kitchen, or where the circuit served more than one room, the limit was just two 13A sockets.

And that was something of relaxation from the previous edition, which imposed similar requirements for kitchens or for a circuit serving two different rooms, but which for a single room allowed a 20A circuit to feed at most three 13A sockets over a floor area not exceeding 200 sq. ft.

In all of the above, a double socket outlet counts as two, thus in a kitchen a 20A branch circuit could feed two single 13A sockets or just one double.

 

[JohnW2]

Under the 14th edition, a 20A circuit could feed a maximum of six 13A sockets, in a single room only, of up to 300 sq. ft. For a kitchen, or where the circuit served more than one room, the limit was just two 13A sockets.
And that was something of relaxation from the previous edition, which imposed similar requirements for kitchens or for a circuit serving two different rooms, but which for a single room allowed a 20A circuit to feed at most three 13A sockets over a floor area not exceeding 200 sq. ft.
In all of the above, a double socket outlet counts as two, thus in a kitchen a 20A branch circuit could feed two single 13A sockets or just one double.

In engineering terms, that obviously brings one closer to 'proper design'. However, I assume that it was subsequently accepted (particularly when numbers of sockets escalated) that far greater degrees of 'diversity' did 'usually work out OK'. It's obviously a bit of an irony, and an additional complication to design, that (in general') the more sockets one has on a circuit, the lower the likely average load per socket. As I always say, unless one is simply going to add up the largest conceivable load that could be plugged into all socket outlets, one has to rely to at least some extent on a 'crystal ball'.

Notwithstanding my earlier comments about BS7671 not liking the idea of loads >In, the OPD is obviously a major guaradian against the more extreme possible situations.

Kind Regards, John.

 

[Paul_C]

However, I assume that it was subsequently accepted (particularly when numbers of sockets escalated) that far greater degrees of 'diversity' did 'usually work out OK'.

Hence, presumably, the later adoption of rules providing for a unlimited number of sockets so long as contained within a given area.

It's obviously a bit of an irony, and an additional complication to design, that (in general') the more sockets one has on a circuit, the lower the likely average load per socket. As I always say, unless one is simply going to add up the largest conceivable load that could be plugged into all socket outlets, one has to rely to at least some extent on a 'crystal ball'.

For sure. An older example of this can be seen from the days when BS546 sockets were still commonly installed in houses. According to the Wiring Regs. of the time, an assumed demand of 5A was to be allowed for each and every 5A socket, restricting a 15A branch circuit to supplying no more than three such sockets. But in practice, installations could be found with many more 5A sockets on a 15A circuit, because in many cases the extra sockets were added for convenience in using existing portable appliances rather than for adding new loads, or for feeding radios, bedside clocks & lamps, TV sets, and similar which obviously did not draw anywhere near the full rated current of the outlet.