Fault protection; no problem. Overload; with a 20A nominal load (allowing for some diversity) no problem. Even with a 26A load, in reality, no problem.
(cue dennis and his evil plan with 15kW of immersion heaters wired into a plug with a nail in place of the fuse)
In many places with the cables buried in wall chases, the cabled capacity on ring will be pushing 55A continuous, and significantly more short term.
They know enough to specify 20A breakers on circuits using the same cable as that on spurs. Just think why its not safe to use 32A in one case and is in the other (no it has nothing to do with rings).
Yes you are right, the heating effect is directly attributable to I^2R losses. However that is only half the equation. If you know the current flowing and the resistance per meter, then you can calculate the number of joules per second (aka Watts) being dissipated in the cable.
However, you now have to work out how to relate that to temperature rise. That will initially be dictated by the heat capacity of the cable. That in turn is dictated by the specific heat capacity of the materials in question and the mass of each.
So 1m of cable will dissipate x Watts. That will go to heat y kg of Copper. Double the length will dissipate 2x watts, but that will go to heat 2y kg of material. So the length actually is not relevant in this instance.
As the cable warms it will lose heat to its surroundings. The temperature will continue to rise until steady state conditions are reached where the heat loss equals the heat gained from I^2R losses.
Only in the sense that you can relate it the CSA of copper involved. Resistance per meter would be a better indicator.
A 2.5mm^2 T&E radial circuit of 5m total length, could be carrying 40A. The voltage drop at 18mV/m would only drop 40 * 0.018 * 5 = 3.6V well within the permitted voltage drop, but exceeding the current capacity of the cable.
That applies to XLPE sheathed armoured cable, which has a maximum conductor temperature of 90 (assuming its terminated in accessories also rated for 90 degree operation). In the case of a PVC clad T&E cable, with a temperature budget of only 70 degrees, the maximum current comes down to 27A when clipped direct.
Indeed, that's because they understand that with a radial circuits the responsibility for overload and fault protection is handled at the origin of the circuit, whereas for a spur from ring circuit the fault protection is handled at the origin, but the overload protection is imposed by the restrictions on the circuit design.
Ring circuits are designed for providing power to multiple portable appliances over a wide floor area with good flexibility and overall capacity, not feeding fixed high current loads. I am not sure why you feel that having circuits appropriate for different applications is in some way a failure of any given circuit design. A dedicated 16A radial is a good circuit for feeding an immersion heater, but a poor one for general purpose sockets in a kitchen. That does not mean the 16A radial is a poor circuit design, just that its more appropriate to some applications than others.
Overload and fault protection are provided by the MCB for the ring. Fault protection for a spur is also provided by the MCB. The possibility for damaging overload on a spur is principally eliminated by the restrictions on the number of sockets. One socket is treated as a 20A load, and 20A will not cause an overload. A user may in reality stick
26A on it, and a good quality one will probably hack supplying that for a reasonable time. Its also unlikely to result in cable damage (given that is in many cases rated for 27A).
Remember it is the responsibility of the specifier and installer to chose appropriate circuit designs. So if installing a ring circuit where the cable is to be de-rated due to the presence of insulation, then you will need to make intelligent choices about where and if you use spurs. A spur to a bedside socket or one in the TV corner is justifiable, whereas one to a utility space in a kitchen likely to take a pair of appliances is not.
How would you propose being able to draw 40A through a double socket for any duration?
The chances of drawing 20A through a 4 way trailing lead for any duration are slim:
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Double sockets are not rated or designed to carry a 40A load, and will not tolerate it for long enough that damage to your spur cable is going to be high in the list of things getting hot at melting.
It will provide fault protection but not overload protection.
There is another mechanism, and its not the plug fuse. So no need to worry.
The main thrust of dennis's argument is obviously bogus since his socket probably won't last 10 mins supplying 40A[1], however
a 32A MCB will carry 40A pretty much indefinitely. Have a look at the thermal / magnetic response curve here:
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considered acceptable (and indeed desirable) that short term overloads do not result in an immediate trip.
[1] And that assumes you could even cobble together 10kW of (non diverse) load and stick it on a pair of extension leads, and then plug them both into a double socket. Your would really have to try very hard to be that dumb.
but the restrictions don't provide overload protection! The only thing that provides overload protection is the user knowing not to do certain things. Its pretty easy to show that the majority of users doesn't even know about these restriction so its poor design, no, its p!ss poor design.
Rings are based on likely domestic usage. When first implemented, any likely high loading would have been from space heating. Nowadays, it's more likely to be kitchen appliances. Hence it being usual to have a separate ring for that.
It's either fortunate or excellent foresight that the plethora of electrical bits and pieces in the home these days that didn't exist 60 years ago are mainly low power consumers.
"Users" can get on and use the circuit as they see fit, and no problems will be encountered.
The fact that you can construct a wholly artificial situation using multiple extension leads, completely unrealistic loads and load durations, and then connect them via one double socket, gloss over the fact that the socket would be a smoking charred mess in very short order, just so that you can claim your innocent user was none the wiser, and this was just an accident waiting to happen, lacks any credibility whatsoever.
Its seems to be a remarkably effective bit of design that works well in practice.
Dennis frequently makes claims that "they have changed" this, that or the other, in the mistaken belief that it helps support his assertion the world must slowly be acquiring dennis' wisdom of ages, and its only a mater of time before everyone has seen the light.
Odd how he never seems able to indicate exactly when and where these "changes" took place.
(also glosses over the fact that guidance on technical matters changes for a multitude of reasons, but the fact that some grumpy twonk on usenet thinks they should probably does not even enter into the equation!)
Even so, apart from tumble driers, most kitchen appliances are fairly short-term / intermittent loads. The higher power they are, the more likely to be intermittent so the average load over time remains much the same.
Electric kettle? Grill? Oven? Clothes iron? And lots more I can't think of now. ;-) They may not be drawing maximum current for long - but long enough to trip the MCB or fuse etc.
As others have said the ring has stood well the test of time. It worked in the era of 30 A rewireable fuses (60 A to blow) and rubber 7/0.029 cable, originally rated at 15 A. Even 3/0.029 was allowed for spurs. In that era most houses had but one ring, few had central heating and the use of 2 and 3 kW fires was very common. In practice the severity and duration of overloads seems to have been insufficient to cause widespread problems.
The ring has an easier life now, with the 'finer' protection of an MCB or BS 1361 fuse, and far less use of portable heaters. Kitchen appliances are the most likely cause of overload now, but heavy loading from ovens, washers, driers, d/washers etc. all operating together will tend again to of be short-duration. Once the thermostats cut in and click away asynchronously the mean load will drop markedly.
You are aware, presumably, of the amendment made during the life of the
16th ed. (carried into the 17th) requiring the /designer/ (not the end user) to consider the likely load distribution in relation to the as-installed cable rating (Iz). That, properly applied, should prevent the situation where all the kitchen sockets are installed close to one end of the ring with an inadequate cable.
And you understand the clear difference between fault and overload protection and how to use the adiabatic equation to verify fault protection? Failure of the OPD to operate in the event of a fault is probably a more significant cause of fires than overloads.
Just how many rings have faults where either the live or the neutral doesn't make a complete ring? There are tens of millions of rings in use and there is no chance they are all fault free. So now instead of a ring you have two radials with numerous sockets *not fully protected* by a 32A breaker. Worse still is there is no way the user will detect the fault, even if he buys a socket tester.
Which bit of the circuit design protects the user from that?
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