On 27/05/2015 22:31, GB wrote:
On 27/05/2015 21:03, ARW wrote:
"GB" wrote in message
...


I did wonder whether the cable is overspecified to allow for this.
There must be loads of ring mains with breaks or dodgy connections,
but the number of electrical fires is fairly small.


Most ring circuits are underloaded. If there is a bad connection at a
socket then the socket usually packs up before the wiring (there may be
some small localised wire damage).
The minimum Iz of the cable for a ring is 20A. That's the maximum a
double socket can supply without burning out.

Let me ask a question to make sure I have understood correctly. A ring
main takes 30A.

A modern one will be protected by a 32A MCB.

Any more and the circuit breaker trips.

That's not the way fuses or MCBs work in reality. They will take a
smallish sustained overload in excess of that indefinitely. Then they
will trip in ever decreasing time in response to a larger overload. You
need to look at the devices response curve to work out how long:

http://wiki.diyfaq.org.uk/index.php?...e-MCBTypeB.png

This curve is roughly designed to mimic the heating effect on the cable.
So to take the 32A device as an example, it will permit 50A of load for
around 1000 secs, 60A for 200 secs, 70A about 100 sec, and 100A for
perhaps 10 sec or so.

Once you get over 160A (i.e. 5x the nominal rating) then the magnetic
"fault" detection part of the trip kicks in, and they disconnect
"instantly"

The cable's current carrying capacity is defined by its maximum
sustained conductor temperature. For a PVC insulated cable this is 70
deg C. The circuit protection should ensure that this temperature is not
exceeded for an extended period (which would start to degrade the life
expectancy of the cable).

If there are no
glitches in the wiring that is carried over two cables with a minimum
current carrying capacity of 20A each, ie 40A total? So, there is a bit
of a safety margin.

The designer needs to ensure that the rated *installed* capacity (i.e.
taking into account the effects of insulation near the cable or other
factors that will lower its ability to shed heat and hence lower its
maximum current capacity) is at least 20A. In many cases it will be
higher than this. For cable buried in plaster or run on the surface, you
would get the highest rating of the cable. (which for 2.5mm^2 T&E is
around 27A). So the safety margin will vary from approaching 30% to
about 90%.

Note also that these are continuous ratings - in reality you will be
able to operate beyond those limits with no adverse effects due to the
nature of diversity.

http://wiki.diyfaq.org.uk/index.php?title=Diversity

I leant an extension lead to a very experienced builder working in my
house stripping some wallpaper with a steamer. I just left him to it. 30
minutes later the power circuit failed. The steamer was well within the
power rating of the extension lead, but only if it was uncoiled, which
the builder had not done. The whole thing had melted, not just the cable
reel but the insulation off the wires. The insulation was the
consistency of custard. It's a bit frightening, actually.

Indeed and a good example of the interrelationship between conductor
temperature, time, and current carrying capacity.


--
Cheers,

John.

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