"GB" wrote in message
...
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. Any more and the circuit breaker trips. 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.

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.



As you said, a normal ring circuit will use 2.5 T&E and a 32A MCB or 30A
fuse. The maximum current a 2.5 T&E can carry indefinitely without damage is
27A depending on how it installed eg if it is plastered into a wall it will
be 27A. Other factors can lower the CCC (grouping of
cables/insulation/temperature). eg A 2.5 T&E cable in an insulated wall but
touching the outer surface has a current carrying capacity of 21A.

Some new builds/refurbs need 4.0mm T&E ring circuits due to the insulation
in the building.


A 30/32A ring circuit must never use a cable that is installed so that it's
CCC drops below 20A. The MCB will also supply 40 odd amps for half an hour
or so before tripping. Short term overloads are allowed but long term small
overloads are not allowed.

Your extenstion lead is a classic example of a grouping factor - although I
would not call him an experienced builder if he fell for that one:-). Once
grouped around itself the CCC of the extention lead cable dropped from 13A
to say 5A.

Of course some people still bang on about balancing the loads on a ring
circuit and mention portable electric heaters to back up their statements.


--
Adam