Date : Fri, 04 Sep 2009 16:18:53 +0100
From : rs423@... (Mick Champion)
Subject: Leccy @ Acorn World '09
Phil Blundell wrote:
> On Thu, 2009-09-03 at 16:18 +0100, Mick Champion wrote:
>
>> Jason Flynn G7OCD wrote:
>>
>>> Mick Champion wrote:
>>>
>>>> A 32 amp breaker is only permissible because each socket
>>>> has two cables feeding it from either side of the
>>>> ring effectively doubling up the rating.
>>>>
>>> It's more complicated than that: the cable that should be used
>>> is dependent on the material it is enclosed in and the length.
>>>
>>
>> I don't doubt any of that. However, what do you mean by "material it is
>> enclosed in"? Do you mean aperture the cable rests in, or the cable
>> covering (ie PVC)? If you refer to the aperture, please tell more.
>>
>
> Both are significant; the rating of the cable depends on its own
> construction materials and on the situation where it's mounted.
>
Okay. The "where it is mounted" bit is news to me, but I don't doubt
what you say.
> The former determines the maximum allowable operating temperature for
> the cable: mineral insulation can run safely at higher temperatures than
> XLPE, which in turn can run hotter than normal PVC.
That sounds about right, but most home installations I've seen use
twin+earth PVC throughout, although not all. My flat uses single 1.5mm
cables for lighting through earthed metal conduits, and ecently, I saw
Pirelli cable being used in a flat in Hersham for both lighting and
sockets. That was very odd.
> The latter
> determines the temperature rise that is likely to occur for any given
> amount of dissipated power in the cable: one that is buried in
> insulating material, or bunched together with many other cables, will
> get hotter at the same current than another cable sitting in free air.
>
I hadn't thought of the enclosure temperature but this makes sense. If
the heat has nowhere to go, it'll just get hotter.
> Incidentally, going back to what you wrote above, the allowable current
> for a ring tends to only be about 150% of the current that would be
> permitted in an equivalent radial circuit, not double.
I need to find some documentation on this purely out of interest. BS7211
or whatever replaced it. If say the 2.5mm twin and Earth is 19amps, then
add half again, you've only got 28.5 amps protected by a 32 amp breaker?
Perhaps I've got the cable rating wrong.
> This is one of
> the reasons that ring circuits are often not an efficient use of
> resources: rather than installing a single 32A ring wired with 2.5mm
> cable, you might well be better off using that same 2.5mm cable to
> install two 20A radial circuits and hence gain 40A capacity in total.
>
I agree and think radials make safer circuits. With a ring, we rely on
the two ends to be continuous. How would you know if they are not? You'd
still get power on all sockets if there were a break was in between. Say
someone "not in the know" does a bit of DIY and fails to connect one end
properly, or a rat needs a quick chew. Your breaker will still not trip
until 32 amps have been exceeded, but now all your power comes from one
end or the other depending on the socket in use. Potential cable
overload!!!! If the same thing happened on a radial, you'd have a big
clue when one of your sockets (or more) stops functioning. Perhaps rings
are preferred simply on a cost basis? Fewer MCBs and less cable?
Mick