Date : Sun, 06 Sep 2009 19:30:49 +0100
From : jgh@... (Jonathan Graham Harston)
Subject: Leccy @ Acorn World '09
Mick Champion wrote:
> > 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
On-Site Guide, Table 7.1 - Conventional Circuits
Ring Circuit, 2.5mm, 30A/32A
Radial Circuit, 2.5mm, 15/16A
Hmmm. Destroys my previous point about rating*SQR(2) - which
electricians round to 1.5 ;) I'll have to dig through the regs
and my lecture notes to find where I've got that from.
> 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
Competant installation and testing. I can do you a whole-house
installation test and certification for 50 quid [plus travel! ;)]
> 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
If only one line has been chewed, an RCD/MCB would detect the
imbalance between lines, and trip. You can get dual RCD/MCBs (or do
it manually) where each end of the ring is connected into its own
connection to guard against the ring becomming two radials, but in
almost all installations that's unneccessary over-engineering.
> clue when one of your sockets (or more) stops functioning. Perhaps rings
> are preferred simply on a cost basis? Fewer MCBs and less cable?
Rings were originally invented to both lower costs (less cable for
the current) and spread the load. Also, with rings you can have as
many outlets as you want, with a radial you may only have a maximum
of three outlets. With radial power you end up with dozens of
fuse-ways and a huuuuuggeee fuseboard.
--
J.G.Harston - jgh@... - mdfs.net/User/JGH
PDP-11 BBC BASIC development - http://mdfs.net/PDP11/BBCBasic/blog