The subject of British ring mains came up in another group
(alt.usage.english). When it was said that nobody really knew the
history, one contributor came up with this explanation, which I
reproduce here for your entertainment.


#[nobody knows the history]
#
#I think I do. (some Brit in the know explained it to me long ago)
#The savings in copper is with respect to a hypothetical situation
#that never existed, and never came to be.
#(sorry this is really to technical and off topic for aue,
#but here we are)
#The idea was that Brits, forever shivering in their damp,
#uninsulated, solid stone walled houses you know,
#needed 5 kW electric heaters to get comfy.
#Now for 5 kW you need 30 A fuses,
#and for safety, having 30 A fuses requires 6 mm^2 wiring.
#(the heavy wire is needed because circuit resistance
#must be low enough to melt a fuse promtly, in case of a fault.)
#Fuses were the only safety available at the time.
#
#Now copper, for a star wiring in 6 mm^2,
#to wherever you may want heat
#was prohibitively expensive, by 1945 standards.
#So the bright idea was to have a ring of 2.5 mm^2 wire,
#fused with 30 A. This is fine if the ring is unbroken,
#with current to the fault going both ways.
#If the ring is interrupted you have a very dangerous situation:
#a short in effectively unprotected wiring.
#(2.5 mm^2 behind a 30 A fuse that requires 150 A to blow promptly)
#
#So it was decided to ameliorate that by having fused plugs as well,
#as back-up for the inherently unsafe main wiring,
#since the mains fuse can't be relied upon to cut out
#when an appliance is faulty.
#
#The sequel: the 5 kW heaters never materialized.
#(the impoverished Brits couldn't afford them,
# and the 'too cheap to meter' power never arrived)
#and Britain is stuck with an inherently unsafe wiring method
#that may actually waste copper.
#(you could save copper and gain safety
#by leaving out part of the ring, and by using the two halves
#as branches of a star, fused with 2x16A instead of 1x30A)
#
#It was a typical case of technocracy runnng wild,
#without informed public debate about the basic assumptions
#underlying it as feedback.
#More briefly, a muddle.

--
Mike Barnes