Re: Why does laptop PSU have an earth wire?


"Chris Whelan" <cawhelan@xxxxxxxxxxxxxxxxxxxxx> wrote in message
news:xJZTf.7531$uT3.5968@xxxxxxxxxxxxxxxxxxxxxxx
Spiny Norman wrote:

[...]

You don't fit a 13 amp fuse in every plug either. Most of the plugs
have a 3 amp fuse in them. 13 amp in kettles and washing machines
etc. The PC I'm using has a 5 amp fuse in it. The monitor has a 3 amp.
I even have 2amp fuses in the drawer but I can't recall what they go
into at the moment:) The fused plug is primarily to protect the lead
but with so much equipment not having internal protection these days
they offer a modicum of protection for the equipment.

Yippee, I've finally found someone on Usenet that understands the UK
wiring
system. Thank you, you've saved my sanity!


Well as he didn't actually explain *why* we have fuses in plugs and everyone
else doesn't, maybe he does or maybe he doesn't understand.

What? Oh if you insist.

Like everyone else, the UK used to wire houses radially. That is, every
socket was individually wired to the fuse box with its own fuse. The UK
used 2 different sizes of socket, a 5 Amp and a 15 Amp, both with just 2
pins. In (I believe) the 1930's, the extra earth pin was added to the 2
existing sizes and a third size introduced (2 Amp), but was not universally
installed. The earth pin activated shuttering was solely to prevent the 2
pin plugs being inserted, and not to prevent fingers being inserted, though
that was a beneficial side effect exploited today.

Following the Second World War, the UK had to embark on a massive rebuilding
programme due to the wartime bombings of civilian homes. Problem was, that
there was a chronic shortage of copper to wire these homes up with. The
solution was to run a single 15 Amp 3 core cable from a 30 Amp fuse in the
fuse box, looped around all the sockets on one floor, and back to the same
fuse in the fuse box (the ring main). The sockets would all be rated at 13
Amp, but each plug was required to be fitted with a fuse appropriate to the
appliance.

Three different designs of plug appeared, one by Wylex (2 rectangular and
one round pin, almost in line, but the live was offset to polarise the
plug); one by D&S electrical (3 round pins in a triangle) and one by MK (3
rectangular pins in a triangle). The Wylex design never proved popular and
sank without a trace (though examples were still on sale in the early
1970's). The MK design is the 13 Amp 'square' pin plug that we know and
love today. The D&S plug, was almost exclusively encountered in council
built property (particularly by the old GLC), where D&S sold the sockets at
highly discounted prices and then recouped the losses through over inflated
plug prices (The D&S plug was five times the price of the MK plug). The D&S
plug had the novel feature that the live pin was the fuse and could be
unscrewed.

Although the fuse is selected to suit the appliance and is widely believed
to protect it, in reality it provides relatively minimal protection to
either the appliance or its lead. It is this minimal protection that has
lead to changes now requiring many appliances to be fitted with thermal
fuses, which offer better local protection.

The UK has stayed with the ring main concept to this day as it offers a
number of significant advantages over a radial scheme. Although mainland
Europe never adopted the ring main, a number of other countries did where
Britain had some influence, notably in the far East. Europe has floated the
idea of harmonising mains plugs with considerable resistance (especially
from the UK which would have to rewire buildings to a radial scheme to
retro-fit them).

BTW, 2A fuses were introduced for things like lamps, but often a failing
filament shorts the lamp out. The 2A plug-top fuse then usually blows
before the lamp's internal fuse.The plug-top fuse generally gets replaced
with the first one A.Consumer can find, which is bound to be a 13A!

Out of interest, (or possibly not :-)), although values of 2, 3, 5, 7, 10
and 13A are available, only 3A and 13A are recognised by BS.


The reason is that originally 13 Amp plugs were fitted with either a 2, 5,
7, 10 or 13 Amp fuse depending on the appliance to which the plug was
fitted. Sometime in the 1950's, the 7 Amp fuse was dropped, and in the
1970's the range was replaced with a new 3 Amp and the existing 13 Amp fuses
only. Items like televisions are often fitted with 5 Amp fuses, which
although a sensible choice, is not an official fitment.


.

Relevant Pages

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