Ooh, I was trying *so* hard to keep out of this one for a while -/

In message ws.net,
":::Jerry::::" wrote:

"Andrew Gabriel" wrote in message
.. .

[big snips, anyway]

What happens instead is, "Oh, I wonder which appliance blew
the fuse?" Start trying them all in another circuit, and eventually
BANG!.

Err, how many appliances are you plugging into a single outlet ?...

That wasn't the point. The point was that there are multiple outlets on
a single circuit. Even with your 120V 15A radial design you might have
half a dozen or more appliances protected only by the common fuse at the
distribution board. When that fuse pops, how do you know which one of
the appliances has caused the problem?

With our 240V 30/32A rings, 30/32A, 20A and 16A radials we are allowed,
effectively, an unlimited number of sockets. My house has (ignoring the
kitchen) 24 double sockets. Because it is relatively small, these
could have been (though they aren't) supplied by a single ring or even a
single 30/32A radial. If the circuit fuse pops, that's a lot of
appliances to check.

If, on the other hand, a 13A fuse manages to correctly discriminate with
the 32A MCB (unfortunately it won't always) then the job's a lot easier.

So protecting something at a 15 A rating is less safe than doing so
at 30 A when someone replaces the fuse in a BS1364 plug with a bolt
or what ever, as has happened ?....


Well, I'm sure it *has* happened, but personally I've not come across it
ever, and I do get to see a few plugs. It was slightly easier to do in
the days of wire-it-yourself plugs, but in these days of moulded plugs
it is difficult to get anything other than a BS1361 1" fuse to fit into
the fuseholder.


The design starts by looking to see where the failures tend to
be. They tend to be in the appliances and appliance cord, i.e.
the parts which get moved around, and not in the fixed wiring
which is generally well protected from degrading effects. So
to localise such failures, protection is applied at the start
of the common failure path, which is the appliance plug. This
has a big advantage that the protection is intimately associated
with the item it protects, so when you have a faulty appliance,
there is never any question of which appliance it is (ignoring
RCD's, which are a later 'problem'), or of moving the faulty
appliance to a different socket and continuing to use it.

So still have local protection if that really is going to be a
problem.


Your point being? We already do have "local protection". It would
require a major redesign of the US system. One further step we *could*
take would be to make all socket outlets RCDs, but this would be
incredibly expensive and rather ugly. RCDs are secondary protection
anyway, and the fuse/MCB is the primary (other than earth faults in TT
systems of course).


Well, that's because prior to around 1970, it wass illegal to
supply an appliance with a plug in case it was the wrong type,
someone cut it off, and then stuck the resulting loose plug
and exposed flex into a socket. That was just one tiny part of
the problem of having multiple socket types. By 1970 (can't
remember the exact year), judgement was that sockets other than
13A were now sufficiently rare that the law could be changed to
require a BS1363 plug to be fitted to all appliances which were
expected to be plugged in to a socket.

Err, it became law in about 1990 IIRC, and as for sticking wires into
sockets, the non fitment of any plug increased that risk !


It is darned nearly impossible to stick bare wires into a BS1364
shuttered socket outlet without at least two pairs of hands. But that
wasn't the problem, the problem was that if the moulded-on plug is
unsuitable it needs to be cut off. It *could* then be inserted into a
socket, leaving live wires exposed to the touch.

Of course, having a removeable fuse in the plug helps even here, as most
appliance instructions I have read include a phrase along the lines of
"if you need to remove the moulded plug, cut it off, remove the fuse and
dispose of with your household waste."

Any responsible person cutting off a moulded plug can remove the fuse,
remove the fuse holder and (if they're feeling particularly paranoid),
bend the fuse carrier with pliers so that even if some neighbour child
digs it out of your wheelie bin from under last Friday's donner kebabs,
it would be nearly impossible for him to electrocute himself.



many people fitted a plug without then changing the supplied 13amp
fuse to the correct lower rating?...

All new appliances nowadays are required to remain safe with 16A
protection, so actually you can leave a 13A fuse in everything
sold in last 10-20 years.

But not 30A when some idiot uses a bolt to bridge the fuse....

Not continuously, no, but for short-term overloads...

....

Hmmm... I'm off to do something less stressful.

Hwyl!

M.

--
Martin Angove: http://www.tridwr.demon.co.uk/
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