Jim Michaels wrote:
On 18 Jun 2005 03:30:22 -0700, wrote:
The basic issue is that
very few people die of electrocution, whilst loads of people die from
electrical fires. The electrical fires largely stem from high currents. If
you halve the voltage, you double the current.
NOT TRUE.
You are making multiple invalid assumptions.
1. That U.S. wiring has the same number of circuits for the same load.
still cant figure out what you mean there. For a given load you do have
higher i with lower v, and each load is on 1 circuit as far as i can
see.
UK Ten 100watt lamps at 240V equal 4.166 amps on circuit rated at 6
amps with 1mm wire.
US Five 100 watt lamps at 120V equals 4.166 amps on 15amp rated
circuit with 14gauge (2.08mm) wire.
In this example the US system has a massively greater safety margin.
Your analysis is too simplistic. The load current / cable rating is not
something that causes any significant number of fires in either case,
it is a nonissue in reality.
Also I assume you realise 1mm2 is capable of much more than 6A, it is
merely fused at 5A or MCBed at 6A.
Safety margin is determined by looking at what in the system causes
safety failures, and how often. Cable rating doesnt come into it. Your
heavy US cables are merely a waste of resources, achieving nothing
afaics.
Unless you can explain how 4A on 32A capable cable tripped at 15A is
safer than 4A on 15A cable tripped at 6A. (figures are examples, not
calculated)
2. That U.S. circuits are not designed for their load.
I dont think that was the assumption:
The proof is that you keep saying "for a given load" and the loads are
NOT the same, we have many more circuits in typical dwelling.
to be honest I dont know what youre referring to, youve snipped the
relevant stuff out.
But I remember just enough to be fairly sure the assumption above didnt
come into it, that IIRC you misunderstood what was being said.
the problem is simply theyre
designed to have a higher incidence of faults.
Simply a system with more smaller circuits each with an equal or
greater degree of safety margin.
Youre not understanding safety margin. Size of cable has nothing to do
with it, once the cables big enough not to overload. Ours are big
enough and much more. Yours are even bigger, but for what? Its just
poor engineering.
As an example a modest 3 bedroom suburban home normally has a 200amp
40 way main panel (CU). That is 200amps in each leg of the incoming
feed using three 85mm cables.
Above ground supply or buried? 85mm2 is awful big, even for 200A. Or is
it aluminium?
this provides 48kW of power. This is a
home with gas space heating, gas water heating, gas clothes drying,,
and often gas cooking.
Even with our maniacal excess it would be hard to overload such a
system to the point of combustion.
Again you miss it. Your systems are overloaded day in day out, not at
the service entrance but at the wall plugs that get too hot, and the
wirenuts that cant reliably maintain their ratings. The result is a
high level of fires.
The practice of push-in
connection on mains sockets is something considered unthinkable here,
for good reason.
They are a homeowner/shoddy contractor item and are frowned upon.
precisely, here theyre illegal and unheard of. There isnt even a black
market in such junk, its just off the scale. Even the occasional 50
year old install isnt that bad.
In US IIUC it would be called 240v, but in fact be 120-0-120, so the
voltage from earth is 120 ac, not 240. In which case the fault
clearance benefit of genuinely 240v would sometimes apply and sometimes
not. Some faults that smoulder at 120 can arc over and trip at 240.
And some that would not cause a fire at 120V will burst into flames at
240V.
yup. the question is which is the greater number. 240 gives much better
clearance rate than 120.
3. Combination of neutral to earth (i.e. effectively TN-C earthing) leading
to electrocution in the event of polarisation swap, or some open circuit
conditions.
snip
Why snip the explanation?
if you could quote all relevant material I might be able to answer.
NT